Regulation of chick provisioning rate in Manx Shearwaters: experimental evidence and implications for nestling obesity

Abstract

1. Procellariiform seabirds such as Manx Shearwater accumulate large quantities of lipid during the nestling period, and a series of recent papers suggests that these lipid reserves provide a buffer against pronounced and unpredictable variation in food provisioning, resulting in part from an inability of parents to regulate food supply to the nest. Supplementary feeding was used to test experimentally whether or not parents were able to adjust their level of food provisioning according to the nutritional requirements of the chick. 2. There was no difference between experimental and control groups in parental provisioning rates before supplementary feeding began. Thereafter, chicks given daily supplements received significantly fewer feeds from their parents, although the mass of food delivered during each feed did not differ between groups. The total amount of food received by chicks in the experimental group (parental delivery plus supplementary food) was only slightly higher than the amount delivered by parents in the control group, indicating that parents adjusted their provisioning rate downwards, although they did not compensate completely for the additional food provided by the supplements. 3. These results indicate that in Manx Shearwaters, nestling obesity is not related to an inability of parents to monitor changes in the chick’s nutritional status. These results are discussed in relation to the optimization of nestlings’ lipid reserves. It is suggested that evidence of a reduction in parental food provisioning in response to a decrease in the food requirements of chicks does not refute the hypothesis that nestling obesity provides a buffer against highly variable food delivery. However, these results do suggest that in species that feed their chicks at comparatively short average intervals, variability in provisioning should be viewed as arising from highly variable and unpredictable foraging success among parents, and not from an inability of parents to assess the nutritional requirements of their offspring.