Abstract
We compared the growth rates of Audouin´s Gull ( Larus audouinii ) chicks from the Ebro Delta and Columbretes Islands. Chicks from the Columbretes Islands were reared in captivity and fed ad libitum. Wild chicks from the Ebro Delta weighed more than chicks hatched from Columbretes eggs but attained a lower weight at fledging. Chicks from Columbretes eggs hatched with a longer wing length but rates did not differ between samples during the early stages of growth. Eggs from the Ebro Delta were significantly larger than eggs from Columbretes. Hence, differences in growth appeared not to be related to differences in egg size (prehatch factor) but only in parental quality (posthatch factor) through the amount of food delivered to chicks. These differences could be due to the incapacity of parents to provide enough food during the last stages of chick development, in part because of reduced food availability at the Ebro Delta during the study period. Alternatively, differences could be attributed to a trade-off between present and future reproduction. Demographic consequences of reduced food availability for the 1993 Ebro Delta cohort should be explored in future work.
Highlights
Growth rates of chicks are important because growth may influence future survival and reproduction (Bolton, 1991)
Nestlings hatched from large eggs were reared under food-stress conditions by wild parents (Oro et al, 1996b)
Eggs were laid under periods of normal food availability (i.e. May) both at Columbretes Islands and the Ebro Delta, since the trawling-fishing moratorium was not implemented until June-July
Summary
Growth rates of chicks are important because growth may influence future survival and reproduction (Bolton, 1991). Chick growth can be affected both by prehatch (i.e. egg size) and posthatch (i.e. parental care) factors (Risch and Rohwer, 2000). The main prehatch factor affecting egg size is food availability CHICK GROWTH IN AUDOUIN’S GULL 113 during the period of egg formation (Bolton et al., 1993). The main posthatch factor is parental quality through the amount and quality of food delivered to chicks We performed an experiment to separate the effects of prehatch and posthatch factors on the growth rates of the chicks of Audouin’s Gull Larus audouinii. Chicks hatched from smaller eggs from the Columbretes Islands were reared during the 2000 breeding season under ad libitum feeding conditions (captivity)
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