Gull Chick Survival: The Significance of Growth Rates, Timing of Breeding and Territory Size

Ecology (1976) 57: pp. 62-75 GULL CHICK SURVIVAL: THE SIGNIFICANCE OF GROWTH RATES, TIMING OF BREEDING AND TERRITORY SIZE 1 GEORGE L. HUNT, JR. AND MOLLY WARNER HUNT Department of Ecology and Evolutionary Biology, University of California, Irvine, California 92717 USA 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 cor- related 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 bad 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 bteeding predicts that when neighbor interference is the major cause of chick mortality, chicks hatche·d 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. Key words: Colonial nesting; fledging success, gulls; Glaucous-winged Gull; growth rates; Larus glaucescens; model, breeding timing and gull chick survival; model, territory size and gull chick survival; mortality; predation. INTRODUCTION 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 abun- dant or unpredictable food resources (Orians 1961, Crook 1964, Horn 1968, Lack 1968, Emlen 1971, Krebs 1974, Hunt and Hunt 1975), to short breed- ing seasons and reduced food resources (Barash 1974), or to predation pressure (Cullen 1960, Ash- mole 1963a, b, 1971, Kruuk 1964, Patterson 1965, Tinbergen et al. 1967, Parsons 1971, Alexander 1974). These are works dealing with factors ex- ternal to the breeding situation; little information is currently available concerning the internal factors that determine spatial and temporal configurations of breeding colonies. The roles of age relative to position or timing (Coulson 1966, 1968, MacRoberts and MacRoberts 1972) and of mutual stimulation (Collias et al. 1971, MacRoberts and MacRoberts 1972, Victoria and Collias 1973) have been examined. Interference between colony members is another internal factor of possible importance. In some social vertebrates interference between colony members can be as extreme as the killing of yow1g by adults. Crushing and biting by adult northern elephant seals (Mirounga angustirostris) Manuscript received 6 December 1973; accepted 25 August 1975. are major factors in pup mortality (LeBoeuf et al. 1972). In large Larus gulls, killing of chicks by neighbors is an important cause of chick mortality (Paynter 1949, Drury and Smith 1968, Parsons 1971, Hunt 1972). Less extreme interference includes be- haviors such as stealing nest material (Ytreberg 1956, Cullen 1957, Sladen 1958) or food (various gulls which we have observed) from other colony mem- bers. Reproductive patterns that might reduce these forms of interference by neighbors sh@uld exist. In the present study we initially set -out to deter- mine the relative importance of various causes of chick mortality, including killing by neighbors, in the Glaucous-winged Gull (Larus glaucescens), and the characteristics of parents successful in rearing young. Parameters examined included territory size, growth rates and timing of hatching. In the second year we attempted to separate the effects of seasonal changes on reproductive success from effects related to the age or experience of the parents through ex- change of eggs between birds laying at different times. We examined the significance of the internal temporal and spatial structure of a colony in rela- tion to food availability for individual reproductive success. Then, we integrated these factors into an hypothesis that predicts optimal timing and spacing of nesting in relation to conflicting needs for pro- tection against external predators and avoidance of interference by neighbors. This hypothesis may have applicability to other colonial vertebrates.