DETERMINANTS OF CHICK SURVIVAL IN THE LESSER BLACK-BACKED GULL - RELATIVE CONTRIBUTIONS OF EGG SIZE AND PARENTAL QUALITY

(1) Avian egg size is commonly held to be an important index of egg 'quality' because it reflects the quantity of yolk reserves available to the chick during embryonic development and on hatching. (2) The assumption that egg size may therefore play an important part in determining chick survival overlooks the confounding variable of parental quality which may influence both egg size and fledging success. (3) In order to test the hypothesis that the abilities to produce large eggs and to rear chicks successfully may be positively related, and to investigate the relative contributions of parental quality (ability to produce large eggs) and egg size per se to chick fledging success, a clutch transfer experiment was performed in which clutches of large eggs were cross fostered with clutches of small eggs. (4) Large eggs produced chicks which were not only skeletally larger (tarsus length) but also heavier for their size than those from smaller eggs, both of which could contribute to increased chances of survival. (5) Logistic regression analysis showed that both egg size and the quality ot the foster parents contributed significantly to the probability of fledging. Parental quality was more important than egg size in determining chick survival, which suggests that correlations between survival and egg size found by other workers in nonexperimental situations may be due to the confounding effect of parental quality. (6) The contribution of large egg size to chick survival in the lesser black-backed gull Larus fuscus L., appears to be mediated via advantages of increased skeletal size, rather than body condition, on hatching. (7) As large egg size is advantageous, selection for increasing egg size is implied. The likely counterbalancing forces are discussed.