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AGE DIFFERENCES IN CATTLE Bubulcus ibis, FORAGING WITH WILD UNGULATES IN KENYA

JOANNA BURGER1 & MICHAEL GOCHFELD2

ABSTRACT The foraging behaviour of adult and juvenile Cattle Egrets was studied at Nairobi Park, Kenya. Egrets fed on insects scared up by fo­ raging Buffalo, Zebra, Wildebeest and Hartebeest. Adults attempted to capture more items, captured more items, and had higher capture success rates than juveniles, although the number of steps taken per minute was equal. Choice of host species and foraging location (by host head, front or hind legs) differed between adults and juveniles. A higher proportion of adult Cattle Egrets fed with Buffalo compared to young.

IDepartment of Biological Sciences, Rutgers University, P.O. Box 1059, Piscataway. New Jersey, U.S.A., 2 Department of Environmental and Community Medicine, Rutgers Medical School.

INTRODUCTION Burger 1981), or dropping shells to crack them (Ingolfsson & Estrella 1978). Foraging tasks that At fledging or shortly thereafter, when parents no involve another , such as piracy, might be longer provision them, juvenile of most spe­ especially difficult since the young must de­ cies must obtain most or all of their own food. cide when to pirate and from whom. Young birds During this transition to independence, young that feed in association with other species might must not only learn where to forage, but how to also have difficulty learning to forage as efficient­ feed. Young may require a long learning period to ly as adults. be able to forage as efficiently as adults, and pre­ In this paper we examine age-related differen­ sumably they improve with age (Espin et al. ces in foraging in Cattle Egrets (Bubulcus ibis). 1983). Age related foraging differences have been Although foraging behaviour of Cattle Egrets fee­ reported for a wide variety of species (see review ding with cows has been extensively studied in Burger 1987, 1988), including and egrets (Heatwole 1965, Dinsmore 1973, Siegfried 1978), (Erwin 1985, Draulans and Vessem 1985, Draulans there are few studies where Cattle Egrets foraged 1987) and White Ibis Endocimus albus (Bildstein with wild ungulates (Burger & Gochfeld 1982). 1983, 1984). Age-related differences have been ci­ We were particularly interested in whether adults ted as a cause for delayed maturity, and are belie­ and young used the same hosts, followed the hosts ved to occur where food is patchily-distributed, in a similar manner, and had equivalent foraging distant, or ephemeral (Lack 1968, Ashmole 1963) success. or where a complex feeding method must be lear­ ned. Although many of the seabirds may not breed until they are over four years old, other spe­ METHODS cies with age-related foraging differences do not especially exhibit delayed maturity. One explana­ The study was conducted in Nairobi National tion is that age-related foraging differences occur Park, Kenya from 11-29 August 1988. Nairobi where the foraging task is difficult to master, in­ Park is 112 km2 and 7 km from downtown Nai­ volving techniques such as plunge-diving (Carl robi. We collected data in one single area of the 1987), piracy (Carroll & Cramer 1985, Gochfeld & park where much of the grass was green and lush

Received 28 February, 1989; accepted 7 July, 1989 202 ARDEA 77 (2),1989 to eliminate differences due to habitat or prey behaviour Kendall tau correlations were used to availability. examine relationship between two variables. Cattle Egrets foraged with ungulates by mo­ ving with them and picking up insects scared up by the movements of the ungulates. We observed RESULTS egrets with African Buffalo Synccrus cafler, Bur­ chell's Zebra, Equus burchelli, Wildebeest Con­ There were significant age-related foraging diffe­ nochaetus taurinus, and Hartebeest Alcelaphus rences (Table I). Adults fed in taller grass with buselaphus. Cattle Egrets primarily fed only with host that took more steps. Adults made these animals. Whenever we encountered Cattle more attempts and obtained more items per minu­ Egrets we recorded foraging data for all individu­ te, and adults had higher capture success than ju­ als in one minute focal samples. Many of the eg­ veniles. Adults obtained more than twice as many rets we encountered were not close enough to be food items as juveniles although they did not take aged and were excluded from the sample. Any eg­ more steps. ret with buffy or rusty feathering on the head was Although the number of steps adults took rela­ classed as an adult. Such birds had yellow bills ted to the number of host steps, this was not true and usually greenish brown legs. Juveniles had for juveniles (Table 2). Further, the success rate dusky-greyish to blackish bills and legs. Their for adults was positively correlated with the num­ head and neck plumage appeared chalky. Data re­ ber of attempts, whereas it was negatively correla­ corded included: date, time of day, grass length, ted for juveniles. Juvenile egrets did not behave host species, age of , location of foraging eg­ consistently with respect to grass length whereas ret (head, by front or hind legs, or shifting), num­ adults did (Table 2); adults moved more slowly in ber of egret and host steps per min., and the num­ tall grass than in shorter grass, and had higher ber of attempts and items obtained per min. Grass capture success in tall grass. length was determined by estimating its length Foraging egrets could select among several next to the hosts legs. When an egret captured prey hosts, and could feed near their head or their legs the swallowing was characteristic and conspi­ (Table 3). Adults generally fed by the head and cuous, allowing us to determine capture success front legs whereas a higher proportion ofjuveniles rate. fed by the host's hind legs. Moreover, adults usu­ We used Kruskal-Wallis tests which yield a X2 ally fed with Buffalo whereas more young fed statistic (Proc N Par one-way, SAS 1985). The X2 with Zebra (Table 3). was partitioned among cells by the same procedure to compare differences between adult and young

Table 1. Effect of age on Cattle Egrets foraging with Buffalo and Zebra in Nairobi National Park, Kenya. NS = not significant

Adult Young X2(P)

Number of egrets 192 86 Grass height (em) 37.6 ± 1.5 32.1 ± 2.1 8.9 (.005) Egret steps min- 1 11.0 ± .6 11.5 ± 1.3 0.17 (NS) Host steps min- 1 13.0 ± .8 8.0 ± 1.2 5.41 (.02) Head movement min- 1 3.7 ± .3 4.0 ± .5 0.76 (NS) Number of Attempts min- 1 3.9 ± 0.2 2.5 ± .3 8.08 (.005) Number of items obtained min- 1 3.5 ± 0.2 1.4 ± .2 7.61 (.005) Percent capture success 90 ± 1 61 ± 3 34.9 (.0001) CATTLE EGRETS IN KENYA 203

Table 2. Relationship among foraging variables of adults and young in Kenya. Given are Kendall tau coefficients (level of significance). NS = Not Significant

Adult Young

Number of egrets 192 86 Host steps with egret steps .43 (.0001) NS attempts min- 1 .36 (.0001) .28 (.01) items min-I .35 (.000 I) .38 (.001) percent success NS NS Attempts with attempts min- 1 .90 (.0001) .73 (.0001) percent success .29 (.0001) -.31 (.01 ) Grass length with host steps -19 (.006) NS egret steps -.31 (.0001) NS attempts min- 1 -.15 (.03) NS items min- 1 NS NS percent success .21 (.007) NS

Table 3. Foraging position and host selection of with wild ungulates, but there were no differences adult and young Cattle Egrets. Shown are percent oc­ in the number of steps juveniles and adults took currence. per minute. Thus for the same relative energy ex­ penditure adults obtained over twice as many food Adult Young items per unit time. Further, capture success rate Number of egrets 192 86 of adults was higher than for young, indicating not Position of egreta by only that juveniles locate fewer items, but that head 49 20 they capture fewer of the items they do locate. front legs 30 42 Presumably young can compensate for their lower hind legs 15 38 foraging success and efficiency by feeding for a shifting 6 0 longer period of the day or by concentrating their Host selectionb foraging where their success more nearly equals Buffalo 73 58 that of adults (Burger & Gochfeld 1983). Zebra 15 35 Young did seem to forage for more of the day Hartebeest 11 3 since they often foraged alone when no host ani­ Wildebeest 1 4 mals were moving about, whereas at such times adults often stood around and rested rather than a X2 = 17.9, df = 3, p < 0.0001. The X2 was partitioned foraging. However all the egrets we observed fed among cells with the main contributor being a deficit in the same habitat, and did not leave to search for of young foraging by the head and an excess of young other habitats or hosts. Since host animals may foraging by the hind legs. themselves begin to move at any time, it may be b In the same herd, X2 = 12.4, df = 3, p < 0.006. The X2 was partitioned among cells and the main contributor adaptive for egrets to remain nearby rather than was the excess of young foraging with Zebra. expend energy in search of new ungulate herds or cows. The lower attempt rate for young may be due DISCUSSION to their choice of host and feeding location. More adults fed near the head and front legs or moved Adult Cattle Egrets made more attempts, obtained about the animal whereas almost 40% of young more food items and had higher capture success remained near the hind legs of hosts. Presumably rates than juveniles when feeding in association food items are more available near the head and 204 ARDEA 77 (2), 1989

front legs because insects would be first encounte­ Carl, R.A. 1987. Age-class variation in foraging techni­ red there. Although young were sometimes exclu­ ques by Brown Pelicans. Condor 89:525-533. ded from the front positions by the presence of Carroll, S.P. & K.L. Cramer 1985. Age differences in kleptoparasitism by Laughing Gulls (Larus atricilla) adults, even when a young egret was the only one on adult and juvenile Brown Pelicans (Pelecanus with its host it sometimes trailed behind, thus ju­ occidentalis). Anim. Behav. 33:201-205. veniles may simply be unable or unwilling to be Dinsmore, U. 1973. Foraging success of Cattle Egrets, close to the head or front legs even though more Bubulcus ibis. Amer. MidI. Nat. 89:242-246. food is available there, or may not recognize the Espin, P.MJ., R.M. Mather & J. Adams 1983. Age and foraging success in Black-winged Stilts Himanto· potential benefit. pushimantopus. Ardea 71:225-228. Adult egrets usually fed with Buffalo, whereas Gochfeld, M. & J. Burger 1981. Age-related differences more young than adults fed with Zebra. Since in piracy of Frigatebirds on Laughing Gulls. Condor Buffalo are heavier than the other ungulates, and 83:79-82. move faster (Burger & Gochfeld 1982), they may Heatwole, H. 1965. Some aspects of the association of Cattle Egrets with cattle. Anim. Behav. 13:79-83. scare up more insects than the other potential Ingolfsson, A. & U. Estrella 1978. The development of hosts. However, Buffalo moved in denser herds, shell-cracking behavior in Herring Gulls. Auk and often moved faster than other potential hosts. 95:577-579. Juveniles may be more reluctant to walk as freely Lack, D. 1968. Ecological adaptations for breeding in among the dense Buffalo than are adults. birds. Methuen & Co. Ltd. London. S.A.S. 1985. Sas User's Guide Statistics. S.A.S. Institute When Cattle Egrets forage with wild ungulates Cary, North Carolina. they continually must decide whether to continue Siegfried, WR. 1978. Habitat and the modem range ex­ foraging with a particular host and individual, or pansion of the Cattle Egret. In: A. Sprunt, J. Ogden, whether to switch hosts. Adult Cattle Egrets se­ & S. Winckler, (eds). Wading birds National Audu· lected hosts and foraging positions to have higher bon Research Report 7:315-324. prey capture rates than juvenile egrets. Juvenile egrets appeared to be expanding as much energy as adults (as judged by the number of steps min· I ), SAMENVATTING yet obtained only half as many food items. This is the first demonstration of age-related foraging dif­ Het fourageergedrag van volwassen en juveniele ferences for Cattle Egret, and for a species fee­ Koereigers werd bestudeerd in het Nairobi Na­ ding in association with other species. tional Park in Kenya. De Koereigers eten insecten die worden opgejaagd door grazende Kaapse buf­ fels, Zebra's, Wildebeesten en Hartebeesten. Vit REFERENCES dit onderzoek bleek dat volwassen reigers per tijdseenheid meer vangpogingen deden en vaker Ashmole, N.P. 1963. The regulation of numbers of tropi­ met succes insecten vingen dan juvenielen. Het cal oceanic birds. Ibis 103:458-473. aantal stappen per minuut (als maat voor energie­ Burger, J. 1987. Foraging efficiency in gulls: A congene­ verbruik) was voor ric comparison of age differences in efficiency and beide categorien gelijk. Er was age of maturity. Studies in Avian Biology 10:83-90. ook een verschil in de keuze tussen het soort hoef­ Burger, 1. 1988. Effects of age on foraging in birds pp. dier waarbij werd gefourageerd en de plaats ten 1127-1140. in Acta XIX Congressus Int. Ornith. (H. opzichte van dit dier. Juveniele reigers zochten Ovellet, Ed.), Univ. Ottawa Press, Ottawa. vaker zebra's op en liepen meer bij de achterpoten Burger, J. & M. Gochfeld 1982. Host selection as an adaptation to host- dependent foraging success in van deze dieren, terwijl volwassen reigers meer the Cattle Egret (Bubulcus ibis). Behaviour 79: bij Kaapse buffels fourageerden en vaker in de 212-229. buurt van de kop en de voorpoten bleven. - H.P. Burger, J. & M. Gochfeld 1983. Feeding behavior in Laughing Gull: compensatory site selection by young. Condor 85:467-473.