Chick Provisioning and Parental Attendance in Cory's Shearwaters: Implications for Nestling Obesity

Abstract

Cory's Shearwaters, in common with other species of Procellariiformes, accumulate large quantities of body fat after hatching. This pattern of development has been related both to temporal variation in feeding conditions around the colony and to stochastic variation in foraging success and food provisioning by individual parents. Chicks need not necessarily always accept food provided by their parents, yet few studies have examined parental attendance independently of the masses of food fed to chicks overnight. This paper examines variability in provisioning and growth of nestling Cory's Shearwaters over three years between 1991 and 1994, and uses observations of individually marked adults coupled with radio-tracking to examine the nest attendance patterns of individual parents. Chicks were fed on 77% of nights on average, with little temporal variation in food delivery at the level of the colony. Individual chicks were nonetheless fed highly variably, and this to some extent supports the notion that nestling obesity results from chronic overfeeding by the parents to reduce the impact of variable food provisioning at an individual level. Chicks were attended by 1.5 parents overnight on average, and chicks were attended but not fed on 16% of occasions. Parents may have returned on these occasions without any food for the chick, or the chick may have refused to accept food from the parents. A minority of nests accounted for most of the occasions when a chick was attended but not fed, and there was evidence to suggest that these chicks were consistently heavy for their size. A refusal by these chicks to accept food on some occasions is consistent with the notion that lipid stores of chicks should be optimized rather than maximized, and this requires further investigation.