Nestling obesity and variability of food delivery in Manx Shearwaters, Puffinus puffinus

Abstract

1. Procellariiform seabirds such as Manx Shearwaters, Puffinus puffinus, accumulate large quantities of lipid during the nestling period, the purpose of which remains unclear. This paper examines temporal and age‐specific variation in the pattern of food delivery to nestling Manx Shearwaters, to determine whether food delivery was likely to be determined primarily by the availability of food to adults at sea or by a chick’s short‐term nutritional requirements. The variability in daily food delivery to chicks in the context of nestling obesity is then discussed. 2. For all but the youngest and oldest age classes, individual meals delivered by adults averaged 49 g, which was 12% of mean adult mass over the same period. More than 80% of chicks were fed each night and the maximum interval between feeding events for any chick was four nights. Feeding conditions did exert some influence over the ability of adults to feed their offspring, but deviations from expected values for both feeding frequency and meal size were restricted to a small number of nights, included values both higher and lower than expected and did not persist for more than one night in succession. These results were not compatible with the notion that large lipid reserves accumulated by chicks during their development provide insurance against prolonged interruptions to feeding. 3. Although adults did not appear to modulate food delivery over the nestling period, there was some evidence of short‐term adjustment of feeding frequency (but not meal size) according to a chick’s immediate nutritional requirements. This suggests that variation in the provisioning rates of chicks did not result solely from stochastic variation in parental foraging success. However, the effect was slight and despite such minor adjustments, the coefficient of variation in daily provisioning rate was not substantially lower than in other species where the chick has no influence on parental food delivery. This suggests that even among those species that feed their chicks at comparatively frequent intervals, lipid accumulation among Procellariiformes may result from chronic overfeeding of chicks in order to reduce the impact of variable food provisioning by individual parents.