FOOD AVAILABILITY AFFECTS ADULT SURVIVAL AS WELL AS BREEDING SUCCESS OF PARASITIC JAEGERS

Abstract

Long-lived species are expected to reduce their current reproductive effort in adverse conditions in order not to jeopardize their future reproduction. Parasitic Jaegers (Stercorarius parasiticus) breeding in Shetland, United Kingdom, have declined substantially in numbers over the last 15 years, although the causes of this decline remain unclear. By carrying out a supplementary feeding experiment during the chick-rearing period, we tested the hypothesis that food availability has contributed to this decline. Data were collected on adult territory attendance, chick growth rates, and breeding success during the experimental season, and adult return rates, body condition, and laying dates during the following season. Differences in carbon and nitrogen isotope ratios in the natural and supplementary food, analyzed in the chicks' feathers, showed that chicks received one-fifth of their protein requirements from the supplementary food, but this amount varied between broods. Compared with controls, supplemented pairs showed higher attendance on the territory, improved breeding success, and higher return rate. This emphasizes that adult return rate is more sensitive to food availability than has been assumed previously. Control birds may have maintained too high a current reproductive effort at the expense of future reproduction, perhaps due to the unpredictability of their main food source, sandeels, which seemed abundant early in the season but scarce during chick rearing. This study suggests that the consequences of poor food availability are shared between offspring and parents in this species.