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Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations
1990 Productivity and habitat preferences of loggerhead shrikes inhabiting roadsides in a midwestern agroenvironment David William DeGeus Iowa State University
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Recommended Citation DeGeus, David William, "Productivity and habitat preferences of loggerhead shrikes inhabiting roadsides in a midwestern agroenvironment" (1990). Retrospective Theses and Dissertations. 16818. https://lib.dr.iastate.edu/rtd/16818
This Thesis is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Productivity and habitat preferences of loggerhead shrikes inhabiting roadsides in a midwestern agroenvironment by David William DeGeus
A Thesis Submitted to the Graduate Faculty in Partial Fulfillment of the Requirements for the Degree of
MASTER OF SCIENCE
Signatures have been redacted for privacy
Iowa state University Ames, Iowa
1990 ii
TABLE OF CONTENTS
PAGE
GENERAL INTRODUCTION 1 Explanation of thesis format 2
ABSTRACT 3 INTRODUCTION 4 STUDY AREA 7 METHODS 9 Nest and breeding pair surveys 9 Habitat measurement 10 Data analysis 12 RESULTS AND DISCUSSION 14 Breeding pair abundance 14 Nests produced per pair 14 Nesting productivity 17 Nest-site and habitat selection 25 Consequences of nest-site and habitat choice 31 Suitability of roadsides as nesting habitat for loggerhead shrikes 38 LITERATURE CITED 41 SUMMARY 50 ACKNOWLEDGEMENTS 51 1
GENERAL INTRODUCTION
Continued declines of loggerhead shrikes in the northcentral region and across much of the continental u.s. have prompted the u.s. Fish and Wildlife Service to consider the shrike for threatened and endangered status (Hands et al. 1989). The most plausible reason for the decline has been the loss of grassland associated with shrubby vegetation, which is required for foraging and nesting (Graber et al. 1973, Kridelbaugh 1981, Luukkonen 1987). Changes in farm practices during the past three decades have left grassland birds like the shrike with very little nesting habitat (Mohlis 1974, Vance 1976). Roadsides now comprise major portions of the available grassland in many intensively farmed regions (Joselyn et al. 1968, Oetting and Cassel 1971, Leedy 1975); therefore, we chose to collect data on the abundance, productivity and habitat preferences of shrikes using these areas. We hoped to determine the extent of use and whether or not such areas were suitable for breeding shrikes. Ultimately, we wanted to collect information that could be used to manage roadsides in ways that would aid the recovery of the loggerhead shrike. We also hope that our results will encourage others to study this over-looked avian wildlife habitat. 2
Explanation of the thesis format
This thesis follows the alternate thesis format described in the 1987 edition of the ISU Graduate College Thesis Manual. Data collection and analysis and the writing and revision of the text were completed by the candidate; guidance and edltorial advice were supplied by Dr. Louis B. Best. The thesis reports on the productivity and habitat preferences of loggerhead shrikes using roadsides in an agricultural landscape. This paper was prepared for publication in the American Midland Naturalist and will be submitted as a co authored publication. 3
ABSTRACT
We measured the abundance, productivity, and habitat preference of loggerhead shrikes (Lanius ludovicianus) nesting in southwestern Iowa roadsides to determine the importance of such habitats to this declining grassland bird. An average of 0.13 pairs/km established territories along gravel roads. Nest success (35%) and productivity (2.2 young/pair) were low due to high rates of nest predation (86% of all losses). Shrikes preferred to nest in white mulberry (Horus alba) and near grassland, alfalfa (Hedicago sativa) or oat fields (Avena sativa). Nest placement within the nesting substrate significantly affected nesting outcome, whereas habitat characteristics of the nest site and the surrounding area had no effect on nest outcome or productivity. Roadsides appear to satisfy the habitat requirements of breeding shrikes, but production in such areas appears to be below levels needed to sustain populations. 4
INTRODUCTION
Continued declines of loggerhead shrikes in the northcentral region and across much of the continental U.s. have prompted the u.s. Fish and Wildlife Service to consider the shrike for threatened and endangered status (Hands et al. 1989). Although unanswered questions remain, there is general agreement that shrike declines have resulted from loss of grasslands associated with shrubby vegetation (Graber et al. 1973, Kridelbaugh 1981, Luukkonen 1987, Brooks and Temple 1990a). Apparantly such habitats are needed to provide the foraging and nesting habitat requirements of this species. Intensification of agricultural practices in the Midwest (e.g., increases in field size, use of herbicides, and confinement of livestock) has resulted in the conversion of pasture land to cultivated cropland and a loss of hedgerows and brushy fencerows (e.g., Mohlis 1974, Vance 1976, National Research Council 1989, Barrett et al. 1990). In large portions of the Midwest the land is privately owned and intensively farmed (Ryan 1986, U.s. Bureau of the Census 1989, Wooley et al. 1988), and roadsides provide much of the only available grassland and shrub cover in these agricultural environments (Joselyn et al. 1968, Oetting and Cassel 1971, Leedy 1975). Thus, roadsides may constitute a major portion of the remaining nesting habitat of loggerhead shrikes in many 5
regions. In addition, roadsides are some of the only areas
that can be managed by public agencies to provide wildlife
habitat.
The potential of roadsides as wildlife habitat has long been recognized (Leopold 1931, Egler 1953), and a few studies have documented use of roadsides by passerine species (see
Leedy 1975 for a review). Basore et ale (1986) and Bryan
(1990) noted that passerine nest densities in other types of
linear cover (i.e., fencerows, waterways, etc.) may be 10-30 times greater than those in adjacent fields. Many other studies also have confirmed the value of linear habitats for nesting birds in agricultural areas, but none have focused on passerine productivity in roadsides (see Wooley et ale 1984
for a review). Instead, evaluations of productivity in roadsides have been restricted to ground-nesting game birds
(Wolfe and Evans 1967, Joselyn et ale 1968, Oetting and Cassel
1971).
Previous studies have indicated that loggerhead shrikes
frequently forage and nest along roadsides (e.g., Miller 1931,
Graber et ale 1973, Craig 1978, Gawlik and Bildstein 1990).
In addition, several studies have documented shrike productivity and habitat use in the Midwest (e.g., Graber et ale 1973, Kridelbaugh 1983, Burton and Whitehead 1990, Brooks and Temple 1990a), but none have focused on the productivity of shrikes breeding in midwestern roadside habitats. 6
We initiated a study of loggerhead shrikes nesting along roadsides in an intensively farmed region of southwestern Iowa. Our objectives were 1) to determine the abundance and productivity of shrikes breeding in roadsides, 2) to characterize habitat preferences of nesting pairs, and 3) to document nest success and the factors influencing nesting outcome and productivity. Ultimately, we hoped to determine if roadsides are suitable breeding habitat for shrikes and to characterize preferred vegetation types so that roadside vegetation management plans can be designed to benefit shrikes and other passerines. 7
STUDY AREA
We studied the breeding ecology of loggerhead shrikes
nesting along roadsides in Adair County, Iowa during three
field seasons, 1987-1989. Surveyor's notes from the General
Land Office (1832-1859) indicate that the level to steeply
rolling topography of Adair County was formerly over 90% tall
grass prairie, but today 90% of the area is farmed. Forty
four percent of the land area in the county is in rowcrops
(corn and soybeans), 24% is pasture, 11% is idle cropland
planted to grass or other cover crops, 10% is planted to small
grains and forage crops (oats and hay), and 2% is woodland
(U.S. Bureau of the Census 1989). Roadsides account for about
1.5% of the non-agricultural land; roadways, residential areas, ponds, and gravel quarries compose the remainder of the area.
This region of the state was chosen because the Fish and
Wildlife Service Breeding Bird Survey (Robbins et al. 1986) and data collected for the Iowa Department of Natural
Resources breeding bird atlas project (D. Reeves, pers. commun., Iowa Department of Natural Resources) indicated that shrike densities were highest in this part of the state. Our preliminary surveys conducted in 1987 confirmed that Adair
County contained a large shrike population and a variety of land-use types in which to measure and compare shrike 8 abundance, productivity and habitat preference. Adair county also contains an abundance of roadsides because the landscape is divided into square 2.59 km 2 sections which are usually bordered on all four sides by a gravel road. Roadsides were about 6 m wide and consisted largely of stands of smooth brome (Bromus inermus) bordered on one side by a limestone gravel road and on the other by a fenceline. Other major herbaceous plant species present were: bluegrass (Poa spp.), prairie sunflower (Helianthus petiolaris), wild parsnJp (Pastinaca sativa), sweet clover (Melilotus spp.), goldenrod (Solidago spp.), common milkweed (Asclepias syriaca), and giant ragweed (Ambrosia trifda). Shrubs and small trees, in various densities, were present in the fencelines that separated the roadsides from adjacent agricultural fields and pastures. utility poles and overhead wires also were present along most of the roadsides. 9
METHODS
Nest and breeding pair surveys.-- Road surveys were repeated five times between late March and mid-May to locate nests placed in roadsides; shrikes arrive in Iowa from late March to early April (Dinsmore et al. 1984). Eleven 31-37 km transects, oriented north-south and located on gravel roads, were driven at 24-32 km/h between sunrise and sunset. The total distance surveyed in 1988 was 377 km. Road construction shortened transect length by 6.4 km in 1989. North-south oriented roads were selected to minimize interference to visibility from the rising and setting sun. Gravel roads were used because the velocity and volume of traffic on blacktop roads created unsafe conditions for survey work. Such roadways are also the predominant type of roadside in midwestern agricultural areas. Host nests were located during road surveys by directly sighting them in non-foliated trees in early spring. Other nests, particularly those found in evergreen trees, were located by searching trees in the vicinity of territorial shrikes. Additional nests were discovered while driving along non-survey roads and by searching nest sites of previous years. All nests located in 1987 were found during non-systematic searches of roadsides in the area. 10
Generally, nests were discovered during nest construction or egg laying and were monitored every 3-4 days. Clutch initiation dates for shrike nests were determined by backdating from known clutch completion, hatching, or fledging dates using an egg laying rate of one egg per day (Kridelbaugh 1983, Luukkonen 1987) and 16 and 17 days for the incubation and nestling periods, respectively (this study). Nest sites were marked by placing yellow flagging on the opposite side of the road, and the locations were plotted on a road map of the area. During each nest visit, the contents and condition of the nest were observed with a mirror mounted on a metal pole. If eggs or young disappeared from a nest between nest visits, the date of nest failure was recorded as the mid-point of the interval between visits. Additionally, if the nest was found empty near the expected date of fledging, the nearby area was searched for 15-20 min or until either a fledgling was sighted or an adult scolded, both indicating a successful nest. Any nest fledging at least one nestling was considered successful. The number of birds surviving to fledging was recorded as the number of live nestlings seen in the nest during the visit just prior to nest abandonment. Areas adjacent to both failed and successful nests were searched to locate subsequent nesting attempts. Habitat measurement.-- Once all nesting activity ended (mid-July) we measured habitat variables at nest sites thought 11
to influence reproductive success. Measured variables included the species, height and canopy diameter of the tree/shrub in which the nest was located; distance of the nest from the ground and the canopy perimeter (measured horizontally); ground cover within a 6 x 6 m area centered on the nest tree/shrub; distance to the nearest roadside tree; amount of woody cover in the roadside and crop type(s) adjacent to the nest on both sides of the road. Crop type categories included were rowcrops, [corn (Zea mays) and soybeans (Glycine max») and grassland [bluegrass pasture, idled cropland (smooth brome, orchard grass (Dactylus glomerata), etc.), alfalfa and oats). Alfalfa and oats were included in the grassland category because their structure is similar to that of grassland. Cultural practices of such areas also resembles that of grassland. Ground cover and the distance to the nearest roadside tree were not measured in
1987. Tree or shrub height was measured with a range finder and clinometer. The diameter of the nest tree, height of the nest, and distance of the nest from the perimeter were measured with a cloth meter tape. The percent coverage of bare ground, grasses, forbs, shrubs, and trees within the 6 x 6-m plots and the percentage of the roadside intercepted by woody cover within 50 m of the nest were visually estimated and assigned to classes: 0-5, 6-20, 21-40, 41-60, 61-80, 81- 12
95, and 96-100%. (Trees were defined as woody plants over 3 m
tall; woody plants under 3 m were classified as shrubs.) The
emphasis on roadside woody cover occurred because most
trees/shrubs were found in road ditches, especially in the
roadside fencerows.
Similar habitat measurements were taken at randomly
selected unused sites to assess habitat selection of shrikes
nesting in roadsides. One tree/shrub within 100 m of every
initial nesting site was randomly selected for comparison to
the initial nest site. Measurement of random variables was
not conducted in 1987.
Data analysis.-- Clutch size, number of nestlings, and
number of young fledging per successful nest were compared
among years and adjacent land-use categories by using the
Statistical Analysis System (SAS) General Linear Model (GLM)
procedures (SAS Institute Inc. 1985). The level of
significance for all tests was set at P i 0.05. T-tests were
used to evaluate differences between means of reproductive
characteristics (clutch size, number of eggs hatching, etc.)
of first and second nesting attempts, habitat characteristics
of successful and failed nests, and habitat parameters of nest and random sites. Preference for tree species used for nest
sites was tested by using chi-square values generated by a simultaneous confidence interval test (Marcum and Loftsgaarden
1980). 13
Nest success vas calculated by using MICROMORT (Heisey and Fuller 1985), a computer program vhich uses Mayfield's (1961a) method to calculate nest survival rates and corresponding sample variances. Comparisons vere made betveen/among daily survival rates because tests betveen/among interval rates are misleading vhen the lengths of intervals differ (Miller and Johnson 1978). Nest success betveen/among years, crop types, tree species, and phases of the nesting cycle vere compared by using chi-square values generated by likelihood ratio comparison tests (Heisey and Fuller 1985). z-tests vere used to compare nest success rates betveen first and subsequent nesting attempts. 14
RESULTS AND DISCUSSION
Breeding pair abundance.-- We located 41 shrike pairs
(0.11 pairs/km) in roadsides along survey routes in 1988 and
53 (0.14 pairs/km) in 1989. Anderson et ale (1985) determined that the probability of sighting a shrike directly adjacent to a roadside transect approaches 100%. The large, bulky shrike nests also were easily detected in the non-foliated trees present during early spring. Therefore, we believe that our surveys represented a near total count of shrikes nesting in roadways along transects.
No other surveys have counted shrike breeding pairs exclusively in roadsides, but, a few roadside surveys have reported the number of breeding pairs using roadsides and adjacent fields and pastures. Brooks and Temple (1990b) located 0.15 and 0.11 pairs/km in Minnesota during 1986 and
1987. Roadside surveys from southern states have reported higher densities. Kridelbaugh (1983) found 0.42 pairs/km along central Missouri roads and Siegel (1980) located 0.62 pairs/km along a road transect in Alabama.
Nests produced per pair.-- Nesting was initiated in Adair
County the first week of April, and the peak laying period occurred the second and third weeks of April. Most young had fledged by mid-June. In 1987, we found 24 breeding pairs which initiated 26 nests. In 1988, 74 nesting pairs produced 15
93 nests, and in 1989, 89 pairs built 119 nests (includes surveyed and unsurveyed roads). The number of nests produced per pair in this study was similar to that reported by others (Table 1). Twenty-two percent of the pairs renested at least once after initial failure, and 2% renested again after a second failure. Other researchers have reported rates of renesting after failure that ranged from 12-67% (Siegel 1980, Kridelbaugh 1982, Luukkonen 1987, Brooks and Temple 1990b, Gawlik and Bildstein 1990, Burton and Whitehead 1990). In our study, only 2% of all successful pairs initiated second broods after initial nest success, whereas rates in other studies ranged from 8 to 74% (Siegel 1980, Kridelbaugh 1982, Luukkonen 1987, Brooks and Temple 1990b, Gawlik and Bildstein 1990, Burton and Whitehead 1990). Double broodedness was highest in southern states where the breeding season is longer (Kridelbaugh 1983). In contrast, studies conducted in Illinois and Colorado (Graber et al. 1973, Porter et ale 1975) reported that nesting attempts following initial success were rare. Thus, latitude and the high incidence of nest failure in our study (Table 1) may account for the lower rates of renesting after initial nest success. Shrikes reoccupied 57 percent of all territories at some time during the study. They reused 38 percent (9/24) of the 1987 sites in 1988 and 58% (42/74) of the 1988 sites in 1989. Five 1987 Table 1.--Comparison of loggerhead shrike productivity in the United states· Ave. Nests Youngfledgea clutch per Nest per succ. per study area N~ Size pair success~nest pair Source Roadside nests cS Iowa 222(3) 5.6 1.3 35.3 4.7 2.2 This study Indiana 107(2) 5.7 1.4 31. 2 4.6 3.6 Burton and Whitehead(1990) Alabama 37(1) 5.0 1.2- 43.2t: 2.0 9 Siegel (1980) Interior 4.0 nests Minnesota 48(2) 5.7 1.3 62.0 3.0 3.7 Brooks and Temple (1990b) Missouri 55(2) 5.7 1.2- 69.1 4.4 h 3.7 9 Kridelbaugh (1982) 25(7) 5.6 71.0 4.6 3.4~Graber ale (1973) Illinois e et Colorado 77(4) 6.4 66.2 5.4 3.6~Porter et ale (1975) .... S. Carolina 50(1) 1.4- 68.0 4.5 4.3 Cely and Corontzes (1986) 0"1 S. Carolina 49(2) 5.3 1.3 65.3 e 4.7 Gawlik and Bildstein (1988) Virqinia 57(2) 5.1 1.6 62.1 4.0 4.0 Luukkone~J9B71 ·Studies with i 20 nests were excluded because sample sizes are too low to accurately estimate nest success (Hensler and Nichols 1981) bNumber of nests (number of years in parentheses) CMayfield (1961a) estimates except where noted cSIncludes studies where a majority of nests were located along roadsides; some of the other studies probably include some roadside nests -Based on reported renesting rates of successful and unsuccessful pairs eApparent nest success (number of successful nests/number nests monitored) 9Nests per pair x nest success x young fledged per successful nest hYoung fledged per nest/nest success ~Representsa minimum value because nests per pair could not be determined 17
sites were used again in 1989 but not in 1988, and six sites were used during all 3 years of the study. Re-occupancy has ranged from 47 to 70% in other studies (Kridelbaugh 1982, Brooks and Temple 1990b, Burton and Whitehead 1990). These rates are thought to provide an index of mortality rates, which average 40-60% in temperate-zone passerines (Ricklefs 1973; but see Haas and Sloane 1989). Return rates of banded male shrikes seem to verify this, as 43-55% have returned to nest sites used in previous years (Kridelbaugh 1982, Brooks
and Temple 1990b, Burton ~nd Whitehead 1990). Our observations of mortality and site re-occupancy of a roadside shrike population do not differ greatly from other habitats. Nesting productlvlty.-- The mean clutch size over our entire study was 5.6 (± 0.9 SO, N=189) (Table 2). Clutch size ranged from 3 to 7, with 6 being the most common size. No significant differences were detected between first and subsequent nesting attempts or among years (attempts: T=1.34, 188 df, P=0.18; years: F=2.18, 2 & 187 df, P=0.12). Only Luukkonen (1987) has reported significantly different clutch sizes between first and second nesting attempts in shrikes, and no study has documented significant yearly differences. Clutch sizes similar to ours have been reported for nearby Minnesota, Missouri, and Illinois (Table 1). Mean nest success was 35% (N=222) over all 3 years (Table 3). Data from 16 nests abandoned before or during laying were 18
Table 2.--Reproductive characteristics of loggerhead shrike nests in Adair County, lova- Second and All nests Variable First nest third nests combined and year N x+SD N x+SD N x+SD Clutch size 1987 - 18 6.0+0.7 1 6. O±.O. 0 9 6.0±.0.6 1988 - 62 5.6+0.8 14 5.4+0.8 76 5.5+0.8 1989 - 72 5. 6±.1. 0 22 5. 4±.0. 9 94 5.6+0.9 Years Combined - 152 5.6±.0.9 37 5.4±.0.8 189 5.6±.0.9 Eggs hatching per nest 1987 - 14 5. 3±.1. 0 1 5.0±.O.O 15 5. 3±.O. 9 1988 - 38 4.9+1.1 9 5.1±.1.0 47 5.0±.1.1 1989 - 45 4. 9±.1. 2 11 4.9±.0.9 56 4.9±.1.1 Years Combined - 97 5.0±.1.1 21 5.0±.0.9 118 5.0±.1.1 Young fledged per successful nest 1987 - 14 5.4+1.1 0 14 5.4±.1.1 1988 - 26 4.5+1.2 6 4.5±.0.5 32 4.5±.1.1 1989 - 42 4. 5±.1. 4 11 4.9±.1.0 53 4.6±.1.4 Years Combined - 82 4.7+1.4 17 4.8±.O.8 99 4.7±.1.3
-Numbers presented represent averages and standard deviations 19 excluded because the date and cause of desertion could not be accurately established. Estimates of nest success did not differ between attempts (X z =2.00, 1 df, P=0.18), among years (X z =1.00, 2 df, P=0.62), or among phases of the nesting cycle (X z =3.00, 2 df, P=0.16). Porter et ale (1975) and Kridelbaugh (1983) reported high year-to-year variability in nesting success, whereas others have reported less yearly variation (Luukkonen 1987, Gawlik and Bildstein 1990, Brooks and Temple 1990b). The variability among yearly nest success was often due to severe weather, i.e., rain, high winds and hail. Predators destroyed eighty-six percent of failed shrike nests in Adair County roadsides. Such losses to predation are typical for most passerines (Ricklefs 1969, Clark and Wilson 1981). Nest losses due to predation may range from 64 to 97\ in open-nesting passerines (e.g., Nolan 1963, Best 1978, Best and Stauffer 1980, Chasko and Gates 1982, Hartin and Roper 1988) and are highest during the beginning of the nesting season. In contrast, predators caused 25-50\ of all nest failures in most other shrike studies (Porter et ale 1975, Kridelbaugh 1983, Luukkonen 1987). These are relatively low predation rates for a passerine and may be attributed to the shrike's aggressive nature (Graber et al. 1973, Goransson et al. 1975). The highest rates of predation in shrike nests seem to be found in roadside habitats. A similar study with substantial 20
Table 3.--Nest success" of loggerhead shrikes in Adair County, Iowa
Days Daily Nest of survival success Year Interval! Failures exposure rate ( %)
1987 Laying 0 Incubation 2 235.0 0.9915 87 Nestling 7 228.5 0.9694 58 Combined 9 463.5 0.9763 40
1988 Laying 6 193.0 0.9689 88 Incubation 31 948.0 0.9673 58 Nestling 19 702.0 0.9729 62 Combined 56 1843.0 0.9696 32
1989 Laying 10 244.0 0.9590 85 Incubation 35 1179.0 0.9703 62 Nestling 15 834.0 0.9820 73 Combined 60 2257.0 0.9734 37
"Calculated using the Mayfield method (1961a) of nest survival estimation (N=222) blnterval length based on averages from our data: Laying = 4 days, Incubation = 16 days, Nestling = 17 days and Combined = 37 days 21 numbers of roadside nests also reported high mortality from predators (71.4%) (Siegel 1960). In addition, Burton and Whitehead (1990) reported high nest mortality in a study where 67 percent of shrike nests were located in roadsides; they also reported high nest mortality due to predation, but did not quantify the causes of failure. Suspected nest predators included blue jays (Cyanocitta cristata), American crows (Corvus brachyrhynchos), common grackles (Quiscalus guiscalus), European starlings (Sturnus vulgaris), raccoons (Procyon lotor), oppossums (Didelphis virginiana), feral cats (Felis domesticus), fox squirrels (Sciurus niger), cricetid mice (Peromyscus spp.) and fox snakes (Elaphe vulpina). All species were abundant in the area and were seen regularly along roadsides. Based on the absence of observable disturbance near most failed nests, avian/snake predation appeared to be more frequent than mammalian predation in our study (Robertson 1972, Thompson and Nolan 1973, Beaver 1975, Best 1978). No nest predation was witnessed, but shrikes were seen chasing a blue jay on one occasion, and mammal tracks and other sign (i.e., fur and scats) were found at the site of several nest failures. Abandonment was another source of nest failure (10%). Suspected causes of abandonment were weather, cowbirds (Mayfield 1961b, Rothstien 1976), and death of adults. The only observed adult mortality resulted from collision with a 22 vehicle. This did not result in nest abandonment because the death took place before nest construction and the dead mate was quickly replaced. Some of the abandonment occurring during laying may have actually been caused by predation because it was often difficult to determine the actual cause of failure during this stage. The remaining nest failures occurred when nests were blown from trees during rain storms (3%), and such losses were often the result of inadequate nest support. Weather related failures were less frequent in our study than in other recent studies because of the drought conditions which existed in 1988 and the concomitant lack of severe storms. Weather has not been reported as a major negative influence on shrike nesting success in most studies, although 67 and 73% of nest mortalities in certain years were attributed to harsh weather in Colorado and Missouri, respectively (Porter et al. 1975, Kride1baugh 1983). Shrike nest success reported for other regions and habitats has typically been much higher than in our study, averaging 60% or more (Table 1). Lower nest success appears to be associated with roadside habitats, as other roadside studies also have reported low nest survival rates. The low nesting success in roadsides may be due to features of roadside habitat that make nests in these areas more vulnerable to predation. Predation rates were much higher in 23 the Iowa, Indiana (Burton and Whitehead 1990) and Alabama studies (Siegel 1980) than in similar nesting studies that focused on areas away from roadsides. Nests found in or near linear habitats and habitat discontinuities such as fencerows and forest edges are often more susceptible to predation (e.g., Gates and Gysel 1978, Rodenhouse and Best 1983, Warner et al. 1987). Predators may be attracted to linear habitats for several reasons. Prey and food items in such habitats are probably located with greater ease and efficiency (Milonski 1958, Jessen et al. 1964, O'Connor and Shrubb 1986) and the density of potential food and prey items (e.g., nests and fruit) is greater in linear habitats than in adjacent crop fields
(Basore et al. 1986i Bryan 1990i D. DeGeus, pers. obs., the author). Linear habitats such as fencerows also serve as travel corridors for major predators, e.g., blue jays and raccoons, in agricultural areas (Johnson and Adkisson 1985, Glueck et ala 1988). An average of 5.0 (±1.1) eggs hatched per nest (Table 2). The number of eggs hatching and young fledging per nest did not differ between attempts or among years (Hatching- attempts: T=0.25, 117 df, P=0.80i years: F=0.56, 2 & 116 df, P=0. 57 i Fledging--attempts: T=O.36, 96 df, P=O.72i years: F=0.92, 2 & 95 df, P=0.40). No other studies have noted differences between nesting attempts, although many have 24 reported yearly differences in hatching and fledging success (Porter et ale 1975, Kridelbaugh 1983, Brooks and Temple 1990b, Gawlik and Bildstein 1990), which are often related to storms and severe weather. Overall hatching success in our study (88\) was similar to an average of 91\ reported for 155 species of birds (Koenig 1982) and to values of 85-93\ documented in previous shrike studies (Porter et ale 1975, Siegel 1980, Luukkonen 1987). The number of young fledging per successful nest also differed little from results of previous studies (Table 1) where most nests were not located in roadsides. We conclude that pesticides or detrimental factors unique to roadsides are not influencing the hatching and fledging success of shrikes. Overall productivity per pair in our study was much lower than that of studies conducted away from roadsides, even though the number of young produced per successful nest did not differ (Table 1). Much of the difference in productivity can be attributed to higher predator pressure in roadsides. The productivity estimates reported in our study and others must be viewed with caution, because true estimates of productivity are difficult to obtain. Haas and Sloane (1989) determined that female shrikes sometimes abandon territories and mates after an initial nest failure and we suspect that renestlng attempts of unseparated pairs are sometimes missed. No one has attempted to monitor nest success of pairs using 25 telemetry, therefore, we assume that productivity estimates from our study and others represent minimum values. Brooks and Temple (1990b) estimated that 5.5 young must be produced per shrike pair to offset losses to mortality. Apparently no populations are maintaining productivity that equals or exceeds replacement levels, and shrikes using roadsides are failing to maintain even half of what is required. Recent breeding population surveys indicate that shrikes are declining at a rate of 5% in the northcentral states and 8% in Iowa (Hands et al. 1989). Brooks and Temple (1990a) felt that breeding habitat was not full and speculated that declines were due to high over-winter mortality resulting from habitat loss on wintering grounds. We suggest that habitat alterations on the breeding grounds and concomitant shifts to marginal habitats, such as roadsides, also are contributing to declines.
Nest-site and babitat selection.-- Shrikes used 10 species of trees for nest sites (Table 4), however, 93% of all nests were found in white mulberry, American plum and eastern red cedar. White mulberry was the preferred and most frequently used nest substrate. American plum, eastern red cedar, and low shrubs were used in proportion to their abundances and followed mulberry in frequency of use. Large, diffusely branched trees (e.g., silver maple, black cherry, elm) were avoided. 26
In general, shrikes use a variety of low shrubby plants, often nesting in red cedar where it is present (Gawlik and Bildstein 1990). Forty-four to 63% of nests found in other studies were located in red cedar (Siegel 1980, Kridelbaugh 1983, Luukkonen 1987, Gawlik and Bildstein 1990, Brooks and Temple 1990a, Burton and Whitehead 1990). Luukkonen (1987), the only previous researcher to measure nest-site preference, found that shrikes preferred red cedar. Only 17% of all nests were located in red cedar in our study (Table 4). Unlike red cedar and plum, extensive use of mulberries as nesting sites by shrikes has not been widely documented. Burton and Whitehead (1990) recorded limited use of mulberries as nest sites in Indiana (3.5%), and Graber et al. (1973) was the only other study to find that deciduous trees were the predominant nesting substrate. Mulberries evidently are not abundant along roadsides in southern and western portions of the shrikes range (Porter et al. 1975, Siegel 1980, Kridelbaugh 1983, Luukkonen 1987, Gawlik and Bildstein 1990), although this introduced species is widely distributed (Fernald 1950). Others have reported that many nests are built in hawthorn (Crataegus spp.) and osage orange (Graber et al. 1973, Kridelbaugh 1983, Luukkonen 1987, Brooks and Temple 1990a), two species whose structure resembles mulberry but which were rarely found in Adair County roadsides. 27
Table 4.--Plant species used as sites of initial nesting attempts· by loggerhead shrikes compared to randomly selected plants within 100 m of nest sites
% Use % Available Species N=159 N=159 Commonly Used White mulberry (Horus alba) 57.9 b (58)C 30.8 American plum (Prunus americana) 19.5(18) 18.2 Eastern red cedar (Juniperus virginiana) 15.7(17) 15.7
Infrequently Usedd Low Shrubs Multiflora rose (Rosa multiflora) 1.3«1) 0.0 Common elderberry (Sambucus canadensis) 0.6«1) 2.5 Dogwood (Cornus spp.) 0.0 ( 0) 2.5 1. 3 b 5.0 Medium Trees Osage orange (Maclura pomerifera) 3.1(2) 0.6 Apple (pyrus malus) 0.6 «1) 0.0 3.7b 0.6 Large Trees Elm (Ulmus spp.) 1.3(2) 11.9 Black cherry (Prunus serotina) 0.0(0) 8.2 Boxelder (Acer negundo) 0.0«1) 2.5 Sandbar willow (Salix interior) 0.0(0) 1.9 Black walnut (Juglans nigra) 0.0(0) 1.3 Silver maple (Acer saccharinum) 0.0(0) 1.3 Green ash (Fraxinus pennsylvanica) 0.0(0) 1.3 Hackberry (Celtis occidentalis) 0.0(0) 0.6 European larch (Larix decidua) 0.0«1) 0.0 Bur oak (Quercus macrocarpa) 0.0 (0 ) 0.6 1.3b 29.6
·Species use figures do not include second and third attempts of 1987 nests bSignificant difference between use and availability based on the simultaneous confidence interval approach, with overall P=O.05 (Harcum and Loftsgaarden 1980) cNumbers in parentheses are total percent used in all attempts dSeveral species were lumped into size categories because of low sample sizes 28
The height ana canopy diameter of nest trees were significantly greater than the same measurements of randomly selected trees (Table 5). Nest trees were almost perfectly circular in profile, whereas the height of random trees was more likely to be greater than the canopy diameter. Only one other study reported the dimensions of shrike nest trees, but it failed to measure the dimensions of a random sample of trees (Gawlik and Bildstein 1990). Shrikes in South Carolina used oval trees whose dimensions averaged 8 m tall and 4 m wide. Selection of larger, circular trees and shrubs may reflect the nest placement preferences of shrikes. Mean nest height in our study was 2.4 m (± 0.8 SD, range=1.0-5.5), and nests were placed an average of 2.6 m (± 1.6 SD, range=1.0- 5.5) from the nest substrate perimeter. This placed nests just below the upper half of the tree (40% of the distance from the ground to the top of the tree) and near the trunk of the tree (15% of the distance from the trunk to the periphery). Other researchers have documented similar trends in the shrike's relatively specific placement of nests within trees. In all studies, nests have been located 1-3 m from the ground and 1-2 m from the canopy perimeter (Porter et ale 1975, Siegel 1980, Kridelbaugh 1983, Gawlik and Bildstein 1990, Brooks and Temple 1990a). 29
Table 5.--comparisons (x~SD) between selected habitat parameters of nest and random sites
Nest Random sites sites Variable (N=159) (N=159) S igni f icance:"
Nest Substrate (m)
canopy diameter 5. 9~1. 8 4. 4~2. 4 T=6.18, P<0.01
Tree height 5.8+1.9 4.8~2.7 T=4.20, P<0.01
Woody cover (m)
Total linear cover 31.8~24.7 29.9+24.8 T=0.69, P=0.49
Nearest tree or shrub 11.6~l9.8 13.2+21.1 T=0.70, P=0.48
Ground cover (%)
Bare 17.0~18.4 10.4~13.6 T=3.67, P Grass 63.1~23.7 65.7±.24.7 T=0.94, P=0.35 Forb 17.6~18.8 20.8±.21.6 T=1.45, P=O.15 Shrub 12.5~21.0 17.9±.20.6 T=2.31, P=0.02 Tree 50.6+33.6 27.6±.32.5 T=6.21, P<0.01 ·student's T-Test 30 Shrub coverage was greater at randomly selected sites, whereas the percentage of bare ground and tree coverage were highest at nest sites (Table 5). Coverage of grass and forbs, amount of woody cover along fencerows, and distance to the nearest tree did not differ between nest and random sites. Luukkonen (1987) also noted no difference in woody plant density between nest and vacant sites, but did note that herbaceous vegetation was higher and more variable at vacant sites. Differences in the amount of shrub, tree and bare ground cover between nest and random sites are all likely related to the shrike's preference for larger trees. Because shrikes chose larger trees, the amount of tree cover also would be greater at nest sites, as would the amount of bare ground, because larger trees shaded greater portions of the ground below. Proportions of grass and forbs were fairly uniform throughout most roadsides, and thus no differences between nest and random sites were detected. Nest patch preference has been noted in other passerines (e.g., Peterson and Best 1985, Martin and Roper 1988) and more pronounced differences may have been noted if we had sampled a more heterogeneous habitat. Grassland preference was evident in shrike territory selection. Seventy-eight percent of all nests were bordered on at least one side by grassland, forage crops or small 31 grains; the remainder were surrounded by row crops. Because grassland and crops with grassland-like structure comprised 55\ of the county's land area (U.S. Bureau of the Census 1989), use of sites adjacent to such areas exceeded availability (X 2 =9.9, 1 df, P<0.05). Several other studies also have documented strong preference for grassland, or territories with larger proportions of grassland, and avoidance of row crops (e.g., Kridelbaugh 1982, Luukkonen 1987, Brooks and Temple 1990a, Gawlik and Bildstein 1990). Shrike abundances also are correlated with regional availability of grassland (Kridelbaugh 1981), and similar trends occur in Iowa. Larger shrike populations are found in the southern third of the state (Dinsmore et al. 1984; S. Droege, pers. commun., U.S. Fish and Wildlife Service), where the greatest amount of pastureland is found (U.S. Bureau of the Census 1989). Consequences of nest site and habitat choice.-- Nesting success dId not differ among nests placed in various tree species (X 2 =7.81, 3df, P=0.41), and, therefore, no reason for selection was evident. Mulberries may have been chosen simply because of their disproportionate availability; a trend noted in other passerines (Reese and Kadlec 1985). However, this species, especially shrubby stages, is densely branched, providing better support for the shrike's loose, bulky nest and more cover than most other woody plant species. Cover 32 over nests and number of supporting branches is often an important factor in nesting outcome (e.g., Nolan 1963, Murphy 1983) and would likely be a major influence in nest-site selection by shrikes. If cover were the major factor controlling nest-site selection, shrikes would be expected to select cedars because they provide cover during the entire nesting season. Indeed, shrikes in other regions used cedars most frequently, although selection was measured only by Luukkonen (1987). However, in shrike nesting studies, as in other similar passerine studies, the effects of cover are unclear (see Best and stauffer 1980). Kridelbaugh (1983) found that cedars provided better nest cover, but nests in deciduous trees were more successful. In contrast, Gawlik and Bildstein (1990) reported higher nesting success in red cedars than deciduous trees. Other factors may be influencing nest site choice. Mulberries may have been chosen because they offered more support branches than red cedars or because their branches were more rigid and move less during the 40-60 km/h winds common in early spring. Thus, their choice may represent a compromise between balancing losses to predators and weather (Murphy 1983). Shrikes also aggressively defend their conspicuous nests and may chose nest sites that facilitate detection of potential predators (Ricklefs 1977, Murphy 1983, Bekoff et al. 1987). Both mulberries and osage orange (a species commonly used in other areas) are among the 33 Table 6.--Comparisons between nest-site characteristics of successful and unsuccessful nests Successful Unsuccessful nests nests ~N=94~ ~N=142} Var iable x+SD x+SD Signi ficance:' Nest Substrate (11 ) Canopy diameter 6.3+2.0 6.0±.1.8 T=0.96, P=0.34 Tree height 6.1+2.0 5. 9±.1. 8 T=0.77, P=0.44 Nest height 2.6+0.9 2. 3±.0. 8 T=2.12, P=0.04 Distance of nest from perimeter 2.8+1.8 2.5±.1.4 T=1. 26, P=0.21 Woody cover (m) Total linear cover 34.0+25.6 29.8±.24.6 T=1. 25, P=0.22 Nearest tree/ shrub 13.5+21.2 12. 2±.20. 2 T=0.45, P=0.65 Ground cover (% ) Bare 19.3+17.9 14.8±.17.7 T=1. 80, P=0.07 Grass 62.9+24.3 63.2±.23.4 T=0.10, P=0.92 Forb 15.8+18.8 18. 9±.19. 3 T=1.13, P=0.26 Shrub 11.0±.21.9 12. 2±.19. 5 T=0.42, P=0.67 Tree 54.8±.32.6 52.7±.34.7 T=0.43, P=0.67 ·Student's T-Test 34 last deciduous trees to foliate (D. DeGeus, pers. obs., Iowa state University). Tree size and the percent coverage of the five ground cover classes also had no effect on nesting outcome (Table 6). Nest placement appeared to be a larger factor in nesting outcome as successful nests were higher than unsuccessful ones. Such placement probably lessened the degree of mammalian predation (Best and Stauffer 1980), although most nests were placed in the lower half of the tree, suggesting that avian predation and weather may have been competing influences on nest success (Murphy 1983). Consequently, circular trees or shrubs may be selected over oval trees because they allow shrikes to place nests at preferred sites that maximize nest protection. Distance to the nearest fencerow tree did not differ between the successful and failed groups of nests (Table 6). In addition, success of nests placed in shrubs that commonly form thickets (i.e., plums) were no more successful than nests in others tree/shrub species (X 2 =7.81, 3 df, P=O.41). Kridelbaugh (1983) and Gawlik and Bildstein (1990) also found that isolated trees did not have significantly higher nest success than nests located in hedges. Nevertheless, numerous studies have demonstrated that the density and structural complexity of cover and the number of possible nest sites surrounding nests is inversely related to degree of predation 35 (Bowman and Harris 1980, Hill and Gates 1988, Martin and Roper 1988, Crabtree et ale 1989). Apparently, differences in cover density and heterogeneity (i.e., presence of greater proportions of forbs, shrub and tree cover) do little to decrease the vulnerability of nests to predators in linear habitat such as roadways. Adjacent crop type had no significant effect on any of the reproductive parameters measured (Table 7). Because nest success did not vary among various crop types this suggests that predator pressure was similar in all areas. Also, no food-induced shortages existed in any of the nests bordered by various land uses. In other insectivorous passerines, smaller clutch sizes, brood reductIon through starvation, and longer nestling periods may result when food shortages exist (Bryant 1975, Drent and Daan 1980, O'Connor 1980, QuInney 1983). Shrikes require adequate supplies of terrestrial insects (Scarabaeidae, Carabidae, Orthoptera, ArachnIda, and Lepidoptera larvae), small rodents, and reptiles (Bent 1950, Krldelbaugh 1982, Burton and Whitehead 1990) and low ground cover that allows detection from perches. Rowcrop habitats provide low vegetation during most of the nesting season (D. DeGeus, pers. obs., the author), however, lower insect abundance and dIversity occur In such habitats because of frequent cultivation, insecticide use and lack of vegetative cover (e.g., Esau and Peters 1975, Harston et ale 1979, 36 Blumberg and Crossley 1983, Blenden et ale 1986, Basore et al. 1987, Lesiewicz 1981). Short-grass habitats provide more persistent and abundant supplies of these insects (e.g. Van Emden 1965, Hill 1976, Esau and Peters 1975, Koskela and Hanski 1977) and low vegetation that allows detection and capture (Mills 1979, Carlson 1985, Janes 1985). Because no differences in clutch size, brood size, length of the nestling period or number of young produced per nest occurred among sites bordered by rowcrops and grassland, we suggest that roadsides provide enough grassland cover to compensate for lower prey abundances found in rowcrop habitats. Nevertheless, strong preference exhibited for grassland in this and other studies indicates that grassland is a necessary component of shrike habitat. Other studies also have demonstrated that shrike productivity, net energy gains, and ability to detect prey are often greater in areas associated with short grass (Mills 1979, Luukkonen 1987, Gawlik and Bildstein 1990, Brooks and Temple 1990a). Miller (1931) and Kridelbaugh (1982) also reported that territory size was inversely proportional to the area of grassland in the territory. Perch availability might be expected to influence habitat selection by shrikes because numerous perches of varying height are critical habitat components of perch-foraging predators (Mills 1979, Janes 1985, Carlson 1985). This factor Table 7.--Mean reproductive parameters compared among three land-use types adjacent to nest sites (Sample sizes are given in parentheses) Reproductive Significance of parameters Grassland Grassland/Rowcrops Rowcrops difference- Clutch size 5.6 5.6 5.5 F=0.27;2,187 d£; (73) (75) (43) P=0.76 Hatch success (\) 85.8 89.6 87.9 F=0.68;2,112 d£; ( 44) ( 44) (27) P=O.51 Number of nestlings 4.9 5.1 4.7 F=0.88;2,116 d£; (47) (45 ) (27) P=0.42 Length of nestling period w (days) 17.3 17.2 17.9 F=O.74;2,81 df; ..,J (32) (31) (21) P=0.48 Number of fledglings produced 4.9 4.6 4.6 F=0.72;2,95 df; (37) (34) (27) P=0.49 Nest success b (\) 33.3 34.3 36.7 X 2 =0.00i2 dfi (86) (84) (49) P=1.00 Young fledged per pair 2.2 2.1 2.6 (73) (70) (41) ·x2 test used to test nest success and an F-test was used to test all other parameters bHayfield (1961a) estimates of nest success 38 probably did not effect selection among various roadside habitats, because nearly all roadsides contained fences and power lines. Nevertheless, the presence of perches may have caused shrikes to select roadsides over other habitats where natural perches were often not available. Bohall-Wood (1987) noted that shrikes used these man-made roadside perches more often when natural perches were absent in the surrounding landscape. Suitability of roadsides as nesting habitat for loggerhead shrikes.-- Roadsides provide major habitat components for shrikes that are often not found in crop fields and open pastures away from roads (i.e., nest, perch and foraging sites) and our surveys indicated that breeding pairs frequently nest in roadsides. Furthermore, Luukkonen (1987) and Burton and Whitehead (1990) both found that shrikes nested closer to roadsides than expected based on the distribution of possible nest sites. Other birds in agricultural areas show similar preferences for fencerows and field edges, often because such areas contain more abundant supplies of insect prey, perches and preferred vegetation (e.g., Rodenhouse and Best 1983, Green 1974, Best et ale 1990). Our results also indicate that successful nests in roadsides were as productive as those in other areas (Table 1). Unfortunately, the productivity per pair was half that of shrikes in other habitats because of high predation rates. Such productivity 39 is far below the estimated 5.5 young per pair needed to offset losses to mortality (Brooks and Temple 1990b). The potential of managing vegetation in road right-of ways for grassland birds has gained considerable attention as grassland habitat in the midwest has disappeared (Warner et ale 1987). Over 180,000 km of roadways are found in Iowa and the acreage in roadsides exceeds 242,000 ha, an area equal to all the land owned by governmental park agencies in the state (Alternative Roadside Vegetation Steering Committee 1989). Comparable amounts of roadside habitat can be found in other states in the region (see Leedy 1975 for a review), therefore, such areas may comprise large and important breeding areas for passerine birds. Results of previous nesting studies on upland game birds are often used to support the contention that roadsides are productive wildlife habitat (e.g., Wolfe and Evans 1967, Joselyn et ale 1968, Oetting and Cassel 1971, Warner et al. 1987). Others have noted that many song birds nest in such areas and have proposed roadside vegetation management plans that would encourage birds to nest in road right-of-ways (Joselyn et al. 1968, Leedy 1975, Alternative Roadside vegetation Steering Committee 1989). Some researchers have suggested that the only disadvantages of roadside use by birds are the threat of collision with vehicles, mowing, or pesticide use (see Banks 40 1979 and Leedy 1975 for review). Unfortunately, our study also demonstrates that roadsides may act as "ecological traps" in a manner similar to that described by Gates and Gysel (1978), Best (1986), and Bollinger (1988). By providing suitable nesting and foraging sites not available in the surrounding agricultural matrix or habitat that is more attractive than other alternatives, roadsides may be drawing birds to areas where heavy predation limits production to levels less than what is need for replacement. In fact, use of these habitats may be contributing to the decline of shrikes in the midwest. Our study clearly demonstrates that further evaluation of bird use of roadside habitat is needed before roadside vegetation management plans are implemented to benefit passerine birds. 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Nest placement significantly affected nesting outcome, whereas habitat characteristics of the nest site and surrounding habitat had no effect on nest outcome or productivity. Roadsides appear to satisfy the habitat requirements of breeding shrikes, however, such areas seem to be "ecological traps" because shrikes were attracted to areas where production was below replacement levels due to heavy predation. We recommend that future studies directly compare the productivity of roadsides to that of other habitats. Studies should also explore ways to lessen the effects predators on roadside nests or to discourage shrikes and similar passerine birds from using such areas. 51 ACKNOWLEDGEMENTS I would like to thank L. Best and the other members of my committee, E. Klaas and D. Glenn-Lewin, for providing advice and guidance on this project. K. Reistroffer and G. Loughran provided assistance with data collection and entry and B. Ballard, B. Frawley, and T. Rosburg offered much appreciated help while out in the field. I am also grateful to the staffs of the Iowa Ecological Services Bureau, Adair County Conservation Board, Adair County Ases office and the office of the Bird Banding Laboratory for providing helpful information and advice. Financial support was provided by the U. S. Fish and Wildlife Service, Iowa Department of Natural Resources, Max McGraw Wildlife Foundation, the Roger Tory Peterson Institute of Natural History and Sigma Xi.