15
NOCTURNAL FEEDING IN MOULTING GREYLAG GEESE ANSER ANSER - AN ANTI-PREDATOR RESPONSE?
JOHNNY KAHLERT, ANTHONY D. FOX & HENNING ETTRUP
Kahlert J., A.D. Fox & H. Ettrup 1996. Nocturnal feeding in moulting Greylag Geese Anser anser - an anti-predator response? Ardea 84: 15-22.
The diurnal activity patterns of Greylag Geese were studied at a Danish moulting site. Before moulting, Greylag Geese foraged throughout the 24 hour period, but changed to nocturnal feeding behaviour exclusively and roosted on offshore islets during the day, as they became flightless. As the moult progressed, geese preferred to feed in coastal habitats and responded more strongly to predator-like stimuli, such as overflying herons and heli copters, even though real predators were absent. This increased suscept ability to commonplace events may be a response to their restricted mobil ity and ability to escape potential predators during moult. Since the fre quency of the predator-like stimuli was lowest at night, we suggest that moulting Greylag Geese compensate for their lowered tolerance threshold to predator-like stimuli by feeding at night when the probability of being disturbed is less. Alternative nutrition-related hypotheses of nocturnal feed ing are discussed, however these could not explain the observed changes in feeding behaviour, when Greylag Geese became flightless. Key words: Anser anser - activity patterns - moult - disturbance - predation - nocturnal feeding Department of Coastal Zone Ecology, National Environmental Research Institute, Kalp, DK-84l0 Rpnde, Denmark, e-mail [email protected].
INTRODUCTION treme wariness to environmental stimuli and the preference for coastal habitats, where rapid es Optimal foraging theory predicts that animals for cape to the water is possible (Madsen & Morten age in such a way as to maximise net energy in sen 1987). Moreover, renewal of flight feathers take per unit time (Krebs & Kacelnik 1991). How neccessitates additional energy aquisition and ever, such a basic strategy may have to be mod thus the presence of suitable food resources (Loo ified by conflicting constraints such as predation nen et al. 1991). Recent observations showed that risk and variable nutrient resources (e.g. Milinski Greylag Geese Anser anser foraged exclusively at & Heller 1978; Caraco 1979; Poysa 1987; McNa night at a Danish moulting site (Fox et al. 1995). mara & Houston 1994). Moult in waterfowl is Nocturnal feeding has been reported sporadically considered a critical period in the annual life cy among various goose species, but only as tempo cle, where these constraints may have particular rary events in tidal zones or during moonlit nights significance for the behaviour and the wayan in (Lebret 1970; Ebbinge et al. 1975). This paper pre dividual performs in energetic terms. Geese, for sents additional data on Greylag activities and example, are flightless for 4-5 weeks, whilst flight dispersal throughout the site during the course of feathers are shed and regrown simultaneously the moult, and discusses the significance of noc (Lebret & Timmerman 1968), and due to this re turnal feeding in relation to moult state, predation stricted mobility, geese are more susceptible to risk, and the available nutrient resources. predators in this period. This is expressed by ex- 16 ARDEA 84(1/2), 1996
STUDY AREA throughout June. After the moult, the Greylag Geese quickly dispersed from the island, and During the late 1980s, a new Greylag Goose most had departed by early July. The moulting moulting site was discovered on Saltholm, an is population on Saltholm was dominated by young land in The Sound between Danish Zealand and and non-breeding birds (Fox et al. 1995). The Swedish Scania (Fig. 1). Numbers have increased study area was divided into three zones (see to become the second most important moult site Fig. 1). (A) the islets which bordered The Sound, in Northern Europe with a maximum of 9000 the lowest of which had no vegetation, and were birds in 1994 (Madsen 1987; Zijlstra et al. 1991; regularly flooded, whereas the larger islets sup Andersson 1992; Fox et al. 1995). The present ported short saltmarsh vegetation; (B) the water study was carried out in the south-east comer of zone referred to shallow water « 1 m) between the island where over 90% of the moulting Grey the islets and the Saltholm mainland; and (C) the lag Geese were concentrated. Before moult, there Saltholm mainland, which was flat and largely was a short settlement phase of about two weeks treeless. At the coast, Saltholm supported a salt in late May, subsequently, moult continued marsh vegetation similar to the larger islets, but inland this graded into a mosaic of halophytic grassland, saltmarsh and salt pans, inundated by periodic sea water flooding and recharged by ac cumulated precipitation. On the east coast, a freshwater drinking site had been provided for the cattle, grazing on the island (Fig. 1). The pre ferred food item was Puccinellia maritima which was present all around the lower Saltmarsh on the island, but Greylag Geese also fed on Festuca ru bra, Plantago maritima, and Elymus repens. The tidal amplitude was less than 20 cm, and changes in waterlevel were mainly due to direction and force of the wind (Eger 1985). Saltholm and adja cent islets have been free of mammalian predators for several years (N. Adamsen, pers. comm.), and the main moulting area lay within a reserve with strictly limited public access.
METHODS
The study was carried out from 8 to 28 June 1993, from 18 May to 2 July 1994, and from 10 May to 29 June 1995, when daylight duration varied be tween 18 and 191/2 hours. Observations were made from a hide six metres above ground level (Fig. 1). Observer disturbance was minimised, Fig. 1. The Saltholm study area with indications of except when transported to and from the hide by habitat zones, dropping transects 1-5 (straight lines), tractor. After such disturbance, no data were sam observation hide (filled square), and freshwater pond pled for at least two hours. Most of the moulting (open square). area could be surveyed from the hide, except Kahlert et al.: NOCTURNAL FEEDING IN GREYLAG GEESE 17 where high vegetation obscured some geese, so Moreover, activity budgets were used to de inevitably Greylag dispersal throughout the main scribe behaviour in the three zones and to assess land would be biased by ordinary counts. Hence, as Greylag responses to stimuli in their environment an index of goose use, dropping counts were made when feeding on the mainland. We have used the in mid-May and in late June, along five transects expression 'spook' to refer to events where over perpendicular to the coastline to supplement Grey 20% of the Greylag Geese showed fear response lag counts (Fig. 1). Droppings were counted in a during the course of a five minute scan by running circle of four square metres at five metres intervals to the water or escaping by flight. Moreover, we along transects inland from the shore. To estimate recorded any obvious stimulus at the time which the duration of the stay in the mainland feeding may have caused the spook. The progress of area for Greylag Geese roosting on the islets dur moult was assessed in 1995, using six subjective ing the day (Fox et at. 1995). The arrival and depar field scores categories based on the visible devel ture time of these birds were estimated by counting opment of the primaries (piersma 1987; Fox et at. the number of Greylag Geese every quarter of an 1995). 'Zero' corresponded to recent complete hour in a defined area beneath the observation to loss of primaries when birds were totally inca wer. As the arrival and departure from the main pable of flight. At 'four' most Greylag Geese land tended to be social events, these periods were could fly, and at 'five' the primaries were full quite conspicious. Arrival and departure time were grown, hence 'four' and 'five' were assigned to determined, by using the 15 minute period, where the category 'capable of flight'. The assessment the biggest relative change in numbers of Greylag ofindividual flight capability was only carried out Geese was observed in the count area. on the mainland feeding grounds and compared to Activity budgets were compiled on the main time of the day throughout the moult period. land by scanning the goose flocks at five minute intervals (Altmann 1974). The scanning of goose flocks were carried out in 1994 and 1995 during RESULTS four specified time periods: (1) one hour before to two hours after sunrise, (2) 11.30-14.30 h, (3) two The majority of Greylag Geese, feeding in the hours before to one hour after sunset, and (4) middle of the day, were able to fly throughout the 23.30-02.30 h. Data were compiled twice a week study period (Fig. 2). By contrast during the hours in each time period. The activities of the geese of darkness, the proportion of flying individuals were assigned to the following categories: feed, declined dramatically to virtually zero towards rest, walk/run, comfort behaviour, swim, alert the end of May, marking the onset of the main ness, fly, and antagonistic behaviour. Comfort be moult period (Fig. 2). This foraging pattern is de haviour included preening, bathing, and wing picted in Fig. 3, showing that the moulting birds flapping. During darkness, we used a 7x60 mm arrived from the islet roosts to the feeding areas night scope with electric light amplification, late in the afternoon, staying there for 11.8 ± 0.7 which had a range of approximately 500 m. A hours (mean ± SE), whereas the individuals ca crude estimate of the effective foraging time was pable of flight probably stayed on the feeding calculated by multiplying duration of stay on the grounds throughout the 24 hour period. This for feeding grounds with the mean foraging time ob aging pattern was also conspicious in the resight tained by the activity budgets. For non-moulting ing patterns of individually marked Greylag birds only data from the day-time block were in Geese in all three study years (moulters: ten case cluded. For moulting birds the mean foraging stories, non-moulters: two, respectively). More time was calculated from the morning-, night-, over, there was evidence that moulting individu and evening blocks in June, weighted equally dur als changed their diurnal rhythm during their stay ing the length of stay. on Saltholm, as three marked birds were watched 18 ARDEA 84(1/2), 1996
1001------::~~~?1
£; • £; 0 80 80 • • £; '" 60 60 '" '" '" •• - transect 1 l/) 40 C> - transect 2 '" c '" '" '0. -- transect 3 '" 0 -- transect 4 o --- transect 5 -0 '0 17 May
hour r------, 12.00 20 27 June LIGHT 06.00 O.!"'"----'-----;-:':-:----'----;::-::-:::----'------;;~---l...____;~----'-____;~ o 100 200 300 400 500 distance from coast (m)
24.00 Fig. 4. Cumulative dropping counts (%) from five transects in the southeast comer of Saltholm in May 18.00 and June 1994. Transect numbers refer to Fig. 1. LIGHT
feed exclusively at night. Moulting Greylag 12.00 Geese fed more intensively (mean ± SE; 74.2% ± 20 25 30 4 9 14 May June 5.2 of the time when present on the feeding grounds, n= 113 scans) than birds capable of Fig. 3. Duration of stay on the Saltholm mainland of flight (67.7% ± 1.1, n= 464). However, assuming moulting Greylag Geese (bars) and geese capable of constant bite size for both sets of birds, this was flight (lines) studied throughout 24 hour periods in May not enough to compensate for less time spent in and June 1995. Light conditions during the study period the feeding area as total time spent feeding are indicated in different background shadings, where amounted to 8.9 ± 0.1 h per 24 hour period for dusk is defined as the period between sunset/sunrise and moulters versus 16.2 ± 0.3 h for birds which were the time when the sun is 6° below the horizon. capable of flight. The presence of the entire popu lation of moulting and non-moulting Greylag feeding by day whilst they were capable of flight. Geese on the mainland from late afternoon (16.00 Later, the same birds were resighted whilst flight h) to the morning (09.00 h) means that the per less when moving between islets and mainland to centage of individuals capable of flight scored in Kahlert et al.: NOCTURNAL FEEDING IN GREYLAG GEESE 19
that period (Fig. 2) gives an overall estimate of DISCUSSION the proportion of moulters and non-moulters. In May, Greylag Geese were more evenly dispersed Predation risk over the study area on the mainland, when the Nocturnal feeding in waterfowl has been sug proportion of flying individuals was high. Later, gested as an adaptation to increased predation risk goose feeding activity was confined to within 200 during the day (e.g. Tamisier 1974; Pedroli 1982; metres of the coastline in June when the majority Fox et al. 1994). Risk of predation can be a local was flightless (Fig. 4). The distribution of drop problem for moulting arctic geese (Madsen & pings in May and June were significantly differ Mortensen 1987), however, in the case of the pre ent to each other in all five transects sampled sent study, predators have been absent from the
(Kolmogorov-Smirnov tests, Dmn= 0.29-0.55, moulting area for several years. Nevertheless, P< 0.001; Fig. 4). The same significant aggrega there are several indications that moulting Grey tion during moult was observed in 1993 and 1995 lag Geese, in particular, behaved in a manner ap (not shown). The frequency of spook events in propriate in the presence of predators. Hence, creased significantly during the course of moult flightless Greylag Geese fed closer to the coast compared to the settlement and dispersal period line than the same geese with powers of flight, (X2 =30.1, df= 1, P< 0.001, Fig. 5). For the major which has been interpreted as an anti-predator re ity of spooks, no obvious reason for the observed sponse by other authors, since the water offers the behaviour was apparent. However, the frequency opportunity of rapid escape (Madsen & Morten of spooks caused by discernable stimuli such as sen 1987; Mayhew & Houston 1989). Moreover, Grey Herons Ardea cinerea and helicopters also only flightless Greylag Geese responded to over increased. The frequency ofhelicopters traversing flying Grey Herons and helicopters by fleeing to the study area and the presence of herons did not the water, and even during undisturbed periods, differ throughout the study period. the frequency of escape events increased dramati cally during moult. Such avoidance behaviour is similar to that reported elsewhere where geese en countered real predators such as White-tailed Ea gle Haliaeetus albicilla (Kotrschal et al. 1992). We included human activity amongst the preda ~ 10 o tor-like stimuli encountered by the geese during -~ the present study which may explain the switch to nocturnal feeding in moulting Greylag Geese as j another instinctive predator avoidance behaviour Cfl ~ (Burger & Gochfeld 1991). Indeed, helicopter '0 flights stopped at night, as did most other activ :.!!o ities in the nearby airport. Furthermore, herons, which also stimulated the Greylag Geese to es cape runs, were largely inactive at night. More over, flightless Greylag Geese definitely lowered their response threshold to such quasi-predator Fig. 5. Percentage of five minute scans where Grey stimuli. Considering that escape runs are costly to lag Goose spooks were observed in different periods during the course of the study period (settlement, moult the geese in terms of lost feeding opportunities period and dispersal). Indicated are factors which cau and the enhanced energy demands of fleeing to sed the spook where discernible (see legend). Digits in water, we suggest that the decision when to feed dicate the number of scans in each period. Pooled data is governed by the susceptability to stimuli from from 1994 and 1995. the environment and by the probability of en- 20 ARDEA 84(1/2), 1996 countering stimuli which provoke spooks. In that ence of waterfowl for coastal habitats (Owen respect, the less susceptible Greylag Geese still 1972, 1973), as observed in the moulting Greylag capable of flight have no reason to change their Geese. In the present study, however, it is not very normal diurnal feeding behaviour, whereas flight likely that the Greylag Geese are water limited as less Greylag Geese may compensate for increased the numerous water pans on the main island, al susceptability to stimuli by feeding during the though brackish, are used as drinking places. The night, when the probability of disruption is low. excretion of salt did not seem to be a problem for In this context the island of Saltholm seems uni the Greylag Geese, otherwise they would prob que in its absence of the usual night active preda ably have shown preference for the freshwater lo tors, such as Red Fox Vulpes vulpes, which would cality on the island, which they did not. otherwise restrict feeding opportunities at night.
Nutritional explanations ACKNOWLEDGEMENTS In other studies, the distribution and abun dance of feeding geese have been primarily ex This study was part of the base-line investigations in plained by the patterns of availability of abundant relation to the fixed link construction between Den and rich food resource (Prop et al. 1978; Loonen mark and Sweden commissioned by the 0resund Con et al. 1991; Zijlstra et al. 1991). Daily variation in sortium. The Danish Nature and Forest Agency and the food quality could therefore explain the observed landowners of Saltholm are thanked for permission to work in the study area. We are also extremely grateful feeding pattern in moulting Greylag Geese. In to the warden N. Adamsen and J.N. Kristiansen for fact, storage polysaccharides of graminiaceous practical help, and to A. Follestad, M. Loonen, and M. food items, especially fructan, is accumulated in Zijlstra for valuable discussions in an early phase ofthe the leaves and stems during the daylight hours by study. P. Clausen, J. Madsen, T. Bregnballe, and H. the normal photosynthetic processes (Salisbury & Noer gave helpful comments on drafts of the manu Ross 1985). This accumulation is further stimu script. lated by falling temperatures (Hendry 1987). So, although it has to remain speculative for the pre sent, there is at least some theoretical evidence REFERENCES that sugar content in grass leaves peaks towards the end of the day when the moulting Greylag Altmann J. 1974. Observational study of behaviour: Geese come ashore to feed. Moreover, other au sampling methods. Behaviour 49: 227-267. Andersson A. 1992. Moulting localities for Greylag thors have shown, although not consistently, that Geese in north-west Europe: an update of the sta even day-feeding geese and sheldgeese intensify tus. IWRB Goose Res. Group Bull. 3: 8-13. their feeding activities just before sunset (e.g. Black J.M., C. Deerenberg & M. Owen 1991. Foraging Summers & Grieve 1982; Black et al. 1991). How behaviour and site selection of Barnacle Geese ever, such a nutrional explanation for the behavi Branta leucopsis in a traditional and newly colon ised spring staging habitat. Ardea 79: 349-358. our only underlines the importance of dusk feed Burger J. & M. Gochfeld 1991. Human activity influ ing, and does not explain the observed change to ence and diurnal and nocturnal foraging of Sander nocturnal feeding in moulting Greylag Geese. For lings (Calidris alba). Condor 93: 259-265. day-feeding geese, intensified dusk feeding may Caraco T. 1979. Time budgeting and group size: a the simply be filling of the gut before they fly to ory. Ecology 60: 611-617. Ebbinge B., K. Canters & R. Drent 1975. Foraging rou roost. Alternatively, if access to drinking water is tines and estimated daily food intake in Barnacle limited in moulting Greylag Geese because of Geese wintering in northern Netherlands. Wild flightlessness, evaporative body water loss may fowl 26: 5-19. be lowered by feeding at night. Moreover, water Eger L. 1985. Vandstand i 0sterspen: varsling af storm limitation has been suggested to explain prefer- flod. M.Sc. thesis, Institute of Mathematics and Operation Research, Technical Univ. of Denmark. Kahlert et al.: NOCTURNAL FEEDING IN GREYLAG GEESE 21
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Na het verliezen van de slag White-fronted Geese at the Newgrounds, Slim pennen, wanneer de ganzen gedurende een aantal we bridge. J. Appl. Ecol. 9: 385-398. ken niet in staat waren te vliegen, werd er nog uitslui Owen M. 1973. The management of grassland areas for tend's nachts gefourageerd. Overdag gebruikten de wintering geese. Wildfowl 24: 123-130. ruiers de kleinere ei1andjes als slaapplaatsen. 's Nachts Pedroli J.-C. 1982. Activity and time budget of Tufted hielden ze zich vooral op in de oeverzone om te foura Ducks on Swiss lakes during winter. Wildfowl 33: geren en vertoonden re1atief veel 'paniekgedrag'. Bij 105-112. nadering van een vermeende predator, zoals grate vo Piersma T. 1987. Population turnover in groups of gels (reigers, grote meeuwen) of helicopters of vlieg- 22 ARDEA 84(1/2), 1996 tuigen, maar vaak ook zonder duidelijke aanleiding, rageren door de miers, zoals een mogelijk betere op stoven grote aantallen miende ganzen te water. Rui brengst van het nachtelijk grazen, of een verminderd ende ganzen bleken extra gevoelig voor de dreiging waterverlies 's nachts worden besproken en verworpen. van predatoren. De auteurs suggereren, dat het relatief (MFL) kostbare vluchtgedrag zo veel mogelijk wordt verme den, door nog uitsluitend 's nachts te fourageren tijdens de mi. 's Nachts waren de reigers veel minder actief en Received 15 September 1995, accepted 20 February 1996 was ook het aantal helicoptervluchten minimaal. Alter Corresponding editor: Johan van Rhijn natieve verklaringen voor het uitsluitend 's nachts fou-