Anim. Behav., 1990, 39, 487495 A test of the information-centre hypothesis in a colony of sandwich terns Sterna sandvicensis FRANK GOTMARK Department of Zoology, University of Gothenburg, P.O. Box 25059, S-400 31 Gothenburg, Sweden Abstract. It has been suggested that colonies of terns function as information centres from which unsuccessful foragers follow other birds to feeding sites. From a sandwich tern colony in south-western Sweden, foragers departed in temporal synchrony and birds leaving at the same time headed preferentially in similar directions. However, unsuccessful foragers did not follow other birds from the colony, and successful birds were not followed more often than other birds. Moreover, a 'leader' and a 'follower' that departed together in the same direction flew to the same feeding site in fewer cases than predicted by the hypothesis. A food shortage was noted twice, but there was no indication that terns sought information within the colony during these periods. Thus, clumped departures are weak evidence for the information- centre hypothesis and in the present case might be explained by clumped arrivals or, possibly, benefits of local enhancement in the foraging area. Sandwich terns usually nest in association with black-headed gulls, Larus ridibundus, and the benefits of this in terms of reduced predation risk may be an important reason for coloniality. Two important reasons why some birds breed in colonies appear to be reduced risk of predation and increased foraging efficiency (Wittenberger & Hunt 1985). Many colonial species feed on food sources that vary unpredictably in time and space, and therefore may be difficult to find. The information- centre hypothesis (Fisher 1954; Ward & Zahavi 1973) suggests that colonies function as sources of information about profitable feeding sites. Unsuccessful foragers may return to the colony and follow other foragers to such feeding sites. Tests of the hypothesis have produced equivocal results (Wittenberger & Hunt 1985; but see Brown 1986). Recently, Waltz (1987) suggested that colonies of common terns, Sterna hirundo, function as information centres. He found that terns tended to depart together from the colony, and that unsuc- cessful foragers were more likely than successful ones to follow others from the colony. In this paper, I present a test of the information-centre hypothesis in a colony of the related sandwich tern. Veen (1977) suggested that colonial nesting in the sandwich tern is partly an adaptation to reduce the risk of predation, and partly an adaptation to improve foraging efficiency. The terns nest in comparatively large colonies and often fly long distances to locate food sources that presumably vary in time and space (e.g. Lack 1968; Cramp 1985). Erwin (1978) therefore considered the sand- wich tern to be a likely candidate for using food 0003-3472/90/030487 + 09 $03.00/0 information sensu Ward & Zahavi (1973). How- ever, foraging adaptations of this species have not been investigated during the breeding season. My objectives in the present study were to examine the following five predictions of the information- centre hypothesis. (1) Terns leaving the colony on foraging trips should depart in temporal synchrony and terns leaving at about the same time should preferentially fly in a similar direction. (2) Unsuc- cessful foragers, compared with other foragers, should be more likely to follow other birds from the colony. (3) Successful foragers, compared with other foragers, should more often be followed by other birds from the colony. (4) Terns leaving together and representing a 'leader' and a 'follower' should preferentially fly to the same feeding site. (5) The information-centre function should be manifest during periods when the colony experi- ences food shortage. In his study of common terns, Waltz (1987) examined predictions 1, 2 and 3. METHODS The study colony is situated on the island of Knarrsk/ir 45 km south of Gothenburg offthe coast of south-western Sweden (57~ ' N, 12~ ' E; Fig. 1). The island is low and rocky but partly covered by grasses and stands of Anthriscus sylvestris. The tern colony was established in 1979 and is at present 9 1990 The Association for the Study of Animal Behaviour 487