CHAPTER 3
AN ANALYSIS OF MORTALITIES INVOLVING THE VULNERABLE AFRICAN GRASS OWL (TYTO CAPENSIS) AS WELL AS IN THE MARSH OWL (ASIO CAPENSIS), BARN OWL (TYTO ALBA) AND THE SPOTTED EAGLE OWL (BUBO AFRICANUS) RELATED TO VEHICLE COLLISIONS IN GAUTENG.
“Owls face many natural hazards anyway, but piled on top of these are the
growing perils of living in an environment increasingly modified by man”
(Tarboton & Erasmus, 1998).
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3.1. INTRODUCTION
Human advancements have had a major impact on nature in numerous and various ways. Most species extinctions are anthropogenically induced. This has been achieved mainly as a result of the expansion of major towns, industries and agriculture into natural habitats. Other factors include fragmentation as well as destruction of forests and pollution of rivers, which, according to Whitford (1985) have together resulted in a changed world.
Little is known about the mortality of raptors in relation to other aspects of their ecology. This is because the actual deaths are seldom witnessed (unless at the hand of man) and because no known method of study gives unbiased information on the ages at death, or on the causes of death. Mortality estimates are important in understanding population turnover and in pinpointing vulnerable age groups or populations (Newton, 1979).
Causes of death to owls vary from starvation during harsh winters, entrapment in man-made structures, and collision with obstacles. Drowning, being deliberately killed, and poisoning by pesticide have also rooted many deaths.
The majority of these are a result of human interference with fairly few owls dieing from natural causes (Newton, 1979). One important impact on owl mortality, which relates to this study, is the prevalence and advancement of roads in developed as well as developing countries throughout the world. This contributes to the great loss and fragmentation of their habitats and placing them in direct threat of being knocked over by a vehicle. In an extensive study of the Barn Owl (De Bruijn, 1994), found that the majority of these owls (56%),
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from ring recoveries, died as road victims, starvation in harsh winters accounted for 15%, being locked up unintentionally (8%), collision with obstacles such as walls, windows and power lines (6%), 2% were found to be killed deliberately and 13% making up other causes not mentioned. Although roads are required to meet various demands of the public and hence also form a part of human progress, they represent one of the commonest and most extensive intrusions of man into natural areas (Whitford, 1985). Roads have also been known to have direct and detrimental effects on wildlife itself and the threat of traffic on wildlife have long been recognised.
There are many examples in the international literature on road mortalities of wildlife and birds throughout the world. There are a number of studies involving general wildlife being killed by traffic, e.g. wildlife destruction by automobile traffic (Dickerson, 1939); vertebrate road mortality (Hodson, 1960); animal victims on Nebraska’s highways (McClure, 1951); and wildlife mortalities in New South Wales (Vestjens, 1973). Studies relating specifically to bird mortalities are reflected in papers by (roadside birds in Punjab)
Dhindsa, Sandhu, & Sandhu, (1988) (Wiltshire) Dunthorn & Errington (1964),
(roadside birds) Finnis (1956), road casualties particularly among House
Sparrows, Blackbird and Song Thrush birds are discussed by Finnis (1960) and Govett (1960), vertebrate road mortality were discussed by Barnes
(1936); Harrison (1954); Hodson (1960); Hodson (1962); Schorger (1954) and
White (1927).
There are only a few published records of road-killed owls, throughout the world and their relationship with roads. These limited studies have been
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conducted on T. alba and A. otus (Baudvin 1997); T. alba, Tyto capensis, Tyto novaehollandiae, Ninox novaeseelandiae and Ninox strenua (Clancy 2002-in press); T. alba (De Bruijn 1994, Newton, Wyllie & Dale 1997, Shawyer and
Dixon 1999); A. noctua (Fajardo, Pividal, Trigo & Jimenez 1998; Hernandez
1988); and Westphalian Owls (Illner 1992).
A survey in East Germany recorded 51 Barn Owl road kills on a 2.5 km stretch of freeway over a seven day period (Uhlenhaut 1976) and the main cause of
Barn Owl deaths in Great Britain (N>1000) were starvation and collision, mostly with road traffic (Newton et al., 1997). Owls and diurnal raptors accounted for 81.5% of all birds killed by vehicles in a study in France
(Baudvin, 1997). In far North Queensland 13 road killed Southern Boobooks were recorded between Mount Garntt and Georgetown on one day
(Emmerson, 1999). A study conducted on mortalities from vehicles on the
Little Owl (Athene noctua) in Spain represents 82% of non-natural deaths of
A. noctua (Hernandez 1988)
Within South Africa there is reference to the subject of vertebrate mortalities by Siegfried (1965,1966) for the Cape Province and Stellenbosch as well as a few others limited to the grey literature mostly reporting on general observations and a number of letters published in newspapers, mentioning species involved, the number of mortalities, their localities and mileage travelled by the observer.
There is a particular paucity of published data regarding the limit of use of road verges by raptors, with few references to the nature of verges, the
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surrounding habitats or the effect of seasons (Knight & Kawashima, 1993;
Meunier, Corbin, Verheyden & Jouventin, 1999; Meunier, Verheyden &
Jouventin, 2000). Yet, the potential importance of roadsides is indicated by the literature on raptors’ associations with other linear man-made habitats like power lines (review in Williams & Colson, 1989).
Not many birds have been known to be killed on roads in the past and is relatively new for developing countries (Dhindsa et al., 1988),. This could probably be attributed to unsophisticated infrastructure of roads and vehicles.
Most vehicles usually did not exceed the speed of 80 kilometers per hour or perhaps vehicles were extremely expensive to own. Further more roads were in exceptionally poor condition and were not maintained as they are today, using more resilient road surface materials. Thus the advancement of technology today has resulted in faster moving vehicles in conjunction with reconditioned and improved surfaces facilitating faster traveling and resulted in higher volumes of birds being killed in this manner. This does not seem to be the case for South Africa, where few reports of bird counts are also due to the lack of mortalities not being formerly reported and such extensive studies have not been scientifically conducted.
While traffic on the N17 highway is not as intense as that on more heavily used motorways, such as the N1 and M1 highways of Gauteng, highway loses to wildlife are greater (pers. obs.). Perhaps owing to the fact that the N17 is a fairly new highway, may have caused an increase in owl mortalities in the area that was previously not threatened. This implication is a major concern. It
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was Paul Jooste’s5 (pers. comm.) experience of traveling along this road and observing high numbers of owls found on or alongside this road that led to the present study. Moreover, the present study draw’s attention to this important ecological aspect that roads have on our wildlife, which has largely been neglected throughout southern Africa.
Considerable losses of birds that are of valuable concern have attracted comments by many observers. For this reason the high numbers of the scarce
Grass Owl initiated the grounds to investigate the cause of such an obstacle that adds to the many other anthropogenic factors causing them to decline.
This chapter reports on how many owls were recorded as road casualties due to vehicle collisions along the N17 and R550 roads, and of which species in particular, would frequent roads and may be prone to mortality from fast- moving vehicles. Additional information on other problem areas throughout
South Africa is also presented.
As far as could be ascertained, no such account has been published on owl mortalities in the South African region especially in the Gauteng Province.
This chapter gives the results of the first study of its kind during a two-year period in the East Rand Highveld, including important information in addition to mere mortality counts.
5 Mr. P. Jooste, Endangered Wildlife Trust, Devon, South Africa.
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3.2. MATERIALS AND METHODS
3.2.1 Study area
The section of the N17 between Springs and Devon (Figure 3.1) was surveyed daily, for two official years (2002 to 2003) and three unofficial years, to determine owl mortality attributable to collisions with motor vehicles. Both the N17 and the adjacent R550, which runs parallel to the N17, were surveyed for a total length of 30 km of fairly flat roadway. Both road surfaces consist of dark tarmac. The N17 is flanked throughout its entire length by grassy verges of 10 to 30 m up to a boundary line, which consists of a 1.2 m, five-stranded barbed wire fence. These verges are interrupted by ditches for the purpose of road drainage, as well as access bridges adjoining farms. The R550 is less complex, with smaller road verges up to 10 m. The north and south of the N17 roadway, adjoining the R550, are flanked by predominately-cultivated fields such as Erogrostis curvula, maize, sunflowers as well as Cosmos bipinnatus with some sections of grassy grazing paddocks used by cattle, similar to the adjacent land of the R550. Small farm dams, connected by small streams undulate through the area. The road verges vary in gradient from being completely flat to 60° angled slopes to accommodate farm bridges. The R550 is flattened throughout.
The road connecting Springs to Devon is generally a single-lane national freeway with one lane approximately five meters in width both ways. The
R550 is a single-lane main road with no emergency lane and narrower road verges. A single-lane secondary road and one arterial route traverse the N17
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and R550 in two different places. All rural link-roads are only single-lane dirt roads. The N17 is generally a busy highway passing or connecting important areas such as Bethal, Ermelo, Leandra, Secunda and Swaziland. In the west it also passes through Nigel, Balfour and Heidelberg. It is used as a fairly heavy trucking route and is used by many to reach the abovementioned destinations.
N ?
Springs
Devon
FIGURE 3.1: Location of the study area within the East Rand of Gauteng Province.
3.2.2. Recording mortalities and sample collection
Daily collections of road casualties were carried out, or if not possible the route was traversed regularly each week. The normal collection was carried out during the early morning between 8h00 and 10h00 of each day, and the return journey later that same morning usually between 11h00 and 12h00. On average five journeys by car were made each week. It was found that the more frequently the area was checked, the easier the various species were to
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identify, as they became almost unrecognizable if left too long upon the road surface. Frequent collection is essential since the possibility of people picking them up out of curiosity and even for medicinal value, in which use of owls in castings or countering spells (Derwent & Mander, 1997) have been known to occur. Another factor to consider was scavenging of carcasses, resulting in an underestimation of the number of mortalities as well as the possible incorrect positions of carcasses due to movement by scavengers, being recorded.
Carcasses were collected and species identification was confirmed by at least two people, which was often difficult due to the poor state of the carcasses.
This was done once on the road and again in the laboratory. Typically it was one of the four species involved in this study that were collected. Fortunately however in no instance except one was it found impossible to identify any of the carcasses. This individual was counted before official collections started and identification procedures became more assured over time and experience with the handling of specimens. Many whole and intact specimens were collected and sent to the Transvaal Museum, Pretoria for mounting.
Carcasses were collected from Mondays to Fridays excluding weekends and this was kept constant throughout counts. It was assumed that traffic is invariably at its lowest during the weekend together with trucking companies, whom only travel this road during weekdays, and it is therefore feasible to expect the higher mortality during weekdays. Collection of carcasses to determine seasonal fluctuations in mortalities, started in April 2002 and ended in March 2003. Continuations of counts are still being carried out at present to determine if trends in seasonality, species occurrence and location continue.
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CHAPTER 3.1: INTRODUCTION
All owls that were seen on the road and within sight of the road verge, were counted while seated in a car moving at 60 to 80 km per hour. Following this all the carcasses spotted were identified and listed. Carcasses not taken back to the laboratory were immediately disposed of by depositing the remains in one of the roadside ditches. This method assured that no casualties were counted twice. Samples that were intact were kept for laboratory analysis, while the feathers of the remainder (others that were squashed but feathers that could still be distinguished) were also gathered and placed in metal free envelopes for future studies involving metal analyses.
In recognition of this, records were kept of all identifiable remains found in relation to the type of road surface, adjacent cover or land use, and time of year together with location of mortalities using a Garmin GPS. Their positions were important to identify any major hotspots that could be present. This would later be determined by plotting points along a map (1:50 000) using the
GIS (Geographical Information System) program ArcInfo (given in Chapter 2).
Mortality positions were also transposed onto habitat, geophysical and soil maps to determine if any related trends existed.
Of the samples that were analysed in the laboratory, analysis of stomach contents, general condition of carcasses, gross number of parasites, and the most important morphometrics were also taken (all discussed previously in
Chapter 2). The gender of the individual, (if possible) and an estimate of age based on reproductive development and general size of each victim, in relation to one another were also noted.
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Weather conditions were noted on the day of collection and sampling was carried out irrespective of prevailing weather conditions. This was also done to establish if weather conditions had an influence on mortalities the night before. This was then related back to weather forecasts, received from the
South African Weather Bureau at station number [0476792 0-Springs] during the same time period as recordings of mortalities. The phases of the moon were also compared to mortalities, to determine if light was a causing factor, since illumination of the road surface is assumed to attract owls to the road at night.
In addition to the surveys on the N17 and R550, volunteers carried out counts in various parts of the country to determine if any other roads had a significant impact of the same magnitude as this study area, on owl populations. The observer counted on all days and areas were also always checked by the same person in their respective area. This insured no overlap of counts.
These results were then handed over to the researcher for analyses. In some cases feather samples for those areas were also collected for future analysis and comparison to those of the study area. In rare cases, whole owls were also collected.
3.2.3. Observations made during sample collection
3.2.3.1 Vegetation, geology and soil type along road verges
A detailed description of the vegetation of the survey site is presented in
Chapter 2. The mortalities recorded were related to the vegetation in the respective transect sites surveyed along road verges. Five different sites were
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CHAPTER 3.1: INTRODUCTION
chosen where, from mortality counts, differences in mortality verses non- mortality hotspots, could be correlated to habitat differences and thus determine if there is any species specificity to habitat alongside roads. An additional four sites were selected to compare if other features besides vegetation had an influence. These included, the Blesbokspruit, a Ramsar wetland crossing; the Rietfontein power line crossing; as well as sites in the
Nooitgedacht and Bosmanskop areas, all indicated in Figure 3.2. For each of the five sites, were randomly selected a further five subplots. Each site ranged in length over 100 m with subplots of 10 m x 10 m quadrates. Species composition of vegetation, percentage cover, average and maximum vegetation height, ground cover, browsing effect and slope were determined.
In addition the presence of any activity of small mammals was also noted.
8 9 Enlar ged area
5 7
4 6 4 3 1
2
S ITE 1-5 S ITE 6-9
FIGURE 3.2: Sites 1-5, with additional sites 6-9, located along the N17 and R550 for the period October 2001 to September 2003.
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A geophysical and soil map was used to determine differences in geology and soil type alongside certain sections of the road and relating it back to owl mortalities. The influence of burning of road verges involving the nesting of the
Marsh and Grass Owls (was discussed in Chapter 2), and were also taken into account relating to mortalities.
3.2.3.2 Human impact and land use
Land-use beyond the road verge boundaries was noted although this generally stayed the same throughout the study area. A comparison of farming practices was noted alongside mortalities. It was important to distinguish between currently used fields and unused fields, and whether these areas consisted of planted crops verses grazing paddocks.
3.2.3.3 Local hazards and obstacles
Certain structures may have played a part in owl mortalities including, presence of fence posts or telephone posts and their distances to the road, the presence of bridges, drainage ditches alongside roads and the degree of slope of the embankments. These features were determined from personal observations, out in the field and related to mortality hotspots.
3.2.4. Behavioural differences that could influence mortalities
It was important to note what appetitive and what consummatory behaviour of owls could play a role in their eventual death on roads. The only way of obtaining data in this regard was to directly observe such behaviour traits, and to relate it to past behavioural studies of owls, which is limited due to their
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CHAPTER 3.1: INTRODUCTION
crepuscular, behaviour and large foraging range. Behavioural traits included, feeding such as the motivation to seek food (appetitive behaviour) and catching and eating the food (consummatory behaviour), as well as differences in flight patterns between species, agonistic behaviour, conflict with others (territoriality), epimeletic behaviour (parental), social and finally investigative behaviour (exploratory). Behaviour may also be endogenous or exogenous (Maclean, 1990) and will assist in explaining age related mortalities.
3.2.5. Information concerning roads and traffic
3.2.5.1. Traffic density and speed
Data obtained from the South African National Roads Agency (Ltd), (Dalpark
Toll Plaza) gave an indication of the type and density of the traffic, at different times, leading into and out of the study road. Only results for one high- mortality and one low-mortality period were needed. In order to obtain an average figure, for each of these periods for comparative purposes, two sets of data were chosen representing a summer and a winter period This was done to distinguish the seasonal differences in mortality counts and car counts. Another comparison was between normal weekdays and normal weekends (normal meaning non-holiday or non-long weekend period) to determine if traffic increased or decreased on certain days, and what time of day these traffic densities peaked. This data could fortunately be collected at any time due to extensive records kept and thus it was possible to first find out which months were of conflicting counts to compare mortality to traffic. An
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approximation of the speed of vehicles travelling on the road was determined by comparing the speed traveled during survey trips to the speed at which other traffic was traveling (pers. obs.). It was also determined at what distance together with speed an owl sitting on the road could be approached before it flew off.
3.2.5.2. Road type, shape and lane width
The differences in road types were recorded. This was a once off observation of the type of surface owls were frequently found on and whether it was a single or double lane section of the highway. It was important to note the shape of roads and whether the actual road was sloped. Condition of the road was also noted.
3.2.5.3. Road surface temperature versus verge and air temperatures
To determine if temperatures on different road surfaces, road verges and ambient temperatures differed, temperatures were recorded using a PTEX
Thermo-Hunter PT-2LD laser and infrared thermometer. This was done at various GPS points (i.e. mortality hotspot areas and non-mortality hotspot areas) on and along tarmac roads and adjacent verges and at various times of the day and night. Ambient temperatures were also taken at the same time using a mercury-filled calibrated thermometer to compare air temperature verses surface temperatures. Two roads were used for comparison, one in the study area itself and the other in Parys, Free State. Weather conditions were also noted when readings were taken.
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CHAPTER 3.1: INTRODUCTION
3.2.6. Statistical analysis
Summary statistics were carried out using SPSS version 11 (SPSS, Inc,
Chicago, IL USA) software package. Homogeneity of variance was tested using the Levene’s test and since homogeneity was found a parametric tests was conducted. A one-way analysis of variance (ANOVA) gave statistical relationships between light and dark phases of the moon to daily mortalities.
Statistical differences in total number of mortalities for each month and total monthly rainfall were determined using one-way ANOVA. A liner regression was then applied to the total monthly rainfall and total monthly mortalities. The difference is significant at the P= 0.05 level.
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3.3. RESULTS AND DISCUSSION
3.3.1. Study area
The complete route is shown in Figure 3.1 and a distance of 30 km was travelled in both directions every day from the Dalpark Toll Plaza to the Devon turn off. A total distance travelled during the two-year study was 15 600 km.
The road consisted of tarmac traversed by one secondary gravel road adjoining the N17, perpendicular to the R550, which ran through predominantly agricultural land. A very small section of the N17 passed through the towns of Brakpan, Boksburg and Springs as well as a small informal settlement.
3.3.2. Mortalities
3.3.2.1. Bird species composition and their relative abundance
During October 2001 to September 2003 a total of 554 owl mortalities were recorded on or alongside the N17 and the R550 collectively. The summation of the mortalities on these roads accounted for four owl species, namely the
Marsh Owl (making up 53.6%) and the second most abundant species was the Grass Owl (27.4%). The Barn Owl (17.5%) was third in order of abundance and lastly the Spotted Eagle Owl with (1.3%). Only one unidentified owl was recorded. These figures are given in Table 3.1 and are also depicted in Figure 3.3.
This study’s results show an average casualty rate of 9.2 owls per year per km along the N17. These averages are very high for South African bird
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CHAPTER 3.3: RESULTS AND DISCUSSION
mortalities when compared with those of Siegfried (1965) who gave averages for the Cape Province as one bird killed every 357 km. More importantly, the numbers given in this study compared with that of the former author’s results only involved owl mortalities, whereas the former study included all birds. An average of 277 owls per year was recorded in this study. All species of owls were found in both years of this study. These owl counts are extreme compared to any other study of owl mortalities in the world, such studies done by Clancy (2002-in press) with 9.1 owls per year found in northeastern New
South Wales. Shawyer & Dixon (1999) found only 68 Barn Owl road deaths per 100 km of road in Britain over an extended period and Vestjens (1973) collected on average 2 birds per year per km.
TABLE 3.1: Total owl mortality counts along the N17 and R550 between October 2001 and September 2003. Species N17 R550 Total A.capensis 234 (52.1%) 63 (60%) 297 (53.6%) T. capensis 125 (27.8%) 27 (25.7) 152 (27.4%) T. alba 86 (19.1%) 11 (10.5%) 97 (17.5%) B. africanus 3 (0.7%) 4 (3.8%) 7 (1.3%) Unknown 1 (0.3%) 0 1 (0.2%) Total 449 (81%) 105 (19%) 554
Many authors (Davis, 1934; Dreyer, 1935, Stoner, 1925 and Warren, 1936) favour observations to an animal-per distance basis. However, Dickerson
(1939) found this method did not give a good indication of mortalities of birds and may result in a relatively small sample, subject to error. A per day mortality rate should compensate for such errors and gives a good indication of seasonal trends in mortalities over time and space, such as used in this study. In terms of occurrence, when considering the N17 and R550 roads individually, mortalities for the N17 were:
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CHAPTER 3.3 RESULTS AND DISCUSSION
FIGURE 3.3: Total comparative monthly mortalities of the four owl species between October 2001 and September 2003
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CHAPTER 3.3: RESULTS AND DISCUSSION
Marsh Owls (52.1%); Grass Owls (27.8%); Barn Owls (19.1%); Spotted Eagle
Owls (0.7%) leaving only (0.3%) comprising of unidentified owls (Figure 3.4).
On the R550 the same four species were dominant with Marsh Owls consisting of 60%; Grass Owls (25.7%); Barn Owls (10.5%) and Spotted
Eagle Owls only constituting 3.8%. No owls were left unidentified on this road
(Figure 3.5). Despite these slight differences in percentage of casualties, there was a similar order of abundance for each of the different species on both roads. This study provides evidence of over-dominance as the four owl species, accounted for almost all raptor species mortalities occurring on these roads, with the few exceptions of one Black-shouldered Kite (Elanus caeruleus); one Lesser Kestrel (Falco naumanni) and one Amur Falcon (Falco amurensis). One non -raptor species found in very few numbers was the
Spotted Thick-knee (Burhinus capensis).
It should be noted from previous chapters that A. capensis outnumber T. capensis and breed in a ratio of 10:1 (Tarboton et al., 1987), which explains the higher numbers of Marsh Owl to Grass Owl deaths. Grass Owls are habitat specific, and are considerably rare in South Africa especially Gauteng as mentioned in previous chapters (Chapter 1 and 2). So lower casualty numbers of this species were expected. It was unexpected to find less Barn
Owls on the roads than Grass Owls as Barn Owls produce clutches from two to four eggs up to as much as 12 during rodent plagues (Harrison et al., 1997;
Wilson, 1970). A pair may produce up to 32 young in one year (Wilson, 1970).
This is higher than both Grass and Marsh Owls, which may produce on average four (rarely six) and three (rarely five), respectively.
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CHAPTER 3.3 RESULTS AND DISCUSSION
FIGURE 3.4: Comparative monthly mortalities of the four owl species along the N17 between October 2001 and September 2003.
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CHAPTER 3.3: RESULTS AND DISCUSSION
FIGURE 3.5: Comparative monthly mortalities of the four owl species along the R550 between October 2001 and September 2003.
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CHAPTER 3.3 RESULTS AND DISCUSSION
At one Barn Owl nest site up to 16 unsuccessful eggs were produced in one breeding season (pers. obs.).
It is important to note that these mortality numbers may not be a true reflection of all mortalities occurring. A large number of carcasses were found on the road itself but there were others that may have been flung into the vegetation on the roadside verges either by speed of impact or air turbulence created by passing vehicles. Road verges may have thus concealed these casualties leading to them never being counted. Injured animals may have left the road and died away from the place of impact thereby not being recorded. Some birds hit by cars may have been transported over unknown distances when their corpses were trapped in front of vehicles. Some animals could have been killed and then removed by nocturnal and diurnal scavengers (including humans) without leaving a trace. The possibility that scavenging animals may have brought old carcasses onto the road was unlikely as most of the casualties that were examined were freshly killed and no such casualties found, had any signs of animals feeding upon them. Vestjens (1973) discovered that after replacing 40 carcasses previously picked up off the road, he found that 31 of these were missing when checked four days later. Seven were partly eaten and two were being consumed by blowfly larvae. Similar results were found in this study, whereby a Grass Owl left in a known position, was later not to be found early the following day and no other Grass Owls were picked up on that day. Hodson (1966) also found similar results and stated that the largest numbers of removals occurred in the first 24 hours following being killed, which correlates with our observations. These factors
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serve to increase the already high counts and what is important to take into account is the fact that so many owls are already being killed, so the relatively few extra numbers that were never counted would not have a significant impact on the collection of data.
Unusual specimens occasionally noted along roads were that of an African
Scops-Owl (Otus senegalensis) found near Bela Bela (Warmbaths), which is the only known individual of its species to be reported as a road casualty. A ringed specimen of a Spotted Eagle Owl was recovered near the town of
Paarl, Western Cape. Upon examination of this owl it was discovered that some injury in the past had resulted in stainless steel needles being inserted into the wing area of this bird. This however was not a causing factor, resulting in its eventual death, as these modifications were fairly old. Feathers, adjoined strongly to the inserted needles, supporting this. However, this bird may not have been able to fly as efficiently as others.
An interesting observation was that of dead owls found within a few meters of each other. These occurrences may be attributed to the many Marsh Owls found sitting in large groups on roads (pers. obs.). Groups of up to 40 have been recorded roosting at one time (Harrison et al., 1997). However, the explanation of Grass Owl carcasses occurring together is far less understood.
One possibility is that parents or juveniles pursuing each other (Kemp and
Calburn, 1987) fall victims. On one occasion two adult Grass Owls were found dead on the road within close proximity to one another. The possibility that these were the foraging parents of a brood of chicks that was being monitored close by, is highly likely. This was due to the fact that these abandoned chicks
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CHAPTER 3.3 RESULTS AND DISCUSSION
were found dead due to exposure to the elements and lack of food, the morning after this incident (Chapter 2). The possibility of different owl species occurring close together cannot be explained except for the fact that owls have been known to gather in mixed groups (pers. obs.), especially Barn and
Marsh Owls, along the road and seemed to be tolerant of one another.
The insignificant number of Spotted Eagle Owls and the absence of other owl species killed on roads, may reflect the species’ habits of preying on an array of animals but mostly insects, as well as small forest mammals, and birds
(Pickford, Pickford & Tarboton, 1989), associated to their habitat preferences.
These species would seldom frequent roads where contact with a vehicle may occur. A Road that runs close to a related area where these owls abundantly inhabit and on rare occasions the Spotted Eagle Owls, may accidentally cross such a road. The Grass Owl, Marsh Owl and Barn Owl, with a few other exceptions, acquire much of their prey from the ground and road corridors that run through open grasslands or wetlands (Del Hoyo et al., 1999). This would in turn, increase their chances of coming into contact with vehicles and making them more vulnerable to collisions.
To employ the use of highway mortalities as a tool to signify distribution and habitat usage by birds and mammals has great potential. McClure (1951) considers it as a random sample of those species that cross open areas. A survey of highway kills can thus be conducted by untrained observers in other more involved methods of wildlife tallying, such as landowners in the area and volunteers.
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3.3.2.2.Temporal occurrences of road mortalities in owls
3.3.2.2.1. Seasonal differences in highway losses
McClure (1951) states that a bird species may frequent highways for several months during its normal periods of activity and at no time make up a large percentage of the mortalities. Other species, during certain months, or even weeks may have a period of extensive movement and make up a large part of the mortality for that month or period. The percentage of losses that occurred for each species are given in Figures 3.6 to 3.9 indicating seasonal activity and casualty densities for each month since the start of this study. The species and numbers found are given for the two complete years of counts from October 2001 to September 2003, however, seasonal differences were determined from March 2002 to February 2003. Of the owls killed along the
N17 and R550, 59% were killed in winter, 20.9% in autumn, 14.5% in spring and 5.6% in summer (Table 3.2). A definite distinction in seasonal trends is shown with a peak kill period for all owls (with the exception of the Spotted
Eagle Owl) occurring during winter (June, July and August). These similar trends continued for the following year of counts. The number of monthly highway mortalities also varied amongst different species.
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CHAPTER 3.3 RESULTS AND DISCUSSION
FIGURES 3.6: Mortalities of Asio capensis and how it relates to the rainfall, during the period of October 2001 to July 2003.
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CHAPTER 3.3: RESULTS AND DISCUSSION
FIGURES 3.7: Mortalities of Tyto capensis and how it relates to the rainfall, during the period of October 2001 to July 2003.
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FIGURES 3.8: Mortalities of Tyto alba and how it relates to the rainfall, during the period of October 2001 to July 2003.
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CHAPTER 3.3: RESULTS AND DISCUSSION
FIGURES 3.9: Mortalities of Bubo africanus and how it relates to the rainfall, during the period of October 2001 to July 2003.
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CHAPTER 3.3 RESULTS AND DISCUSSION
TABLE 3.2: Seasonal variations for road mortalities of four owl species along the N17 and R550 between December 2001 and August 2003.
Species Summer Autumn Winter Spring Summer Autumn Winter D J F M A M J J A S O N D J F M A M J J A Asio capensis 4 2 6 23 9 19 23 46 21 9 2 1 0 2 2 5 5 11 21 51 23 12 51 90 12 4 21 95 Tyto capensis 3 9 11 2 1 1 7 12 10 5 3 8 2 1 2 0 5 8 7 16 11 23 4 29 16 5 13 34 Tyto alba 0 2 0 0 0 1 8 17 13 5 2 4 4 2 0 0 0 4 4 18 11 2 1 38 11 6 4 33 Bubo africanus 0 1 0 0 0 0 0 1 0 0 0 0 0 0 0 4 1 0 0 0 0 1 0 1 0 0 5 0 Monthly Total 7 14 17 25 10 21 38 76 44 19 7 13 6 3 4 9 11 23 32 85 45 Seasonal Total 38 56 158 39 15 43 162 % 20.90% 59% 14.50% 5.60%
From Figure 3.3 it can be seen that the Marsh Owl mortalities occurred in high
numbers during winter months (June, July and August) with 90 occurring in
the first year and 95 in the second. A smaller peak occurred in the autumn
months of both years, with 51 and 21 in the first and second years
respectively. During 2003, 29.3% of all Marsh Owls were killed in July alone,
but only made up 0.6% of the Marsh Owl highway losses for the month of
November, and no casualties found during the December month. Spring and
summer are very similar regarding mortality counts, with lower numbers
occurring in each of these seasons. Thus from Figure 3.3 a distinct bell
shaped graph can be seen, where from the end of winter until the beginning of
summer a decline in casualty numbers of Marsh Owls occurred, picking up
again in autumn. Similar results were found when comparing the N17 to the
R550 although the R550 had fewer numbers altogether (Figures 3.4 and 3.5).
The Grass Owl gave similar results to that of the Marsh Owl with regard to the
winter months being the highest peak mortality season with 53.7% Grass Owl
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mortalities occurring (Figure 3.3). Interestingly enough, 2001/2002-summer period, resulted in almost as high casualty counts than that of the 2002-winter period (Table 3.2). Although the winter and spring months of both years remained relatively constant, there was a change over in mortalities of the autumn and summer months, with higher mortalities occurring in the
2001/2002-summer than that of the 2002/2003-summer period, and reverse being true for the autumn periods of consecutive years. A reason for this obscure result is the possibility that varying rainfall and late veld fires occurred in 2003. This will be elaborated on in Section 3.3.2.2.2.
Both the Grass and Marsh Owl casualties mainly appeared on the roads during the winter period of 2002 and 2003, just after the breeding season, which occurs from March to May (Dean 1971; Irwin 1981; Tarboton &
Erasmus, 1998, Tarboton et al. 1987 – see Chapter 2). The breeding season coincides with maximum grass cover (Steyn, 1985) and thus again with rainfall and fire regimes. The reason for this seasonal bias is due to the fact that these birds were killed possibly in the first three months (June, July and
August) after leaving the nest and is therefore responsible for the peaks seen after the breeding season. This is evident from the casualties examined that appeared to be generally young owls. This is similar to the results of
Maciejewski (1997) and other authors (Clancy 2002-in press; Newton et al.,
1997; Shawyer & Dixon, 1999), where owl casualties were found throughout the year, but were more evident during certain seasons and suggested that the seasonal increase was probably attributed largely to the dispersal of juveniles, after fledging in autumn-winter period. At this time, owls are also
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nesting in dryer areas and leaving nest and roosting sites due to the increase in the typical highveld winter fires, which are the possible reasons for adult deaths during winter. Marsh Owl mortalities also appeared again in high numbers in autumn and this could be owing to their earlier breeding cycles, known to occur in this species (Dean, 1971; Irwin, 1981; Maclean, 1993;
Mendelsohn, 1989; Tarboton et al., 1987). Clancy (2002-in press) found immature owls in particular would find hunting during winter more difficult. At this time of the year roadsides offer or attract a more profitable hunting ground when insects and rodents are scarce further away from roads. Although birds foraging on roads get easily obtainable food, they are more at risk to being hit by a vehicle compared with those foraging away from roads.
Grass Owl mortalities were also more abundant during October 2001, but whether these owls were adults or youngsters could not be determined as there were insufficient carcasses collected for laboratory analysis during this period. The occurrences of Grass Owl mortalities are more irregular and are relatively more difficult to understand. However, if we compare these results to rainfall, it may make it easier to identify possible causes. By comparing
Figures 3.6 to 3.9, there seems to be an indirect correlation of mortalities to rainfall. Higher rainfall seemed to coincide with lower mortalities and lower rainfall, with higher mortalities. Analysis of variance (ANOVA) supports these observations with a significance level of P =0.015 (F =7.493) between mortalities and rainfall. A linear regression was performed to investigate this relationship and also confirms that a significantly correlation (r2 =0.319) between total mortalities and total rainfall occurs (Figure 3.10).
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CHAPTER 3.3: RESULTS AND DISCUSSION
100
75 r2 = 0.319 p = 0.015 50
Mortality 25
0
-25 0 25 50 75 100 125 Rainfall (mm)
FIGURE 3.10: Linear regression indicating a negative significant correlation between rainfall and mortalities.
This relationship could clearly be seen regarding the Grass Owl mortalities and is very prominent in the winter season. However, if we start at the first month of counts (October 2001), which had a lower rainfall season than
October 2002, mortalities were higher in the first year and far less in the second year of counts. This may also explain the drop in mortalities in autumn
2002 where rainfall was lower in the summer of 2002 than that of summer
2003, possibly influencing the mortalities in the months to follow (March, April,
May). The more rain in summer 2003 may have resulted in later veld fires and thus a later peak of mortalities. This factor works when comparing counts to rainfall throughout the study period. When comparing rainfall to other species’ mortalities besides the Spotted Eagle Owl (which we do not have enough data to compare), a negligible influence of rainfall on mortalities can be seen but not to the extent it influences Grass Owl mortalities. This, together with breeding season of these owls, explains why certain seasons dominate in the occurrences of owl mortalities.
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The seasonal prevalence of the Barn Owl is more difficult to explain. This species is known to breed at any time of the year (Tarboton & Erasmus,
1998), usually in response to periods after rain when rodent numbers are highest (Harrison, et al., 1997) and second broods may occur when there is a prolific abundance of food supply in the vicinity. According to Tarboton et al.
(1987) the Barn Owls’ main breeding season and main egg-laying month was between March and May for the old Transvaal Province, coinciding with that time of the year when rodent numbers are at their highest levels. It was personally witnessed that a pair of Barn Owls produced eggs in January 2002 and again in May 2003 (see Chapter 2).
Newton et al. (1997) found that British Barn Owl (T. alba alba) deaths from starvation and collision were much higher outside the breeding season with peaks in autumn (November), comprising of mainly juveniles and adults and juveniles in late winter (March). The breeding season for T. a. alba occurs between the months of April to August. De Bruijn (1994) also found similar results to these. Clancy (2002-in press) recorded most Barn Owls in late winter to early spring. Shawyer & Dixon (1999) showed that 90% of Barn Owl road mortalities occurred during autumn and winter, also supporting findings of this study and previous work. The high mortality of the first period, just like in all other owl species in this study, may be related to the many risks, which the inexperienced owls have to face during the post-fledging dispersal (De
Bruijn, 1994). The second smaller peak probably was attributed to reduced prey (such as small rodents and shrews) supply in spring due to the increase in number of first year owls’ demand for available prey.
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The Spotted Eagle Owl did not appear on the study road in substantial numbers due to lack of suitable habitat for this species, except for a few farmyards where they have been known to breed (pers. comm. 6Harry van der
Merwe, 7Johan van der Merwe, 2002) during August. The Spotted Eagle Owl is known to breed in early summer, specifically from August to September
(Tarboton & Erasmus, 1998). This could explain the difference in occurrence of mortalities. They also prefer insects as a large portion of their diet, and the need to frequent dark roads does not form an adequate foraging ground for the prey species. Insects also occur in greater abundances at the onset of the rainy season.
3.3.2.2.2. Mortality occurrence in relation to rainfall, temperature and moonlight
Dickerson (1939) found that climatic condition may be expected to have a marked influence on the occurrence of animals on highways. It has been observed repeatedly that relatively fewer dead animals are encountered on mornings following rain during the latter part of the night. One explanation why rain influences mortalities is the possibility that drivers are forced to travel slower and therefore do not pose a serious hazard to wildlife during rainy nights. Another possibility that will be discussed further is the influence rain may have on the different hunting techniques. Dickerson (1939) also noted that more dead animals are encountered when afternoon rains are followed by moonlit nights than when such rainy afternoons are followed by dark nights.
There does not appear to be any sort of this occurrence in our study, and a
6 Harry van der Merwe, Commercial farmer, Devon. 7 Johan van der Merwe, Commercial farmer, Devon.
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ANOVA did not give a significant relationship between daily mortalities and light or dark evenings (P =0.067, F =3.357). One explanation for this may be that Grass and Barn Owls are known to rely more on hearing than on sight for locating and capturing prey (Burton, 1973). Marsh Owls on the other hand hunt earlier in the evening and later in the morning than the Grass and Barn
Owls, when plenty of light is available and need not rely on light during dark evenings.
Extremes of low temperature reduce the movements of many forms of wildlife.
For owls, to minimise energy expenditure, roadsides may be selected as a hunting site in relation to the density of perches and abundance of prey. This is an energetically economic hunting technique during cold winters when energy needs to be spared, instead of having to travel far distances over expanses of grassland in flight, in search for food. This explains the possible mortality peaks in winter and autumn when temperatures drop (Figure 3.11).
This was also confirmed in Meunier et al. (2000) study, involving the use of roadsides by Buzzards and Kestrels.
Although the data presented here are in large measure self-explanatory, many raise more questions than they answer. The curves do show pronounced seasonal peaks; however, they have to be related to both additional statistical data and adequate field notes to explain obscure results and species most definitely need to be viewed separately in such studies as seen from the above hindrances.
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Many active animals were observed at night throughout the study period and many more observed on the road during the winter months, specifically July, when the greatest overall abundance were noted (pers. obs.). This was reflected in records of living individuals during evening surveys (pers. obs.).
The few other highway mortalities that were most numerous in this month also indicated this. Various small mammals were observed flattened on the road surface. In one location during July 2002, many rodents could be seen crossing the road (pers. obs.) possibly due to the excessive veld fires, which are common at this time of the year.
30
25
20
15 MAX 10 MIN
5 Temperature
0
-5
JAN '01APR '01 JUL '01OCT '01 JAN '02 APR '02 JUL '02OCT '02 JAN '03 Month
FIGURE 3.11: Minimum and maximum temperatures (°C) taken from station number [0476792 0-Springs] during the period between January 2001 and March 2003.
Bird species are known to have losses that commensurate with their migratory habits and population patterns (McClure, 1951). Even though these owls are not migratory and are permanent residents there is a definite pattern in population or seasonal trends in mortalities due to other more important
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factors (discussed below). Too few owl carcasses were obtained from other important areas throughout South Africa to verify if local cycles in owl mortality were apparent in these areas as well.
3.3.2.2.3. Anthropogenic activities in relation to seasonal occurrences of mortalities
i. Road verge maintenance
Another important factor, already briefly mentioned, was the direct relationship to season and to roadside cover, which perhaps could relate back to the high mortalities during certain times as well. During the winter periods roadside cover is mostly burnt. At the end of the summer period, after heavy rains, it is at its highest. However, roadside cover due to seasonal changes had minimal effect on abundance of owls but rather effecting species diversity occurrence of owls. It is significant that during this study period most Grass Owls were killed in sections grown over entirely with grass right to the edge of road. This could also indicate a shift in hotspot distributions in different sections of the road over time, while vegetation is constantly changing and burning regimes altered.
Controlled burning of grasslands and road verges is also carried out during the winter/autumn period, flushing the owls from roosting and nesting sites
(Chapter 2). This may also in turn flush rodents from their habitat alongside roads, where they would move to a safe haven such as the road itself, allowing owls to take advantage of the situation and feed upon them. Grass
Owls have been known to benefit from such a situation (Del Hoyo et al.,
1999). Numbers of other animal mortalities also remained low in summer and
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spring months. The expected heavy casualties only materialised in winter
(pers. obs.) when rodents together with the owls, were found on roads in abundant numbers after veld fires.
ii. Agriculture
Some of the casualties invite further comment: of the 297 Marsh Owls killed,
109 (36.7%) deaths occurred between May and August in the first year and
106 (35.7%) occurring during the same period the following year. This is the time of ripening and harvesting of the maize when owl species visited the various fields in large numbers looking for food. Specifically insects and granivorous rodents, which take advantage of the easy accessible food attract many of their predators to these areas as well. Up to 17 Marsh Owls were found roosting together in a small field (pers. obs.) and were also found in early mornings foraging around harvested maize fields. It is not uncommon to find groups of up to 40 birds gathering together (Steyn, 1985). In a study conducted on the use of roadsides by diurnal raptors in agricultural landscapes Meunier et al. (1999) found that there was a seasonal shift in the use of roadsides by buzzards (Buteo buteo) and kestrels (Circus sp.), with a high use of motorway verges in winter and a low use in summer. Raptor species used verges in preference to cultivated fields, during certain seasons
(Meunier et al., 1999). As indicated in Chapter 2, granivorous rodents were found in high abundance along road verges as opposed to the herbivorous
Otomys sp. of rodent and the insectivorous Crocidura sp. of shrew, which were found in higher abundances away from roads. The granivorous species feed upon grass seeds during the time of year when vegetation is flowering, i.e. during spring and summer (Brooks, 1974). However, when the grass
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species are not producing seeds, such as during winter, the granivorous species rely on another source of food, i.e. the spilled grain found alongside the road. Depending on whether these were summer or winter crops may have influenced both prey and owls’ choice of preferred foraging grounds, and may indicate that raptors in general are influenced by their preys’ seasonal activities.
Eragrostis curvula is a grass species used by farmers for fodder of cattle.
However, because of the structure of this grass species, Grass and Marsh owls also use this type of grass for construction of their nests. Unfortunately the cutting of such grass occurs during the same period as the breeding season of these two grassland owl species are thus directly affected before the rainy season approaches. Even if some expanse of this grass is left for the owls, it still exposes them to the many predators such as Black-backed Jackal
(C. mesomelas) occurring in the area (pers. obs.). The movement of grassland owls due to the bulldozing or ploughing of land causes these owls, to vacate their nests and daytime hiding places and to frequent roads.
3.3.3. Major hotspots and other locations along the N17
In each year of this study, road casualties were found to be concentrated at specific locations on the N17 and R550. The grass where bird mortalities distinctly congregated are classified as “hotspots” or “black-spots” and these hotspots seem to not only apply to this study but also those of Dunthorn &
Errington (1964), Finnis (1960), Hodson (1960) and Shawyer & Dixon (1999) amongst others. It can be seen that the mortalities on the N17 were
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CHAPTER 3.3: RESULTS AND DISCUSSION
concentrated mainly in the eastern part of the surveyed road in two distinct regions. The first group forming the largest proportion of deaths is considered to be the primary hotspot. The second group was considered subsidiary and found to the west of that of the first and had slightly less owl mortalities than the first. An examination of Figure 3.12 illustrates the plotted GPS positions of individual mortalities, indicating the two specific hotspots along the N17 that were formed. The first hotspot indicated by site 3 and the second by site 5.
From this figure it is also noteworthy that each of these hotspots was more prone to Marsh Owl casualties for hotspot 1 and Grass Owl casualties for hotspot 2. This indicates that a species-specific factor was involved in affecting certain mortalities.
3.3.3.1. Species composition in hotspots: recorded from April 2002 to April 2003
The primary hotspot consisted of 37.4% Marsh Owls, 7.1% Barn Owls and
4.6% Grass Owls contributing to 49 % of the total owl mortalities found on the
N17. This hotspot accounted for 69.9% of all Marsh Owls killed on the N17 followed by 29.2% of the Barn Owls and 21.4% of all the Grass Owls. Spotted
Eagle Owls were not collected in this hotspot. The subsidiary hotspot was made up of more Grass Owls than any of the other species involved. Of the total percentage (15.1%) of owls found in this hotspot 8.1% were Grass Owls, with equal numbers of Marsh and Barn Owls (3.5% of the total). Of all Grass
Owls on the N17, 38.1% occurred in this hotspot with only 14.6% Barn Owls and 6.6% Marsh Owls (Refer to Table 3.3).
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CHAPTER 3.3 RESULTS AND DISCUSSION
FIGURE 3.12: Owl mortalities in relation to hotspot 1 (Site 3) and hotspot 2 (Site 5) along the N17 for the period October 2001 to September 2003.
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CHAPTER 3.3: RESULTS AND DISCUSSION
TABLE 3.4: Owl mortality hotspots occurring along the N17 and *R550 during October 2001 to September 2003. A. capensis T. capensis T. alba B. africanus Site 1 % Species 3.80% 4.80% 14.60% 0% % Total 2.00% 1.00% 3.50% 0% Site 2* % Species 33.30% 36.40% 0% 0% % Total 19.60% 7.80% 0% 0% Site 3 % Species 69.90% 21.40% 29.20% 0% % Total 37.40% 4.60% 7.10% 0% Site 4 % Species 0% 0% 0% 0% % Total 0% 0% 0% 0% Site 5 % Species 6.60% 38.10% 14.60% 0% % Total 3.50% 8.10% 3.50% 0% Site 6 % Species 3.80% 11.90% 2.10% 0% % Total 2.00% 2.50% 0.50% 0% Site 7 % Species 10.40% 16.70% 10.40% 50% % Total 5.60% 3.50% 2.50% 0.50% Site 8 % Species 0.90% 0% 6.30% 0% % Total 0.50% 0% 1.50% 0% Site 9 % Species 1.90% 4.80% 0% 50% % Total 1.00% 1.00% 0% 0.50%
3.3.3.2. Features associated with hotspots
Land use adjacent to the hotspots consisted of cattle grazing and Eragrostis sp. planted lands, with a perennial stream running almost perpendicular to the
N17. The grassy verges alongside the first hotspot zone was very diverse with its long seeding grasses (H. hirta, E. curvula, T. triandra), wild flowers and various sedges (Chapter 2). This proved to be a popular feeding ground for owls, as witnessed at night, where many Marsh and Barn Owls could be found sitting on the road and fence posts respectively. The secondary hotspot was found to consist mainly of C. dactylon E. curvula and T. triandra grass found in
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close proximity to the roads edge and was slightly less diverse in vegetation
(Chapter 2). Most observations at night were found to occur at the first hotspot section where Marsh Owls could be seen passing almost continuously back and forth over the road between the suitable habitats on either side. This was also witnessed during early mornings and late afternoons in winter (pers. obs.). It is thus not surprising that, upon at least two occasions, a double fatality was caused when a harassed or hungry parent, possibly with chicks, was closely pursued by one of its kind (possible an offspring or pair) and on other occasions different species flew into a passing vehicle and were found within 3 m of each other. This also perhaps could have occurred in quick succession as opposed to being hit at the same time. A pair of Grass Owls was found within close proximity of one another on the road,
3.3.3.3. Reasons for occurrence of hotspots
3.3.3.3.1. Natural features associated to mortalities
It is in these hotspots that a significant proportion of the total owls killed were found. Possible reasons, adapted from Dunthorn & Errington (1964), may include: i) the large numbers of owls cross this section of road and although few are killed they form a significant proportion of the total found within the given area, ii) fewer birds cross at these points, but because of some feature of the terrain, the risk of being killed is great and many deaths occur. Based on the findings of other authors, it seems that Finnis (1960) favours the latter and Hodson (1960) the former. Our study seems to favour the former with owls favouring this specific locality of the hotspot section and occurring in
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CHAPTER 3.3: RESULTS AND DISCUSSION
abundance within this area. Reasons may also be a combination of both possibilities and specific features within this hotspot region may also be inducing mortalities. There are many active owls present in the primary hotspot area as witnessed from evening surveys. When one owl dies, it seems that another, present in the area, replaces it. This is shown by the continual death of these owls within a period of three years, without changes in abundance of mortalities throughout the study period. The habitat alongside roads also seems very suitable to these owls in this specific reach as nests and roost sites were located within 200 m from the hotspot. On the other hand, in areas where fewer owl mortalities occurred, fewer owls were found roosting or nesting during the day or active during the evening. Hodson (1960) established that most deaths occurred opposite gaps along stretches of roadway with uniform borders. He also demonstrated an association between mortality and habitat. This aspect is discussed further in Section 3.3.4.
Dunthorn & Errington (1964) also suggests that the hotspots may reflect habitat preferences to specific species, which agrees with the findings of this study.
Most of the dead owls were found on roads adjacent to grazing fields and cosmos and especially associated with planted E. curvula and water, with fewer found in areas of dry maize fields. It was also noted that within these hotspot zones where regular mortalities occurred, rodent holes were present in abundance (pers. obs.). Shawyer & Dixon (1999) found that hotspots were associated with the lowest lying parts of their study road and at specific points where grassy corridors associated with river, stream and drainage lines,
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intersected the road. These findings were very much similar to those found in this study and in direct comparison to a stream running perpendicular to the
N17. The Boesmanspruit, channels its way from the south side making its way north and thus intersecting the road at the exact section where the primary hotspot is positioned (Figure 3.13). It was concluded by Shawyer & Dixon
(1999) that Barn Owls concentrate much of their movements along the prey- rich banks of these watercourses (immature owls for dispersal and adults when extending their breeding range in winter), with the result that they are funneled onto the road where the watercourse or other linear corridor of grassland intersect the road. Together with evidence that rodents prefer this area because of the softer rich soil made the owls frequent these areas and were more at risk to being hit by a vehicle. This is in contrast to the non- mortality zone (site 4) where no such water occurred and where rodents were less likely to occur due to the hard granite type geology.
3.3.3.3.2. Artificial features associated to mortalities
Artificial features specific to certain areas may be responsible for owls crossing at these points. These features include power-lines, bridges, road intersections and proximity of fence posts. Four lesser hotspots were found in association with such features. These included the Blesbokspruit crossing
(site 8) where the N17 crosses over the Ramsar wetland and was associated with the occurrence of mostly Barn Owls and Marsh Owls.
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CHAPTER 3.3: RESULTS AND DISCUSSION
FIGURE 3.13: Habitat map indicating location of the interception of a watercourse at the hotspot site (Site 3) in comparison to the non-hotspot site (Site 4).
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CHAPTER 3.3 RESULTS AND DISCUSSION
The Rietfontein power line crossing (site 9) was associated with the occurrence of Marsh, Grass, and Spotted Eagle Owl mortalities in greater abundances and the Nooitgedacht area (site 7) with all four species of owl mortalities occurring in this reach. Lastly the Bosmanskop area (site 6) consisted of mostly Grass Owl mortalities as well as Marsh and Barn Owl mortalities (refer to Figure 3.2). Only two owl remains were found along the 5 km sand road and very few were found in various towns, with one unusual
Grass Owl collected in an built up residential area in Boksburg. This is unusual for Grass Owls that are not known to frequent such areas (Fry et al.,
1988). Owl carcases frequently occurred at the four way stop at the
Bosmanskop N17 intersection (Figure 3.14). Barn Owls especially were found in higher abundances close to low bridge crossings (site 1) and where narrow verges with fences close to the road were present. The supply of suitable perching sites, allowing a less energy-demanding hunting behaviour than flight-hunting for that of the Marsh and Barn Owl are also very important structures. On two occasions Marsh Owls were found tangled in barbed wire fences, indicating the necessary use of such structures. Irwin & Lorber (1984) discuss barbed wire fences as a hazard to owls.
FIGURE 3.14: A Grass Owl mortality discovered at the Bosmanskop - N17 four way stop intersection.
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3.3.3.3.3. Other features associated to hotspots
Mortalities could also be related to the gradient of the road in relation to the adjacent verge and land. No mortalities occurred where the road was lower than the adjacent land, but where the road was higher than its surroundings many carcases were located and formed hotspot zones for mortalities.
Perhaps this is because the steep embankment slopping up off the road forms a barrier to owls and rodents who would have to pass over such a structure.
These embankments are also normally found in association with concrete structures such as bridges. The ground is harder with sparse vegetation growing in the form of few clumps of H. hirta thus making it unsuitable for both owls and rodents. This is represented at site 4 of the study sites. Cover and access was important for prey feeding on grain spilled on the road and was easier to reach when the road was raised rather than sunken. Mortalities remained high with some very obvious hotspots near the scene of prey activity together with grain spillage.
3.3.4. Characteristics of road verges attracting birds
3.3.4.1. Road verges
The nature of road corridors along highways and major roads could also provide attractive foraging habitat for owls, thus increasing the chances of owl mortality in the vicinity. A closer look at the characteristics of the vegetation as well as the relationship between roadside habitat structure and owl mortalities, in this area differed only slightly to those of previous studies. Hodson (1960) found higher mortalities occurring opposite gaps and openings than along
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CHAPTER 3.3 RESULTS AND DISCUSSION
those stretches of roadway with a uniform border. In this study area there was no apparent habitat structure factor effecting mortalities, such as those of previous studies, which involved trees, large hedges and shrubs alongside the road. The habitat structure along all surveyed road-segments and adjoining lands in the study area displayed homogenous vegetation structures, namely that of reclaimed grassland and intensive agriculture. However, different species of vegetation grow in different proportions to one another, leaving a mosaic structure along the side of the road. Although vegetation at the edge of the road increased in height during spring and summer and was at its highest in autumn just before burning, it never reached a height above three meters. Road verges are cleared mechanically, once a year in the area and no spraying is carried out. Cut grass strips of about 3 m are maintained. At the time just before burning, locals cut and collect grass piles. Local farmers also burn firebreaks each year and belong to a ‘controlled fire organization’. Since the inception of this practice, fires on farmlands and along roadsides have declined by 50% (pers. comm. 8Mr. J. Haywood, 2003).
3.3.4.2. Vegetation
There was a clear distinction in vegetation groupings along different vegetation sites along the study road (Chapter 2). According to Dickerson
(1939) three types of roadside cover have been considered in relation to bird mortalities.
8 Jurg Haywood, Commercial farmer, Devon
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Type 1: Include roads, which are bordered by varied vegetation in which tall grasses or sedges (30 cm or more in height) are common and shrubby cover either predominates or at least occurs as scattered clumps. Trees may be present as scattered individuals or in woodlands between cultivated fields.
Type 2: Include roads along which the dominating vegetation is of tree or shrub size.
Grassy margins or low herbaceous cover also may occur in varying degree.
Bare cut and fill banks are common. Cultivated land and pasture may occupy a considerable part of the roadside land but sufficient scattered trees are usually present to serve as lookouts for raptors and perches for passerine birds.
Type 3: Include roads where the dominating feature of the roadside cover is either grass or coarse herbs. Shrub-sized vegetation may occur in isolated clumps.
Considerable areas of shrubby plants and even open tree growth may be important features of this type but usually trees are of such remote relationship to the roadside cover that fence posts become favourite perches for raptors and other birds.
When relating these vegetation types to this study we find that the vegetation found along road verges of the N17 conforms to Type 3. This type of vegetation is stated by Dickerson (1939) as the most common habitat type involved in the destruction of wildlife compared to the other two types.
However, a more intensive study of individuals whose range overlap a road is
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needed, to determine precisely how specific verges are selected, forming mortality hotspots.
3.3.4.3. Habitat, soil type and geology along road verges
Habitat types were divided into two major groups occurring in the area, namely grassland vertic high clay and grassland sandy loam. However, these two types formed a mosaic pattern throughout the area and especially alongside the road. Two major soil groups occurred in the area, namely non- calcarious and mesotrophic and eutrophic soils. Geological types consisted mainly of arenite, dolerite and granite (Figure 3.15 to 3.17). It appeared that the more varied the vegetation, soils and land use in the farmlands produced a larger number of owl mortalities.
3.3.4.4. The importance of road verge vegetation in relation to behaviour
The distance from the road to escape cover apparently is of considerable importance for wildlife (Dickerson, 1939). Where vegetation was not too tall but extended right up to the border of the road, the hazard was greatest. This may be due to the fact that once a bird has sensed danger the first reaction would be to fly to a spot visible to the bird. If the grass cover was too thick alongside the road the bird would have to fly abruptly above this vegetation to safety. This appears to be difficult for an owl with such a large wingspan to body ratio. It however, does not explain the mass mortalities in the winter months when the roadside vegetation is burnt and where owl mortalities have been known to occur. Perhaps its not the tall grass, which influences other birds and wildlife that need to escape above the vegetation but in actual fact
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the possibility of flying up towards a fence post on which for raptor species, such as the Barn and Marsh Owls, to perch.
The majority of Grass Owl carcasses were found near habitat extending right to the edge of the road that was in excess of a meter tall. However, there is an explanation for Grass Owl mortalities occurring where grass cover was tall and thick. Not only is it dangerous for owls to escape from the road, some owls are also prone to approach the road in flight, heading straight into an oncoming car. Several times while travelling and observing owls in the study area, Grass Owls were witnessed to burst out of the overgrown cover and almost strike the car travelling at 80 km per hour. This however indicated that this feature of verge habitat was once again species specific and was related to the different hunting habits of each species. By comparing the three sites more prone to mortalities of each species along the N17 (Chapter 2), it was possible to determine if this feature was species specific to both owls and vegetation and whether height and cover of vegetation had an influence (see
Chapter 2, section 2.3.2.).
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FIGURE 3.15: Positions of owl mortalities during October 2001 to September 2003 in relation to type of habitat. Source: DACEL Nature Conservation
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FIGURE 3.16: Positions of owl mortalities during October 2001 to September 2003 in relation to soil type. Source: DACEL Nature Conservation
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FIGURE 3.17: Positions of owl mortalities during October 2001 to September 2003 in relation to type of geology. Source: DACEL Nature Conservation
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3.3.5. Behaviour
There are four classes of raptor behaviour (adapted from Masman, Daan &
Diijkstra, 1988): (1) perching, (2) walking or alighting on the ground, (3) flight- hunting, and (4) other behaviour (e.g. such as traveling). The owls in this study were observed to occur on roads in three different ways; sitting on roads, perched along fence posts and flying over the highways. These observations were based on behavioural habits discussed previously in
Chapter 2 for each species. Grass Owls were observed on the N17, R550 as well as the adjoining dirt road crossing these roads in direct flight, hunting and probably moving from one part of their home range to another. The little that is known on the patterns in movement of this owl species is discussed in
Chapter 2. The other two species namely, Barn and Marsh owls were found on or close to the road, with less movement occurring. Barn Owls and Marsh
Owls were often found sitting on roadside posts (well known in the Barn Owls and to a lesser extent the Marsh Owls), and Marsh Owls with a fewer number of Barn Owls were found sitting on the road or road verge itself, possibly waiting for a potential meal.
Grass Owls rarely hunt from a perch, mainly because of the absence of suitable perches in its open habitat (Steyn, 1985). This can be seen by its adaptation of having longer legs and wings that are suited to catching prey from flight rather than a perch (Burton, 1973). Grass Owls were however known to utilise perches in the Barberspan Nature Reserve (pers. obs.), but were never seen to perch along fences in the study area. They are also known to spend lengthy spells on the ground only after a successful hunting
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strike as it prepares to swallow its prey (Hume & Broyer, 1991). Upon approaching this type of activity, the owl is reluctant to fly away (pers. obs.).
Marsh Owls have also been known to pursue prey on foot (Del Hoyo et al.,
1999).
Many of these behavioural traits attract birds to their specific types of environments and are what are responsible for owls frequenting roads. Such behaviour is appetitive i.e. the act of seeking food in this regard attracting them to roads, while the actual hunting technique of catching and eating prey is the consummatory behaviour. A raptor on the hunt for prey may undergo a prolonged period of appetitive behaviour and a short period of consummatory behaviour as it respectively searches for and catches its prey (Maclean,
1990). There are a number of possible reasons why owls spend many hours either perched on fence posts or are seen in low quartering flight over fields.
The degree of hunger that a bird experiences will govern the threshold for food seeking. The greater the hunger the lower the threshold for the appetitive act of seeking food and the more motivated it is (Maclean, 1990). This would include the need to feed both young and other parental parties, usually the female as well as themselves, and thus a much higher demand of hunting would be needed.
Observation of these owls at night showed no signs of agonistic behaviour.
Whilst observing an individuals’ behaviour on one side of the road, the spot light revealed more owls sitting on the ground in the road verge vegetation on opposite ends of the road, specifically Marsh Owls. Many Barn and Marsh
Owls were also observed to be in close proximity to one another along the
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road, indicating a tolerance for other species of owls as well as certain features attracting both of these species to the same area in abundance. The probability that owls are killed in association with another due to one or more of the previous behavioural aspects, best explain the double fatalities. This was best explained in terms of observing owl behaviour at night on roads as well as examining their corpses found on roads.
3.3.5.1. Appetitive behaviour: seeking food
It is generally assumed that owls die whilst in the actual process of capturing their prey directly off the road and at the same time being hit by a vehicle.
However, from the examination of the intact carcasses that were collected from the road, in a small number of cases, rodents were found attached to the talons of only two Grass Owl carcasses or insects were found in the gape of a few other species of owls. It is suggested that this is not a direct cause of fatality. Instead this probably related to hunting techniques and possibly why
Grass Owls are not found sitting in the road at night, as they prefer to swoop down in low pursuit of their prey. Grass Owls were assumed to be killed whilst flying across the road at a low level and then being caught in the thermal by the airflow of a passing vehicle. Turbulence in turn produces instability and this produces drag, which overcomes lift (Maclean, 1990; R?ppell, 1977).
Whether the appetitive behaviour was associated to hunting or to traveling is irrespective, but the actual behaviour of flight is what seems to be important in this species specifically. Marsh and Barn owls were found sitting on fence posts or on the road themselves with no signs of distinct appetitive behaviour, they could however have been awaiting for prey to appear or perhaps their
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behaviour was not related to hunting behaviour but rather to more of a maintenance behaviour such as thermoregulation, which will be discussed in section 3.3.9.5.
3.3.5.2. Consummatory behaviour: feeding
It should be noted that upon analysis of prey contents of owls it was discovered that most prey found consisted of more than 36% rodents and
26% insects. Most prey was almost completely digested to partly bone, exoskeletons and fur, indicating successful hunts had already occurred. Prey therefore was not instantly caught on the road just as an oncoming vehicle made contact with the owl, whilst possibly preying on such a food source. This made up only 3% of causes of Grass Owl death from vehicle collisions, indicated from rodents still present in the talons of the owls after impact
(Figure 3.18).
FIGURE 3.18: Rhabdomys pumilio attached to a Grass Owl carcass. Instead the Barn and Marsh owls are believed to be sitting on the road or on a perch hours after their first catch of the evening, digesting what was already caught, almost ready to regurgitate and possibly awaiting another feeding
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opportunity. Owls regurgitate pellets six to 12 hours after prey has been caught (Tarboton & Erasmus, 1998). These four species usually produce two pellets in one evening associated with two hunting bouts. It is possible that the remaining owls (35%), containing no prey in their gut, sit on the road in anticipation of finding the prey and are consequently hit without having been successful.
Sutton (1927) comments upon mortality among Screech Owls in Pennsylvania remarking that many were attracted to roads by small mammals, birds and insects killed there. However, this study found no signs of owls scavenging on other road-kill fauna. This is made evident from examinations of prey caught by owls, having fully intact bone and skull fragments and none being crushed.
As stated previously in Chapter 2, owls swallow prey whole and avoid compressing the prey.
3.3.5.3. Timing of hunting activity
Timing of hunting activity varied significantly between owl species. Marsh
Owls are known to frequently hunt during daylight, approximately two hours after sunrise and before sunset (Steyn, 1985; Tarboton & Erasmus, 1998) especially on dull days or in autumn and winter months. This has been observed in the study area as well as in the Barberspan Nature Reserve
(pers. obs.). The Marsh Owls are probably southern Africa’s least nocturnal owl species (Tarboton & Erasmus, 1998). Grass Owls however, are among the most nocturnal of owls and they rarely leave their roosts to hunt until at least half an hour after sunset and they are only seen in the daylight when
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they have been flushed from a roost by veld fires and have taken the opportunity to hunt rodents escaping the flames (Del Hoyo et al., 1999;
Tarboton & Erasmus, 1998). In contrast the Barn Owls usually start earlier in the evening and finish later in the morning (Del Hoyo et al., 1999). Barn Owls are also strictly nocturnal with few exceptions where Barn Owls have also been known to hunt in early mornings and late afternoons during winter when food is scarce (Hume & Broyer, 1991). Although this was never observed in this study, possibly due to the abundance of food and would only have occurred when disturbed. Shawyer & Dixon (1999) found that timing of hunting activity varied significantly between Barn Owls foraging on the roadside and those in open farmlands nearby. Activity peaks on farmlands occurred between 16h00 and 20h00 and coincided with the hours around dusk. On the road however, hunting activities peaked between 22h00 and
03h00. These times coincided with the time when traffic movements were at their lowest. Most timing of activity in this study area also peaked later in the evenings between 00h00 and 03h00 and was quieter and almost deserted earlier in the evenings from 20h00 to 00h00 when traffic was still heavy and again dropping from 03h00 to 04h00 when traffic picked up once again.
Together with findings of Shawyer & Dixon (1999) it can be confirmed that traffic density and the high level of background noise emitted by the vehicles definitely plays a role in disrupting owls hunting activities by interfering with owls keen hearing abilities to locate and capture food especially on dark nights on roads. Shawyer & Dixon (1999) also found that when roadside hunting was at its peak around midnight, it coincided with the time when the proportion of trucks to cars increased.
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3.3.5.4. Awareness of traffic
3.3.5.4.1. Awareness on roads
Finnis (1960) stated that there does not appear to be any sign of traffic awareness among most species of birds, in so much that they do not recognise traffic as a potential danger. This may be true but there is however evidence to show that birds are continuously aware of traffic such as the example just given concerning hunting activity and traffic volume (Shawyer &
Dixon, 1999). Dunthorn & Errington (1964) devised a method for measuring the alertness of birds to oncoming traffic. This awareness was termed the
‘reaction time’. Put briefly this method consists of measuring the time interval between bird’s leaving the road surface and the time an approaching vehicle reaches that vacated position on the road. This gives a measure of how close a bird on the road surface allows a vehicle to approach it. If observations are made from a car, then a note of the car’s speed allows a further measure of the bird’s responsiveness at that speed. These observations suggest that birds on the road surface are well aware of oncoming traffic and get out of the way in good time, so that the bird on the road surface is in a particularly safe position to avoid approaching traffic. This observation is contrary to the view of many authors who consider that road deaths frequently occur among birds, which are unaware of an approaching car and reaction times are slow. More universal opinions however, are that the probable cause is that most diurnal birds get killed whilst attempting to fly from one side of the road to the other and are struck because they do not see the oncoming vehicle, or ignore it, or misjudge its speed. In this regard there does not seem to be any signs of
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traffic awareness amongst Barn and Marsh owls particularly on rural, quieter roads. Hooting and dimming headlights do not seem to distract the birds in any way and only by approaching the bird to within meters of its reach will it then only fly off.
As previously stated, owls on roads would find it difficult to take off with such a high wing to body ratio especially on colder evenings when they are less active and would thus need a faster reaction time. These birds having to lift up off the ground may have to use different flight patterns and directions to avoid a collision. Most raptor species that use perches, utilise these structures by dropping forward to gain momentum The heavier the bird is the greater the effort it will need for take off from these structures (Steyn, 1984).
The direction of flight when a bird takes off usually follows the way that they were facing when still on the ground. They take-off when the vehicle reaches a particular distance from them, to which, they seem to become habituated
(Dhindsa et al., 1988). This distance may depend upon the body size of birds and appears to be species specific. Although there is not much specific data published on the minimum distance to which the vehicles are tolerated by different species of birds, some observations suggest that this distance is comparatively shorter in the case of smaller than that of larger birds. This might be because smaller birds, being more active, can apparently attain sufficient height to be clear of the vehicle and thus cross the road in less time than larger birds. Cooke (1980) studied the distance to which certain passerine species tolerate an approaching human and found a significant
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association between the degree of tolerance and body size of birds with smaller species allowing a closer approach than larger species.
Personal observations showed owls that crouched in the roadway in the path of traffic did not move from the road when the vehicle approached until just 5-
10 m away, not even the sound of a hooter caught their attention. Although they were aware of the vehicle’s presence they did not seem to sense the danger of passing vehicles, which suggests that they are aware of the usual passage of vehicles.
3.3.5.4.2. Awareness associated to flight
Flying birds, even when not directly intimidated by traffic, have been frequently noticed by Finnis (1960) to twist and turn in panic and fly along a road for some distance if they happen to be crossing a road when a car or a cyclist were beneath them. Undoubtedly these habits are similar to Grass
Owls’ typical hunting habits, where its large head turns from side to side, scanning for prey. A dive is made every ten minutes but nine out of ten strikes yields nothing and the Grass Owl is quickly back in the air (Hume & Broyer,
1991). Unaware it has just entered a dangerous crossing, it would be difficult to gain speed and lift to manoeuvre itself away from the danger of a vehicle.
A bird’s brain is small and incapable of attending to various things all at once and only those of immediate importance are contend with first (Dhindsa et al.,
1988). Dhindsa et al. (1988) state that for a bird to distinguish any moving object, big and near enough to be dangerous, the image of the object on the birds retina must be moving with respect to other images, all of which should
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appear to be moving when a bird is in flight. When the image of an object is not moving on the retina of a bird, which is flying in a straight line, the object may be taken as a distant object and ignored by the bird. Unfortunately when a bird is flying on a diagonal collision course with a vehicle and sees it with only one eye, the image of the vehicle on the bird’s retina will be stationary as if it were that of a very distinct object. As a result birds do not take into account the distance of the vehicle until it is very close to it. In such situations a collision is unavoidable. According to Dhindsa et al. (1988) birds in flight presumably judge distance on the basis of parallax – a phenomenon by which the distant object to one side of the direction of movement of a moving person appear to fall behind slowly, while the nearer objects fall behind rapidly.
Another factor associated to flight pattern occurs when a bird tends to fly low down such as those in studies by Finnis (1960) and Hodson (1960), involving the Wren, Robin, Song Thrush, Blackbird and Whitethroat. The owls involved in this study can be included in this category since most owls (especially the
Marsh Owl, Grass Owl and to a lesser extent the Barn Owl) are known to hunt in low quartering flight close to the ground (3 m for Barn Owls and 1 to 3 m for the Marsh and Grass owls) in search of food (Del Hoyo et al., 1999; Steyn,
1985) dropping suddenly onto its prey with their talons extended.
Conversations with regular drivers indicate that in almost all instances when owls are killed or are seen to be killed, they have appeared out of nowhere onto the road in front of their oncoming vehicle. Some are overtaken when trying to return to the cover from whence they came.
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An interesting comment given in Finnis (1960) on bird behaviour in traffic referred to the habits of birds when menaced by an oncoming car of diving down to ground level. It was stated that - birds in the course of evolution have apparently gained survival value by using this maneuver against the stoop of a hawk. To get out of the way of a car all the birds need do is to fly vertically a few meters, but they never do, it is a matter of speculation how long it will take for this habit to be reversed if ever.
3.3.5.5. Temporary blindness
It does not appear to be to be only awareness that relates to owls’ delayed reactions to an oncoming vehicle but more so another possibility being the light emanating from a vehicle at night, causing temporary blindness (Labisky,
1960). This may account for the large mortalities occurring among nocturnal species such as owls sitting on the road in this study and the nightjar mortalities in the study by Jackson (2002). The blinding of animals in the glare of approaching headlights is undoubtedly an important factor in their destruction. This can particularly be observed on dirt roads, in many smaller mammals such as Springhare (Pedetes capensis) and Scrub hares (Lepus saxatilis) that do not move away from the light. Observations given by
Dickerson (1939) suggests that the sharpness of the shadow at the margin of the light zone may be an important factor in determining the promptness or tardiness with which the animal moves out of the path of an approaching car.
The blinding of an oncoming vehicle causing other oncoming traffic not to see the owls sitting in the path of their vehicle, which was observed during this study may have also been a major factor involved. Perhaps the owls are
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unsure which direction to fly depending on which car will reach it first. Sutton
(1927) suggest this is a cause of Screech Owls being killed by cars and a possible reason why owls do not move off the road immediately.
3.3.5.6. Roadside attractiveness related to food resources and hunting behaviour
Roads isolate animal habitats (Mader, 1984) and thus may serve as an ‘edge’.
Any ecotone exhibiting an abrupt structural change in vegetation between adjacent habitats and/or the point at which the change occurs constitutes such an edge (Gates & Gysel, 1978). Gates & Gysel (1978) also termed these ecotones ‘ecological traps’. Accordingly many species of birds with different food habits are attracted to roads apparently because of the edge effect, as there is a tendency for variety and density of organisms to increase at the border between different plant communities (Odum, 1997). In this study the
‘edge’ was related to cultivated fields and road verges. Roadside habitats are known to contain a wealth of small mammals, especially in very wide verges
(Adams & Geis, 1983; Meunier et al., 1999; Meunier et al., 2000). Roads therefore may act as such ecological traps for owls utilising verges as a result of their foraging range in suitable habitat with abundant food.
It was found that rodent holes were in abundance along the road verges, where soil was soft enough (as indicated from soil and geophysical maps – refer to Figures 3.15 to 3.17) and food abundance was plentiful and permanent. It was also found that during the rodent survey the highest numbers of rodents captured was at the site located at the roads edge (Table
2.21). The behaviour of most raptors along roadsides indicated that verges
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were used as principal hunting grounds. This was seen from the many diurnal raptors such as the Black-shouldered Kites (E. caeruleus) perched on fence posts with rodents in their talons and the many pellets from owls accumulated along these fence posts.
Although many authors have suggested that birds of prey use road verges for scavenging (Haug, 1985; Watson, 1986) and for hunting (Fajardo et al., 1998;
Michael & Kosten, 1981) they did not compare the abundance of both raptors and prey in roadsides to that in adjacent areas. The results from this study indicate that small mammals were in fact found to be in higher abundances in road verges than opposed to fields in adjacent areas (Table 2.21). This is perhaps due to them not being able to establish permanent residence in farmlands during autumn, where farming practices such as harvesting and plowing have been carried out. Small mammals would take the opportunity to feed on the remaining crops, however would not be able to establish residence in such areas. Only during a small period in springtime are they able to establish some form of habitat close these better feeding grounds.
Roadsides might be poor but reliable food sources in winter (Meunier et al.,
2000) for small mammals. This may be another possible reason for seasonal fluctuations in rodent occurrences, and ultimately owls, on and close to the road. Meunier et al. (2000) found this feature to be true in his study. By studying the behaviour of rodent plagues one could determine peaks in different years alongside roads and in fields and what effect they have on owl mortalities but that would be an long term study on population trends, forming a study on its own.
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At several places maize covered vast areas at both sides of the road. The southeastern Highveld of Gauteng is known to be a popular producer of grain within South Africa. The farmers produce large quantities of grain, the transport of which continues throughout the season mainly in the time after harvesting from farms to silos and silos distribute to other parts of the country.
Spilled grain and crushed animals make roads and their verges a rich source of food for granivorous and omnivorous birds. In our study granivorous rodents too make use of this important and easy available food source, on which the owls will feed themselves for this simple reason that the majority of the total recorded dead owls contained grain-eating or mixed-feeding rodents in their gizzards. Insects and other rodents made up the rest of the prey items eaten by the owls dissected and no shrews (insectivores) and Vlei rats
(herbivores) were found amongst the gizzard contents. These prey were only found in collected pellets at roosting and nesting sites, indicating that these owls may have fed somewhere else and those found on roads fed upon the rodents occurring on or alongside the road itself (Chapter 2). While each of the species would tend to specialise in a particular type of prey, it is important to remember the dictum “opportunity makes the meal” (Steyn, 1992). This would specifically apply to the more inexperienced and weaker individuals.
The four most common small mammals in our study area (Chapter 2) were the Striped Mouse (R. pumilio), the Mutimammate Mouse (M. natalensis); Vlei
Rat (O. irroratus) and the Swamp Musk Shrew (C. mariquensis). They all exhibit both diurnal and nocturnal activities (De Graaff, 1981) and constitute the preferred prey of diurnal raptors as well as the nocturnal owls, constituting
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the most abundant prey in their diet. These results seem to confirm assumptions made by several authors on the role of road verges as a food source for most raptors (Michael & Kosten, 1981; Williams & Colson, 1989).
Another hypothesis is related to the size of roadside verge on the hunting behaviour of raptors. Harriers are rarely seen hunting on roadsides, despite the importance of rodents in their diet (Butet & Leroux, 1993). This may be because verges are too narrow for their low-flying hunting technique. Perch hunting or hovering is better suited to these habitats (e.g. Barn Owls, Marsh
Owls, Black-shouldered kites and Kestrels).
Thus, in addition to the density of prey, their availability to suit raptors hunting techniques is crucial. Although the density of perches in the different zones were not determined during this study, they were obviously greater along motorways owing to the presence of the continuous fences in addition to fewer trees and shrubs typical of open grasslands.
3.3.5.7. Some uses of roads to other bird species
Many authors have completed studies of roadside use in birds and revealed a host of reasons why they were drawn to roads in particular. One of the most obvious uses of roads by other birds is to obtain small particles of grit and are thought to be eaten off the road surface to aid in digestion of food (Dhinida et al., 1988; Meinertzhagen, 1954). However, owls are in no need for such a function, so this seems to be an unlikely cause for owls to be sitting on the road. Insects are attracted to droppings of livestock, or by the shinny surface of the road and swarming beetles and migrating grasshoppers are killed in
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large numbers by cars (Dhinida et al., 1988). Birds alight on the road to feed on these insects (Vestjens, 1973). The numerous insects that birds find washed on to the roads by heavy rains attract many species of birds to roads, but there is no indication of rain affecting mortality counts in this regard as the
N17 has a runoff curve that most highways possess to allow rainwater to runoff the surface.
Dhindsa et al. (1988) mention many other reasons why birds were drawn to roadways, such as standing water puddles, a supply of carrion, abundant nest and perch sites and a source of solar radiance during colder winter months.
Other attractions roads offer to birds are drinking pools, soft mud patches
(becoming increasingly rare) and dust bathing sites, none of which owls are in need of. Perhaps the most interesting suggestion of the attractions that roads have for birds was reported by Finnis (1960) where many species alighted on roads to keep their feet dry when there was heavy dew in the field. This is not an unlikely suggestion as most casualties were found in autumn and winter when the study area is particularly prone to frost and particularly dealing with grassland owls, which roost and nest in such a habitat that is prone to frost during the evening and early morning. But this only potentially explains why
Marsh Owls may have been sitting on the roads Grass Owls were not found sitting on the tarmac roads and Barn Owls prefer to nest and roost in protected man-made structures or trees and thus frost would not affect them in any way.
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3.3.6. Mortalities of owls versus other taxa
The relative numbers of owls killed compared to other vertebrate species was considerable. This contradicts Clancy’s (2002-in press) statement that owls are high order carnivores and, as such, would be expected to occur at lower population densities on roads than lower order carnivores, insectivores and herbivores. The numbers and species of small mammals found killed by traffic are unknown but it is known that they occurred in abundance during the winter season as the same peak time as the owls (pers. obs.). A large number of domestic (Felis domesticus) and wild cats (Felis lybica), domestic dogs (Canis domesticus), and Black-backed Jackal (C. mesomelas) were also found killed, some while feeding on other road casualties but were not in as high abundance as the owls. Owls have not been known to scavenge on road kills during this study. This was confirmed from examining pellets and stomach contents of their small mammal prey, which were found intact and not crushed as was likely to happen when impacted and flattened by a vehicle.
Other birds included four species, which made up a small number of casualties, namely, one Black-shouldered Kite (E. caeruleus), one Amur
Falcon (F. amurensis), one Lesser Kestrel (F. naumanni), and four Spotted
Thick-Knees (B. capensis) These birds also frequently forage along roadsides or fly low across the roads, with the exception of the Spotted Thick-Knee, which have a unique and unknown attraction to roads. Although kestrels and buzzards clearly used verges for hunting, their abundance along roads was not directly related to the relative abundance of small mammals as stated by
Meunier et al. (1999). The supply of perching sites, allowing for less energy-
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demanding hunting behaviour than flight-hunting, and the width of the verges, appeared to be more important factors in the attractiveness of roadsides for these species.
3.3.7. Nocturnal versus diurnal species
Nocturnal animals seem to frequent roads more often than diurnal species, when traffic densities are low and roads are supposedly warmer but it still does not explain the higher number of owls to those of other nocturnal vertebrates, only the behavioural aspects discussed could explain this phenomenon. During the study period not a single Common Duiker
(Sylvicapra grimmia) was found dead on the road, however, they were found in abundance in November during the evenings, over a seven day period eight were counted in one night. However, common duikers do not have the same feeding strategy as owls and generally stick close to thick grass cover away from the roads. Many rodents and Lagomorphs were found especially in winter after veld fires, mainly in the hotspot area.
Even though migratory flocks of diurnal birds of prey, such as Lesser Kestrels and Amur Falcons, as well as Kites, become concentrated along the highways’ road margins, since it affords the most attractive or the only cover and food available in the area, there still is no mass mortality of these birds due to reasons given above. Most Kestrels and Falcons, hawk aerially catching their food in their feet and transferring it to the bill in flight (Maclean,
1993; Steyn, 1984), and do not settle on the ground to consume their prey like owls do.
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There do not appear to be any nightjars in this area as found in other parts of the country where they become frequent road casualties. Information relating to this may be found in publications by Harrisson (1954); Jackson (2002) and
Winterbottom (1954). Most nightjars occur in habitats associated to woodlands and forests (Maclean, 1993). Daneel (1943) stated that at night, owls rarely misjudge cars traveling at 40 mph or over, but nightjars are frequent victims no doubt to being blinded by the headlights. This author obviously did not take into account the vast owl mortalities that do occur on roads.
3.3.8. Biology: breeding, age and movements related to mortalities
Roadside areas also serve as nesting sites for many species of birds. In this study no owls were found to be nesting alongside roads and the closest nest was located 200 m from the road itself (see Chapter 2). This excluded the pair of Barn Owls nesting in a silo situated 50 m from the R550 road. As previously stated the choice of roadside habitat for young post fledglings, following the peak nestling season, may also result in increased mortality of younger owls from traffic (Brown, Brown & Pesotto, 1986; Dhindsa et al., 1988; Hernandez,
1988; Vestjens, 1973). Most of these authors suggested that this was due to the inexperience of young owls with moving vehicles. The age of birds also affects their sensitivity to moving objects (Dhindsa et al., 1988) with older birds being more sensitive than younger ones to an approaching human.
Therefore one would expect the juvenile bird to be more prone than adults to mortality from vehicles. McClure (1951) found birds inexperienced in the speed of approach of vehicles and in the judgment of speeds and were more
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apt to be killed. Highway losses due to inexperienced animals are further borne out by a comparison of the monthly highway traffic and monthly losses.
If the highway kills were directly proportional to travel, the losses would be expected to follow more closely the trend of traffic. Instead the losses rose precipitously with the appearance of inexperienced individuals.
Ringing recoveries of owls showed that mainly juveniles were found dead during a study by Shawyer & Dixon (1999). These authors indicated that they had originated from nest sites both within and outside the 50 km by 5 km corridor of their study road. It is assumed that most owl mortalities occurring on this study road originated from youngsters originating from nests in similar proximity (those nests found as close as 200 m) to the above findings and with a few occurring outside the area.
The actual patterns of movement of different owl species as well as dispersal between adults and juveniles are important to understand. Most juvenile
Grass and Marsh owls disperse in late autumn early winter and these populations would comprise of a percentage of inexperienced birds in their first year of life. Adults feeding broods have to fly further distances in search of food and if scarce would probably take more risks to find it. Although the
Marsh Owl is stated to be a resident species, this species is undoubtedly nomadic to a certain extent, especially when its habitat is destroyed by grass fires, drought and overgrazing (Del Hoyo et al., 1999; Steyn, 1985). Young usually fledge at about 35 days (Del Hoyo et al., 1999). The Grass Owl is however more sedentary than the Marsh Owl and is also resident unless driven out by the same factors to those of Marsh Owls (fires, drought and
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overgrazing). If however the land recovers, it promptly returns to its old hunts
(Hume & Broyer, 1991). The Grass Owl also prefers to return to regular roost sites every morning whereas the Marsh Owl may only use a roost site once
(Steyn, 1985). Some local movements are likely depending the food supply as well as the post breeding dispersal of juveniles (Del Hoyo et al., 1999). Grass
Owl juveniles usually fledge at about the same age as the Marsh Owl, at 35 days. Young Barn Owls develop slowly and may be a month old before they begin to explore the surroundings of the nest and up to 9-10 weeks old before they fly from it. They would have to seek out an existence, often in poor habitat (especially in the case of heavily farmed areas), learning to hunt and hopefully finding suitable terrain before the onset of cold weather. Adult Barn
Owl’s are however sedentary throughout the year and throughout their lives and if a suitable nesting or roosting (usually the same) is found a pair of Barn
Owls will usually remain there until removed or die of natural or unnatural causes. This has been confirmed in this study from observations made at the nest site of a pair of Barn Owls, whereby a pair remained throughout the study as could be seen from the many accumulated prey remains found on the floor beneath the nest (a layer about 20 cm deep). This pair will probably remain there until something happens and another will automatically replace them.
Movement as a consequence of range expansion, which is known to occur in adults during the winter season, is likely to occur when defense of the nest and the delivery of food to it, has ceased. Most of the adult Barn Owls killed in the study by Shawyer & Dixon (1999) also originated from breeding territories near to the fringes of the study area, 2.5 km from their study road. It is assumed then that if the breeding cycles change, so will the peaks in
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mortality, which again is related to food availability. Small mammals too, can become reduced and have a more patchy distribution at this time of the year
(viz. winter) encouraging owls to move out of their traditional breeding territories, following prey dispersals.
Roads attract an abundance of younger birds due the presence of easy available prey not too far from their breeding grounds and result in less youngsters surviving their first season. However, this may allow the population trends to be stabilised and parents compensating for such losses by increasing clutch size or perhaps breeding more than once in a breeding season. This however, still needs to be studied further to verify such assumptions. This reason could explain the same number of owls being hit every day and every season in the same spot over time, and not necessarily have a declining influence on the population numbers. Note that 76 owls were counted in July 2002 and the following year numbers did not differ too extensively with a count of 85 owls carcasses for July 2003. Owls are not too territorial; it’s more a dependence on space and food availability, so if one owl is removed in time from a certain space with a good food supply, the likelyhood that another will replace it within days is high (De Bruijn, 1994).
Unfortunately the study was unable to compare breeding seasons to the number of birds killed along the same stretch of road during 2002. But during
2003 owls seemed to be doing very well with good broods and many nests in the vicinity being located (see Chapter 2). There appeared to be no reduction in numbers in the breeding population in 2003 in all species suffering heavy mortalities the previous year.
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3.3.9. Mortalities and highway use
Hodson (1962) suggested that speed and density of traffic are major factors governing bird casualty figures and implied that the increase in number of cars on the road is parallel to an increase in the number of diurnal birds killed.
Finnis’ (1960) view that the need for wider, straighter and faster roads is likely to be more helpful in conserving a passerine population as opposed to
Hodson’s (1962) suggstion. These two suggestions merit further investigation concerning density and speed of traffic as well as the design of roads as factors of owl mortality.
3.3.9.1. Traffic density and types of vehicles involved
There was a large difference in owl mortalities when comparing different seasons and may have been influenced by highway usage by motorists.
Information on vehicle counts together with type of vehicle, received from the
National Roads Agency are given in Figures 3.19 to 3.21 where traffic volume is measured in vehicles per hour. Of the two data sets reported for this study, one represented a summer period and one for a winter period, to distinguish between seasonal differences. These data were fortunately available for any time due to extensive records being kept at the Dalpark Toll Plaza, which leads into and out of the study road. It was thus possible to first find out which months were of conflicting mortality counts to compare these mortality to traffic variables.
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The following dates were chosen based on the results of mortalities:
i). January; 14th to 20th 2002, which was a normal week including a
weekend, starting on Monday and ending on a Sunday It is during a
summer period where mortalities were: seven Marsh Owls, 17 Grass Owls,
two Barn Owls and one Spotted Eagle Owl.
ii). July; 15th to 21st 2002 which was also a normal week and a weekend
from Monday to Sunday, similar to (i) except occurring during a winter
period, with higher mortalities of 72 Marsh Owls, 24 Grass Owls, 33 Barn
Owls and one Spotted Eagle Owl.
The mean annual traffic (cars per year) differ little from year to year (pers. comm. 9Mr. G. Ackermann, 2003) with less overall traffic occurring on the
R550 and even less on the rural, dirt road. However, certain adverse weather conditions, such as rain in summer periods and fog in the winter periods, most definitely would have influenced results of car density, speed of traffic and occurrences of mortalities since it is known to have an approximate reduction in numbers of vehicles using the road.
Reductions in traffic counts were also expected to be lower on weekends as opposed to weekdays. The reason for this would be purely motoring on weekdays by business traffic that appeared to slow down during weekends.
Traffic was also expected and shown (Figure 3.19) to be lower in the evenings as opposed to that during the day.
9 Geoff Ackermann, SA national Roads Agency, Pretoria
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CHAPTER 3.3: RESULTS AND DISCUSSION
FIGURE 3.19: Traffic peak flow for total counts along the N17 during a 24-hour period.
Traffic density was also noted to be different for vehicles going towards
Johannesburg (Figure 3.20A) to those moving away from the city (Figure
3.20B). During weekdays in January the total volume of traffic to
Johannesburg peaked between 05h00 and 08h00 with 1000 vehicles per hour. The vehicles moving in the opposite direction at the same time were less with only 500 vehicles per hour traveling that route. A second peak occurred for vehicles once again traveling away from Johannesburg, between
15h00 and 19h00, with almost 800 vehicles per hour and only a smaller rise in traffic flow going to Johannesburg at this time with 500 vehicles per hour.
Without exception all low peaks occurred after sunset and before sunrise in both seasons. During the evenings in both directions, traffic densities became somewhat reduced, which was expected and was similar in both seasons.
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Traffic during the evenings dropped to its minimum between 01h00 and 03h00 with less than 20 vehicles per hour. These results in traffic times generally followed this pattern throughout the week, as well as following the general trends of working hours to and from Johannesburg, with a slightly earlier peak on a Friday afternoon for those going or returning from Johannesburg, which is typical for the end of a working week. However, Friday evenings also gave some interesting results. Traffic was higher in both directions between 20h00 and 01h00 the next day, with the start of the weekend. Weekends gave a slightly different pattern of traffic flow to that of a weekday. Traffic peaked (350 vehicles per hour) between 06h00 and 19h00 in both directions and once again was higher during late evening and early morning than weekdays.
Unfortunately mortality counts on weekends were not undertaken, so a direct comparison of weekday mortalities to weekend mortalities could not be established. However, on a few occasions observation surveys were undertaken on a Friday evening and the following day did not reveal many casualties on the road, possibly indicating less mortalities occurring on weekends due to a slightly increased traffic flow and speed during the evenings on weekends.
Weekdays during the July period were much the same as that of the summer period, with peaks to and from Johannesburg occurring at the same time, with the same volume of vehicles. Evenings also seemed to be very similar indicating that traffic volume may not have an influence on seasonal mortalities (Figure 3.21).
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A.
B.
FIGURE 3.20: Weekday traffic flow along the N17 to Johannesburg (A) and going towards Springs (B) on the same day.
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CHAPTER 3.3 RESULTS AND DISCUSSION
A.
B.
FIGURE 3.21: Weekday traffic flow along the N17 during (A) January 2003 and (B) July 2003 on the same day of the week.
As mentioned previously, speed may be an important factor during different times of the year coupled to weather conditions. Heavy vehicle densities were however slightly higher on weekends and to a lesser extent on weekdays
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CHAPTER 3.3: RESULTS AND DISCUSSION
during the winter period than that of the summer period. An explanation for this would be the need for more transportation of crops during time of harvesting, that being mainly during the winter period in this area. This route is known to be more of a business route and less of a holiday destination route thus traffic during holidays was not expected to change.
The volume of traffic is directly proportional to the amount of both noise and chemical pollution emanating from the moving vehicles (Dhindsa et al. 1988).
The density of traffic may therefore result in distracting and repelling some diurnal birds away from roads; a reason why more nocturnal species of birds are found on roads, unlike diurnal species, which are unable to settle down on heavily used roads. It has been reported that the effect of road disturbance elsewhere may be so significant that some birds avoid roads for distances varying from 500 m-600 m from quiet rural roads to 1600-1800 m from busy highways (Dhindsa et al., 1988).
Although there is no historical data for traffic densities, it is obvious that traffic volume has increased within South Africa. Hodson (1960) found in his study area during 1938 less than two million cars were on the road but by 1958 this figure had doubled and stood at over four and a quarter million. The upkeep of roads has also improved over time. This is evident from the maintenance that is continually being carried out on the road and will continue to be cared for due to the demand of public safety. The traffic flow along the N17 is also expected to increase once plans have been approved for future restoration of the road. It is however not known what impact such actions will have on the
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CHAPTER 3.3 RESULTS AND DISCUSSION
owl populations and mortalities in the area during procedures and in the future when completed.
3.3.9.2. Traffic speed
That the speed of motor vehicles was a major factor in determining the number of bird casualties quickly became apparent and in almost all cases, this was found to be closely allied to the existing weather conditions. Most birds can take evasive action from the path of vehicles traveling up 60 km per hour. (Dhindsa et al., 1988) Hodson (1960) found this speed to be 35 miles per hour, equivalent to just over 56 km per hour (pers. obs.), and very similar to our findings, thus making weather conditions that reduced traveling speed of prime importance. Rain would give rise to treacherous road surfaces as did mist with the attendant bad visibility, which would automatically cause a reduction in the speed, compared to drier, clear days.
Vehicles travelling at speeds above 80 km per hour appeared to be of critical hazard for all species of wildlife but especially owls (pers. obs.). This speed was found to be similar to the study of Dhindsa et al. (1988) and Nankinov &
Todorov (1983) for bird strikes in India and Bulgaria respectively. At speeds estimated at 80 km per hour and over owls have been observed to escape less frequently unless they rise to the height of the car top or higher before the car approaches to within half a car length. This would be more difficult for owls with large wingspans. Observations of the birds’ behaviour in flight (discussed previously) suggest that at these higher traveling speeds the draft from the fan and turbulence may be, in part responsible for the increased hazard in that situation. Maclean (1990) indicated that turbulence might produce instability,
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CHAPTER 3.3: RESULTS AND DISCUSSION
which then produces drag and ultimately overcomes lift. They have apparently learned what the minimum distance is at which they must fly to avoid collision with a vehicle (Dhindsa et al., 1988). However, when some vehicles move very fast, birds seem to misjudge the distance and a strike often occurs. As previously mentioned, birds that cross roads while flying also misjudge their distance from moving vehicles (Fremlin, 1985).
The drivers during this study were all, conscientious about owls occurring on this particular road and managed to avoid hitting many of these owls, driving at a speed of 60 km per hour, which would otherwise have been killed if driving at a faster speed. Nankinov & Todorov (1983) have also noted that most bird-strikes on the road in Bulgaria occurred when vehicles’ speed was close to 100 km per hour. A study in Germany by Illner (1992) found that a car speed of more than 80 km per hour resulted in about 21 times more owls being killed by cars and found that the density of traffic had little effect on road death rates. The prevalence of heavy vehicles on the study road is almost certainly an additional factor. A large number of heavy vehicle movements are made at night on this route.
This study agrees with observations in previous studies and dry, clear weather may incite various bird species to increase activity apart from also leading to an increase in the speed of traffic using the roads. A combination of the two invariably gave rise to an increase in the casualty rate. But unfortunately little can be done regarding these aspects of speed and density. This brings us to the next subject concerning traffic, involving the different features of roads.
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CHAPTER 3.3 RESULTS AND DISCUSSION
3.3.9.3. Road category
Clancy (2002-in press) found that the number of owl mortalities and various other animals killed on roads is influenced by road type. Roads are generally divided into four categories, that being highways, other major roads, minor roads and dirt roads. In our study area only three categories are present, namely national road (N17), other major roads, i.e. main road (R550) and a secondary dirt road (Boesmanskop road) joining the aforementioned roads.
Almost all owls were killed on highways (80.7%) with a smaller amount on major roads (18.9%), only a few have been known to be killed on dirt roads
(0.4% - two Marsh Owls during our two-year study). Vestjens (1973) and
Clancy (2002-in press) found similar results, in which the latter author found most owls were killed on highways (82.2%), with a smaller number on other major roads (10.7%) and minor sealed roads having 7.1% of owl mortalities occurring on it and no owl were found in his study along unsealed roads.
In our study a comparison can be made between the owl mortalities occurring on the N17 highway to that of a major road the R550 together with the adjoining sand road, as these were the roads frequently checked. In terms of occurrence as previously discussed (Figure 3.4 and 3.5) the individual mortalities for the N17 were 52.1% Marsh Owls, 27.8% Grass Owls, 19.1%
Barn Owls, 0.7% Spotted Eagle Owls and 0.3% unknown and for the R550, the same four species were dominant with 60% Marsh Owls, 25.7% Grass
Owls, 10.5% Barn Owls and 3.8% Spotted Eagle Owls. The two Marsh Owls collected on the dirt roads are insufficient to make assumptions about species
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CHAPTER 3.3: RESULTS AND DISCUSSION
using these types of roads. It is thus apparent that road type has no influence on species preference to certain roads.
It is clear from total casualty counts of the N17 and R550 (Table 3.2) that more owl mortalities occurred on the N17 than that on the R550. The speed of traffic on the R550 is less than that of the N17 and the N17 would expect to have more owl mortalities given the reasons previously discussed. However, the lower counts may be because there are fewer cars at night traveling such a road or better still, other features may be preventing massive mortality.
Other features such as condition of the road, that of the R550 being in a worse condition (such as potholes) than the N17 and thus slowing down the few cars that do travel this road at night. The greater the average speed of vehicles on highways results in larger numbers of owl road kills. However this does not explain why mortalities have been known to occur on dirt roads. This may be attributed purely to chance.
From the data of Clancy (2002- in press) it is obvious that the larger the road, and the higher speed of vehicles the greater the chance of an owl road kill occurring. The explanation for this is that the larger roads may have been constructed through more optimal owl habitat than unsealed roads and that the higher traffic speed on these roads increases the risk of owls being killed.
The speed of cars is a major factor on fast highways, unlike dirt roads. It is believed that owls wait until the road is not too busy, indicated from the time they seem to come out at night, and sit on the road (pers. obs.) and it only takes one car at a high speed to catch an owl unaware. The higher traffic density and less suitable habitats alongside other major highways in Gauteng
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CHAPTER 3.3 RESULTS AND DISCUSSION
(e.g. M1 and N1) and the fact that these roads are extremely wide and lit up are reasons why owls and other fauna do not frequent such roads. This study is in agreement with the findings of Clancy (2002-in press) concerning roads intersecting optimum owl habitat as discussed in Chapter 2.
Most roads within South Africa are made of either sand, concrete, gravel, or tarmac. The owls in this study area live mainly in open grasslands traversed by sand and tarmac surfaced roads. Only a small percentage of animals would be expected to be killed on dirt roads because of the reduced velocity of vehicles (McClure, 1951). Conversely, McClure (1951) stated that mortality on concrete highways would be greater because of high vehicle velocity on this surface. Apparently an animal crossing concrete has one-eighth the chance of survival as it would crossing other types of road surfaces (McClure,
1951). In New Mexico, Knobloch (1939) found that in summer months the loss of wildlife along concrete roads was 4.1 animals per mile, and along gravel roads, 0.45 per mile. Unfortunately this study area did not contain concrete roads and a comparison with this type of road was thus impossible. However, it had been suggested that concrete roads might reflect light better and increase the visibility of a driver to oncoming traffic (pers. comm. 10Mr. G.
Ronald, 2003). Another feature is whether prey may be present on the different types of roads. From the small mammals survey it was found that rodents caught along the N17 highway were in higher abundance than those caught at site nine, which was located next to the dirt road, however, this was not extensively studied.
10 Gary Ronald, Automobile Association of South Africa, Johannesburg.
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3.3.9.4. Road width in relation to owl mortalities
The initial stretches of the study road (N17) started as a three-lane highway becoming a two and eventually a one-lane highway consecutively. Yet very few owl mortalities occurred where the roads were wide. There was also less lively activity at night on wide sections of road. The probable reason is that this section generally ran through more of a built up area than did the narrower section of the study road and because of it generally carrying more traffic volume than the narrower roads with larger road verges. The study by
Dhindsa et al. (1988) revealed 18 vehicles per minute occurring on a four-lane road, nine vehicles per minute on a two-lane road and only four vehicles per minute on a one-lane road. Species diversity, or the relative abundance and number of species in a community, declined as roads were widened from one to two lanes, and again from two to four lanes. Equitability, or the evenness in species abundance and species-richness, was also found to decrease with an increase in road width. All three of the raptor species noted in his study, Black
Kite (Milvus migrans), the Eurasian Kestrel (Falco tinnunculus) and the
Spotted Little Owl (Athene brama) were inclined to occur further away from wider roads. Owl mortalities and live owls in this study also decreased with the increase in road width and owl mortalities were also more abundant in single lane section of the study road (pers. obs.). As roads are broadened traffic intensifies, resulting in raised levels of activity and noise acting as a deterrent to many avian species, including most raptors (including owls) from the road
(Postelli, 1927).
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CHAPTER 3.3 RESULTS AND DISCUSSION
It was also important to note that the bends in the study road (non-hotspot) had less mortality than the straighter sections of road (hotspots 1 and 2). In actual fact no mortalities occurred at site four, which was found to have a distinct bend in the road with steep slopes on either side of unsuitable habitat for smaller mammals and no extensive cover or perching sites for the owls.
3.3.9.5. Temperature variations of roads
Harsh weather can accentuate most kinds of mortality (Newton, 1979). It can increase the food needs of the bird or make food harder to obtain; it can make birds more susceptible to disease; or it can force them nearer to human settlements, including roads, so that they are vulnerable to human induced factors. Another factor separating types of roads associated with owl mortality, may be temperature induced with some roads being warmer than others.
Starrett (1938) considers roadside cover to be of less significance, as compared to temperature, humidity, and precipitation. Whitford (1985) suggests that black-surface roads absorb and store large quantities of solar heat and attract birds by providing them with warmth when the air is cool and direct solar energy is least available. This habit of sitting on the side of the roadway to warm them at dusk may also have contributed to some fatal accidents.
From our results it was determined that the average air temperatures verses tarmac and ground temperatures only deferred by approximately 1.32ºC between tarmac and ground and 0.56ºC between air and tarmac temperatures. From Figure 3.22, it is evident that temperatures varied little
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CHAPTER 3.3: RESULTS AND DISCUSSION
between three different localities (Site A, B, and C) and did not vary amongst different days either. Daytime tarmac temperatures, reaching a maximum of
30ºC was much higher than the evening temperatures with a minimum temperature of 5ºC as opposed to a maximum ground temperature of 33ºC and a minimum of -3ºC (Figure 3.22). This indicates that ground temperatures fluctuate more than tarmac temperatures in the area. However, since owls are only active during the evenings, it is only relevant to note the difference in temperatures from 19h00 to 03h30 as these were the times that coincided to most activity. The hotspot zone proved to be cooler in the evenings compared to the non-hotspot zone. This was found in all three variable temperature namely, ground (Figure 3.23A), tarmac (Figure 3.23B) and ambient (Figure
3.23C) temperatures. This could be attributed to the area occurring in a dip with surrounding moist areas. During the day however, tarmac temperatures were cooler in the non-hotspot area than that of the hotspot.
3.3.10. Other forms of mortality
Of the numerous causes of mortality that occur such as starvation, disease, predation, electrocution, shooting, trapping, poisoning, drowning, collisions with other obstacles and other accidents that are all known to kill birds of prey, only some are likely to regulate populations.
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CHAPTER 3.3 RESULTS AND DISCUSSION
A.
35 30 25 AT 20 TT 15 GT
Temperature 10 5 0 13:15 16:18 20:00 2:40 Time
B. 35 30 25 20 AT 15 TT 10 GT Temperature 5 0 -5 13:30 16:00 20:30 3:00 Time
C.
35 30 25 AT 20 TT 15 GT
Temperature 10 5 0 13:15 16:18 20:00 2:40 Time
FIGURE 3.22: Ambient (AT), tarmac (TT) and ground (GT) temperatures (ºC) between three different localities along the N17. (A) S 26º 16’15.3”, E 28º 29’ 69.1”; (B) S 26 º 21’ 19.5”, E 28º 38’ 90.2”; (C) S 26º 21’ 88.0”, E 28º 42’ 74.6”.
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CHAPTER 3.3: RESULTS AND DISCUSSION
A. 35
30
25
20 Non Hotspot 15 Hotspot 10
Ground temperature 5
0 13:00 16:40 19:45 02:30 -5 Time
B. 35
30
25
20 Non Hotspot 15 Hotspot
10 Tarmac temperature 5
0 13:00 16:40 19:45 2:30 Time
C. 20 18 16 14 12 Non Hotspot 10 Hotspot 8 6
Ambient temperature 4 2 0 13:00 16:40 19:45 02:30 Time
FIGURE 3.23: Tarmac (A), ground (B) and ambient (C) temperatures in the non- hotspot and hotpot zones along the N17.
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CHAPTER 3.3 RESULTS AND DISCUSSION
Starvation, predation and population dependent diseases are those that are most likely to act in a density dependent way (Newton, 1979). On the other hand, population independent diseases and some accidental forms of death are density independent and are unrelated to population density.
Animals colliding with vehicles can have an impact on both the mutilation of wildlife as well as the risk of accidents causing damage to the vehicles themselves or possibly even the loss of human lives due to such collisions.
The N17 is known to be prone to high car accidents (high accident zone), and although none have been reported to be owl related, this is a possible cause for many accidents (pers. comm. 11Capt. E. Prinsloo, 2002).
Roads are indirectly responsible for the electrocution of millions of birds worldwide through their close relationship with utility rights-of-way. Power line corridors are usually left untouched by landowners and may allow owls to utilise such paths. All power lines need to cross over roads at some stage and where such a situation occurs it may create a deadly situation for birds of prey utilizing these paths, leading them to cross roads. Power lines have also been known to have direct impacts on birds of prey (Postelli, 1927) however, it does not seem to have a major impact on the smaller owls such as those in this study. Owls have that been known to die from such a cause include the Barn
Owl, Tawny Owl, and the Little Owl (Bevanger, 1994).
Nocturnal species and birds active during twilight periods such as Grass,
Marsh and Barn Owls are also expected to be vulnerable to crashing into
11 Capt. Elise Prinsloo, Devon Police Station.
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CHAPTER 3.3: RESULTS AND DISCUSSION
artificial obstacles e.g. where 9% of Barn Owls collided with all sorts of obstacles such as walls, windows, electric power cables and barbed wire (De
Bruijn, 1994). No owls in our study period were known to have collided with other obstacles besides vehicles, and with the exception for a few Marsh Owls found caught in barbed wire fences along roadsides. The fences attract these birds by providing hunting perches, in grassland areas where natural perches such as trees are rare. Finally, attention should be drawn to the indirect mortality of nestlings, which can result from the death of parent birds, when they are killed on foraging expeditions. This may have tremendous effects if the owl populations are to sustain losses due to vehicle collision.
These factors all contribute to the high occurrence of mortalities found on roads, however, they do not make up all variables involved, and different areas may have different features involved depending on the species one is dealing with.
3.3.11. Other owl mortalities occurring within South Africa
Owl mortality counts are currently being conducted nationally by volunteers
(Table 3.4 and Figure 3.24 to 3.29). The Mpumalanga roads being monitored include R29-Secunda-Trichardt (23 owls in three months). In Gauteng,
Limpopo Province and North West: the N1-between Hammanskraal and Bela
Bela (ten owls in six months); R42/R50 Delmas (11 owls in eight months);
R510-Lephalale (Ellisras) (four owls in eight months); R505-Zeerust (four owls in six months); R28-Mogale City (Krugersdorp) to Randfontein (two owls in three years). In the Free State: R59-Sasolburg to Parys (40 owl mortalities in
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CHAPTER 3.3 RESULTS AND DISCUSSION
only two months); R716-Deneysville (eight owls in eight months); R701-
Bethulie (No owl mortalities in six months, only one injured owl). In the
Eastern Cape: N2-Grahamstown (No owls in six months). nI KwaZulu-Natal:
R33-Dundee to Vryheid (ten owls in seven months). Western Cape: N7-
Malmesbury (four owls in six months); R62-Ladismith (No owls in six months);
R326-Gansbaai (three owls in six months). In the Northern Cape: N14-
Upington, R359-Augrabies and Kakamas, two owl mortalities in six months occurred.
Other areas occasionally being monitored, but where volunteers are lacking include on the R562 - Midrand (only one Grass Owl collected so far); the N12-
Potchefstroom (one Barn Owl collected in September); R25-
Bapsfontein/Bronkhorstspruit (two owls so far, One Grass and one Spotted
Eagle Owl); N2- Mossel Bay to Vleesbaai and Gouritsmond (three Spotted
Eagle Owls); N2 from Port Elizabeth to Jeffrey’s Bay (one Barn Owl). Owl carcasses have also be collected from N14/R47-Ventersdorp (one Marsh Owl) and N14/R375-Biesiesvlei (one Barn and one Grass Owl).
Besides the current study roads (N17 and R550), another area of concern is indicated by the large counts of 23 owl mortalities occurring in the Secunda area in only three months as well as the Sasolburg to Parys road, on which 40 owls have died in two months. The fact that these areas also consisted of many Grass Owl mortalities was also a concern. These areas are fairly similar in structure, appearance and habitat to that of the study area. Most other areas have insignificant mortalities and thus of less concern to the conservation of owls.
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CHAPTER 3.3: RESULTS AND DISCUSSION
§
FIGURE 3.24: Map indicating locations of owl counts conducted nationally.
Western Cape Malmesbury4 owls in 6 months Ladismith 0 owls in 6 months Gansbaai 3 owls in 6 months
Malmesbury Ladismith
Gansbaai
FIGURE 3.25: Map indicating locations and numbers of owl carcasses collected in the Western Cape in 6 months.
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CHAPTER 3.3 RESULTS AND DISCUSSION
§
Gauteng, Limpopo Province, North West & Mpumalanga Bela bela 10 owls in 6 months Tzaneen 3 owls in 4 months Ellisras 4 owls in 8 months Zeerust 4 owls in 6 months Secunda 23 owls in 3 months Delmas 11 owls in 8 months
FIGURE 3.26: Map indicating locations and numbers of owl carcasses collected in the Gauteng, Limpopo Province, North West and Mpumalanga areas over a few months.
Sasolburg
Deneysville
Bethuli Free State & Eastern Cape e Sasolburg 40 owls in 2 months Denysville 8 owls in 8 months Bethulie 0 owls in 6 months Grahamstown 0 owls in 6 months
Grahamstown
FIGURE 3.27: Map indicating locations and numbers of owl carcasses collected in the Free State and Eastern Cape over a few months.
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CHAPTER 3.3: RESULTS AND DISCUSSION
Upington
Augrabies Kakamas
Northern Cape Upington Augrabies Kakamas 2 owls in 6 months
FIGURE 3.28: Map indicating locations and numbers of owl carcasses collected in the Northern Cape in 6 months.
Kwa-Zulu Natal Vryheid Dundee Vryheid 10 owls in 7 months Dundee
FIGURE 3.29: Map indicating locations and numbers of owl carcasses collected in Kwa-Zulu Natal in 7 months.
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CHAPTER 3.3 RESULTS AND DISCUSSION
TABLE 3.4: Owl mortalities occurring on other South Africa roads during the study period between October 2001 and September 2003. Area Road Date Species Area Road Date Species Midrand R562 2002/04/04 T. capensis Secunda R29 2002/08/26 A.capensis Denysville R716 2002/06/28 T.alba R29 2002/08/26 A.capensis Ventersdorp N14 2002/05/14 A.capensis R29 2002/08/26 T.capensis Bessiespruit R375/N14 2002/05/14 T. alba R29 2002/08/26 T. alba N14 2002/02/20 T. capensis R29 2002/08/26 T.alba Potchestroom R29 2002/09/03 T.alba R29 2002/08/26 T.alba R51 2002/07/24 A. capensis R29 2002/08/26 T.capensis R58 2002/03/26 B. africanus Sasol/Parys R59 2002/08/28 T.capensis Delmas R42 2002/08/27 T. capensis R59 2002/08/28 T.alba R50 2002/05/21 A.capensis R59 2002/08/28 T.alba R50 2002/05/24 T.alba R59 2002/08/28 T.alba R50 2002/05/24 T. capensis R59 2002/09/23 A.capensis R50 2002/07/03 A.capensis R59 2002/12/10 T.capensis R50 2002/07/14 T. capensis R59 2002/12/09 T.capensis R50 2002/07/14 A.capensis R59 2002/11/20 T.alba R50 2002/12/06 B. africanus R59 2002/11/15 T.capensis R50 2002/09/14 A.capensis R59 2002/11/13 A.capensis R50 2002/09/14 T. alba R59 2002/11/06 T.alba R50 2002/09/14 T.alba R59 T.capensis Brokhorstpruit R25 2002/06/01 T. capensis R59 2002/09/09 T.capensis R25 2003/02/07 B. africanus R59 2002/09/26 A.capensis Bela Bela N1 2002/07/01 T.alba R59 2002/09/25 T.alba N1 2002/09/04 T.alba R59 2002/09/26 T.capensis N1 2002/04/09 Scops R59 2002/09/25 T.capensis Randfontein Plot 2002/09/11 T.alba R59 2002/09/23 A.capensis R28 2002/11/26 A.capensis Ladismith R62 2003/02/09 B. africanus R28 B.africanus Gouritsmond N2 2002/01/18 B. africanus Mosselbay 2002/12/30 B. africanus 2002/01/18 B. africanus Jeffriesbay N2 2002/01/06 T. alba
Thus far records that have been submitted, indicate the there is no other area
in South Africa that compares to this area.
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CHAPTER 3.3: RESULTS AND DISCUSSION
3.4. CONCLUSION
The number of bird traffic mortalities worldwide must be vast considering how much land roads intercept. With the improvement of many busy highways facilitating faster driving, highway kills of wildlife can be expected to increase.
This may be counteracted somewhat by the greater width and lowered attractiveness of the roads to wild animals through the modification of road verges. Although where highways of any kind traverse suitable owl habitats, great losses due to collision with vehicles can be expected.
It has been established that owls of the four species (A. capensis, T. alba, T. capensis and B. africanus) are regularly killed along this section of road within the study area with the numbers of records in this study representing a considerable unknown proportion of all owl road kills nationally. This may be an important factor facilitating the decline, specifically of the Grass Owl, being relative scarce within South Africa. The impact could have a tremendous effect, especially at a local level. Therefore, the following factors all jointly need to be addressed: prey control; vegetation management alongside the road, management of fire regimes, traffic control and road design; and most importantly, monitoring the owl populations themselves, if we want to avoid wiping out the owl populations under threat. This study however, is only a starting point for future studies and much more is needed, especially time.
It is important to note that the number of mortalities of a given species is irrelevant and rather the numbers associated with the population in question may be more important. An example is the larger numbers of Marsh Owl,
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CHAPTER 3.4 CONCLUSION
which fell victim more readily than other species however, they also naturally occurring in higher numbers. It is interesting to note that Dickerson (1939), referring to the work of Linsdale (1939) in the United States, observed that the set conclusions which go along with roadsides are such that a larger bird population can live there, and although many birds are killed by traffic and by striking wires, these negative factors are not serious, and on the whole the beneficial influences of the roads more than offset the harmful influences. This last sentence is very important to take into account and thus it would be imperative to first weigh up the negative and positive factors involved. Future studies should take place to determine what effect these road mortalities have on the actual owl population.
It is thus important for one to know the population (agent) one is dealing with and know the risks involved for that individual species in question, for example what is the chance of that species making a single crossing of the road, being killed. This in turn may then be coupled to other factors such as traffic density and speed of traffic (hosts), as well as the environmental factors involving season, weather, character of road verges, roads and surrounding environments.
Barnes (1936) was clearly impressed by the effects of variables such as type of road and roadside verge, time of year and age and sex of species involved.
Dunthorn & Errington’s (1964) investigations in Wiltshire drew attention to
‘hotspots’ on the route and also paid attention to roadside and crops growing beyond these points. For South Africa, Siegfried (1965) appears to have written the most on bird mortalities in general and an examination of his work
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CHAPTER 3.3: RESULTS AND DISCUSSION
and others previously mentioned, clearly showed a similarity both of approach and technique to those used in the present study.
However, the one common finding in almost all owl mortality studies (Baudvin,
1997; Clancy, 2002-in press; Fajardo, 1998; Hernandez, 1988; Illner, 1992
Newton et al., 1997; Shawyer & Dixon, 1999; Uhlenhaut, 1976) was the noticeable seasonal pattern in recorded deaths, with peaks in autumn (mainly consisting of juvenile birds) and in late winter (due to both adults and juveniles). The seasonal patterns of mortality is expected due to the various seasonal changes in population such as breeding, which is related to the need of an increase in food supply. During peak mortalities, owl populations have most probably reached their peak in numbers for that season, as this is the time after breeding when most juveniles are becoming independent and dispersing into the nearest available habitat, with a good and readily available food source being mainly on or close to roads.
Mortalities can thus also be examined separately for different gender and age groups, but a thorough knowledge of the species under observation is needed first. This may be achieved with the aid of ringing individuals of known age and monitoring their life spans and following their course of movements. In no species was mortality examined separating the genders, but in most birds a difference would be expected from what is known of sex ratios in birds.
Newton (1979) found during his study of the Barn Owl, those sectors of a population that were most on the move, mostly consisted of first-year birds of both genders and second-year males. Adult birds tended to remain near their breeding areas, with the under-representation of second-year females being
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CHAPTER 3.4 CONCLUSION
because by that age more females than males had become resident within breeding areas. It is thus assumed that it would probably cause more damage to the population if more females than males had died.
The concern of this study along with many other studies was, whether a trend in low numbers of mortalities would be indicative of a decline in population numbers overall. Schorger (1954) stated that it couldn’t be said that the data on road kills are indices of population. Whether ongoing killing leads to long- term population decline depends on whether it replaces the natural mortality or adds to it. Newton (1979) pointed out that if mortalities occurring from vehicles collisions replace natural mortality, then the population will not decline. If however, mortalities from vehicle collisions as well as natural causes outweigh that which would otherwise occur from natural causes alone, then the population is more likely to decline. To a large extent this also depends on the period at which killing takes place. It is less likely to have a significant impact in months following the breeding season, for the population is at its seasonal peak, consisting largely of juveniles, most of which are known to die or disperse before they reach their first year of life (Newton,
1979). From the results given in this study, this seems to be the situation at hand, and it is therefore assumed to have little affect on the population in question. The effect of mortality is greatest if it occurs at the start of a breeding season instead, for the population is then at its seasonal low, after most natural mortality has occurred already (Newton, 1979). During this period mortalities will occur mostly among breeding adults, which as Newton
(1979) stated, “are the most valuable sector of the population”.
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CHAPTER 3.3: RESULTS AND DISCUSSION
It is important to know whether the population is numerically stable in the long- term or is altering, because mortalities due to unnatural causes such as vehicle collision may be greater than replacements with yearlings.
Determining the actual trends in wildlife populations from highway mortalities would necessitate further study, but there is no doubt in this study that the losses are proportional to the population densities and that owls are producing enough offspring as well those migrating from other areas, to stabilise future generations. The fact that they persisted each year implies that the mortality of the recovered birds was less than that of the whole population occurring in the area. Without intervention, a decline in mortality counts along with less individuals breeding in the area is likely to indicate a decline in the population in the area.
There are also ‘hotspots’ where large numbers of animals are killed on the road, either single species or multiple species. Where mortalities involve scarce species, such as the Grass Owl, careful attention should be focused on such areas. The impact of the unnatural mortality on the Grass Owl population as a whole could be significant as these species, as already mentioned have small populations left within southern Africa. Although vehicle collisions only seem to be a problem in this particular area where the population seems to be doing well in regards to breeding success, other areas where this species occurs with less success need to be managed and monitored closely. Illner (1992) considered that road deaths in Germany could have contributed to the observed long-term decline in Barn Owl and Little Owl
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CHAPTER 3.4 CONCLUSION
populations. Therefore it is desirable to reduce the incidence of road-killed owls, particularly at sites with multiple deaths.
By looking at the main species casualties in this study, it is clear that hunting habits of each individual is a reason for increasing species-specific hazards resulting in death and indicates that different types of road habitats that each bird utilises, affects the outcome of mortality. For the Grass Owl, it seems without a flight path, except roads flanked by heavy grass cover were the most dangerous for this species. This species hunts by quartering low over the ground in search for prey and certain gaps in the grassy cover may lead this bird onto the road and straight into an oncoming vehicle, or it may be caught in the thermal with no chance of escape. Naturally these are species which are unlikely to occur in urban situations, due to their necessity for vast open land to hunt, and therefore if killed will not be found on such roads except for the single unusual case found in this study still to be explained. It was found that the narrow cover flanked roads and not the larger, wider, busier open type road, which is most dangerous to these owl species. It thus seems as though the demand for busier, wider and straighter roads may in actual fact benefit owl populations and prevent owls wanting to sit on roads in future.
Other raptor species although abundant, by their feeding habits and general behaviour are much less frequently found as casualties than might be expected. Black-shouldered Kites, Falcons and Kestrels are diurnal, which indicates that there is a distinction between diurnal and nocturnal mortalities.
Possible reasons for this being, the high density and low speed of traffic at
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CHAPTER 3.3: RESULTS AND DISCUSSION
these times respectively, as well as some behavioural differences such as flight patterns and feeding grounds along with abundance of prey. The blinding lights emanating from nighttime traffic is also a factor distinguishing nocturnal mortalities from diurnal mortalities and should not be overlooked.
Certain circumstances influencing the extent of highway losses can be summarised as follows:
1. Highway mortality counts are species specific and season specific.
2. Highway mortalities are related to the specific habitats and habits of
each species involved. (Grass Owls for adjoining cover and Barn Owls
for perching sites).
3. Highway mortalities are directly proportional to the status of the road
and therefore directly proportional to the speed of traffic on that road.
4. Highway mortalities seem to be directly proportional to the
interspersion of rodents, as prey, occurring in the road verge habitat, as
indicated by trapping and observations of rodent burrows and thus are
an indication of abundant food supplies.
5. Highway mortalities are directly proportional to an abundance of sink
populations beyond roads.
6. Highway mortalities are inversely proportional to the experience of
wildlife with fast moving vehicles.
Owl mortalities due to vehicle collisions may not be that much of a concern from an agricultural point of view. But what is important to note is that a predominately agricultural area, such as the study area, can ill afford to lose its predatory birds such as these owls. Such predators can in a natural way
318
CHAPTER 3.4 CONCLUSION
control granivorous rodents, the majority of which are considered to be agricultural pests (Brooks, 1974).
Therefore, large numbers of owls may occasionally be killed by motor vehicles, but the impact on local populations would depend upon a number of factors such as the original population size, the age of the birds killed and whether a high percentage of mature breeding adults were involved. The highway losses did not seem to be very closely related with the visible population but definitely appeared to be more related to the feeding habits of the birds or to their age and experience.
While it is difficult to argue that avifauna road mortalities do not significantly impact species populations, it is equally difficult to determine whether roads are threatening species survival rates (Postelli, 1927). While road death would seem to present evidence of population disruption by its sheer numbers, a number of authors (Hodson, 1962; Postelli, 1927) pointed out that avian road casualty figures may be inflated due to the ease of finding such specimens as compared to owls who died in other ways. However, it is difficult to comprehend that the amount of owls found along the road, being a relatively small area, could be comparable to natural deaths of the owls occurring in a natural area of the same size.
Some bird species are apparently better adapted to the life on roads. Vestjens
(1973) argues that the killing of a number of animals by traffic on roads is unfortunate but unavoidable. Unlike the study of Vestjens (1973) the species involved in the study, namely the Grass Owl, is considered to be vulnerable
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CHAPTER 3.3: RESULTS AND DISCUSSION
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