Abstract
In a 2—yr study of the survival of Glaucous—winged Gull (Larus glaucescens) chicks, pecking of trespassing chicks by neighboring adults was the major cause of chick mortality. In years of both low and high food availability chick survival was strongly correlated with growth rates. Chicks that grew slowly were more likely to be killed by neighbors than fast—growing chicks. In the year of low food availability, among slow—growing chicks, those hatched early in the season on large territories had better survival rates than chicks hatched late in the season on small territories. In the year of high food availability, timing of breeding and territory size had little bearing on chick survival. A model of chick survival in relation to timing of breeding predicts that when neighbor interference is the major cause of chick mortality, chicks hatched early in the season will have the highest probability of surviving. When predators are the major cause of chick mortality, chicks hatching in the middle of the breeding season will have highest survival. A second model relates chick survival to territory size. Chicks raised on large territories will most likely avoid neighbor interference, while predation will select for either small or large territory size, depending upon the effectiveness of group mobbing against the predator.
Highlights
Social systems and spacing patterns of breeding vertebrates have received considerable attention in recent years
In 1973, 57% of the chick mortality occurred before chicks were more than I0-days-old, and at least 49% of chick deaths was caused by neighbors
Only minor differences existed in chick survival between experimental and control pairs
Summary
Social systems and spacing patterns of breeding vertebrates have received considerable attention in recent years (see reviews by Brown 1964, Lack 1968, Orians 1969a, Brown and Orians 1970). Colonial breeding has been described as a response to abundant or unpredictable food resources (Orians 1961, Crook 1964, Horn 1968, Lack 1968, Emlen 1971, Krebs 1974, Hunt and Hunt 1975), to short breeding seasons and reduced food resources (Barash 1974), or to predation pressure (Cullen 1960, Ashmole 1963a, b, 1971, Kruuk 1964, Patterson 1965, Tinbergen et al 1967, Parsons 1971, Alexander 1974). Reproductive patterns that might reduce these forms of interference by neighbors sh@uld exist
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