To understand the adaptive significance of colonial breeding, evaluating the costs and benefits that colonial breeding provides to individuals is necessary. To study some of these costs and benefits of colonial breeding, over 24,000 nest-hour observations of kleptoparasitism (stealing of food being fed to young), intraspecific attacks on and killing of chicks, and chick adoption were conducted in colonial breeding Black-tailed Gulls (Larus crassirostris) during two breeding seasons. Although kleptoparasitism was rarely observed (under 0.001 events/nest/observational hour), the occurrence of chick-attacks was 5-10 times higher (0.005-0.01 events/nest/observational hour) than that of kleptoparasitism, with over 10% of the attacks resulting in the death or disappearance of chicks. More than 60% of all attacks on chicks were by failed breeders and non-breeding adult floaters. The survival of attacked chicks reached 98-100% if they escaped into other nests and were accepted by "foster" adults, or if they were defended by non-parental adults. Chick adoption and non-parental defense were more likely to be observed among breeding neighbors (52-60% of chick adoptions and almost all examples of non-parental defense) than among non-neighbors. An increased chick survival rate, resulting from nesting in close to proximity to conspecific neighbors, was considered as a factor promoting colonial breeding among the Laridae.
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