Foraging-Flock Recruitment at a Black-Billed Gull Colony: Implications for the Information Center Hypothesis

-The extent to which a flock leader advertises its departure from a colony and recruits flock mates is an important issue of the Information Center hypothesis. At a colony of Black-billed Gulls (Larus bulleri), I found that attractive calls were given by some leaders, that leaders called more often than followers, and that calling leaders recruited followers more often than silent leaders. Playback experiments demonstrated that these "contact" calls were attractive. The results indicate that some benefits, most likely related to group foraging, result from flock membership when the gulls are away from the colony. Benefits of group foraging away from the colony provide a more likely mechanism for explaining food-related selection pressures favoring a colonial foraging system than do the presumed benefits derived from the more complex, information-transfer mechanism envisaged by the Information Center hypothesis. Received 11 December 1980, accepted 1 July 1981. ACCORDING to the Information Center hypothesis described by Ward and Zahavi (1973), certain avian assemblages, such as communal roosts and breeding colonies, evolved primarily because they function as information centers where birds that have been unsuccessful in locating a good food patch can benefit by following other, more successful birds to the latters' food. The hypothesis is considered to apply mainly to birds that utilize food distributed in unpredictable but temporarily rich patches. Data consistent with the Information Center hypothesis have been obtained in studies of the Red-billed Dioch [Quelea] (Quelea quelea) (Ward 1965), the Great Blue Heron (Ardea herodias) (Krebs 1974, 1978), and the Bank Swallow (Riparia riparia) (Emlen and Demong 1975). The central concept of the Information Center hypothesis involves information transfer between or among birds at the roost or colony. Of particular relevance are signals whereby a potential leader might identify itself to followers. Although information transfer of the sort envisaged by the Information Center hypothesis could occur in the absence of specialized signals devoted to that function (Bertram 1978, Waltz in press), the extent to which a leader advertises its departure remains highly relevant to the hypothesis (Ward and Zahavi 1973). In this report, I examine some of the contexts and evolutionary implications of information transfer as it pertains to signals that could function as a means of flock recruitment at a colony and describe observations and experiments designed to assess the occurrence and probable function of these signals in Blackbilled Gulls (Larus bulleri). Black-billed Gulls nest in dense inland colonies in New Zealand (Stead 1932, Beer 1966, Evans 1970). They typically forage in flocks and utilize unpredictable but often temporarily rich food patches. Food items include fish, as well as worms and other small invertebrates that are exposed on open pasture land after rains or turned up on fields by farm implements (Dawson 1958, pers. obs.). SOME CONTEXTS FOR INFORMATION TRANSFER AT COLONIES Parasitic relationship. -A parasitic relationship would arise if followers that did not know where food was ("ignorant" birds) were able to victimize leaders that did know where food was ("knowledgeable" birds) by following them to their food patches, even though such information transfer were of no advantage to the leader (cf. Krebs 1978, Waltz in press). This situation could arise if a colony evolved and were maintained for reasons unrelated to information transfer, for example, if the colony provided protection against predators (Lack 1968). Possible costs to the leader might include those associated with leading potential competitors to the leader's food source. Even 24 The Auk 99: 24-30. January 1982 This content downloaded from 207.46. 13.160 on Mon, 17 Oct 2016 05:19:47 UTC All use subject to http://about.jstor. org/terms January 1982] Flock Recruitment in Gulls 25 if such costs to the leaders were negligible, it is expected that knowledgeable leaders would not be selected to expend time and/or energy in advertising their departures. The occurrence of calls or other conspicuous recruitment displays by leaders would not be predicted in a parasitic system. Attempts by leaders to slink away quietly and inconspicuously would be more likely (Bertram 1978). Reciprocal altruism. -Information transfer could occur at a colony in a context of reciprocal altruism if a bird A leads another bird B to food on one trip, and bird B reciprocates and leads bird A to a food patch on some later trip (Krebs 1978). As pointed out by several investigators (Trivers 1971, Maynard Smith 1978, Davies and Krebs 1978, Vehrencamp 1979), however, systems based on reciprocal altruism are usually open to invasion by "cheaters. " A cheater in the present situation can be defined as a bird that follows others to their food but does not lead others to any rich patches it might find itself. As in the parasitic situation, leaders in a system based on reciprocal altruism could face costs associated with leading competitors to the food supply and would not be predicted to expend time or energy in advertising their departures with calls or other recruitment displays. Cooperation between selfish leaders and followers. -A system in which knowledgeable leaders and ignorant followers all benefit is a third context in which information transfer at colonies could occur. This situation could arise whenever leaders would benefit directly from the company of flock members away from the colony site, for example, if flocking en route to the foraging grounds or while foraging were beneficial as an antipredator adaptation (Bertram 1978), or if group foraging were beneficial (Rand 1954, Fisher 1958). When these awayfrom-colony benefits of flocking occur, leaders and followers should all benefit by leaving in a flock. If leaving as a member of a flock is beneficial, leaders should be selected to recruit followers. This prediction is opposite to that derived from the previous two situations (parasitism and reciprocal altruism). TESTS OF PREDICTIONS The main predictions derived from the above theoretical considerations deal with the extent to which a potential flock leader should be selected to render itself conspicuous or inconspicuous to others when it leaves the colony on a foraging trip. Whenever a bird flies away from a colony, its actions necessarily provide information in the form of visual cues to any observers that might be watching. Vocalizations, in contrast, need not be given and, when present, can be taken as evidence for the emission of ritualized signals subject to positive selection pressures (Smith 1977) of the sort expected from a bird selected to make itself conspicuous, but not from one that is selected to be inconspicuous. Gulls, including the Black-billed Gull, are highly vocal, and preliminary observations indicated that loud calls are sometimes associated with foraging flocks. The occurrence of calls and their function when emitted were used to test the above predic-