Food provisioning to nestling shearwaters: why parental behaviour should be monitored?

We examined temporal variation in food delivery to nestling Cory's shearwaters Calonectris diomedea, by repeated periodic weighings during the night. We tested whether the magnitude and frequency of meals were influenced by the condition of chicks. In contrast to previous studies of chick provisioning in petrels and shearwaters, the evidence of feeding derived from chick weight gains was complemented by data provided by an electronic system, which logged the entry of each parent to the nest. Estimates of feed size and visiting frequency obtained from chick weighing alone differed from similar estimates obtained using the automatic logging equipment. The data obtained with the logging system combined with chick weighing also showed that, to some extent, food provisioning was regulated, chicks left in poorer condition being more likely to receive food the next night than those left in better condition. The methods based on chick weight gains alone did not detect this regulation effect. Our findings suggest that resolving parental visits to the nest is crucial to obtain accurate parameter estimates, and to address the problem of regulation of provisioning rates in Procellariiformes. Our results do not support the hypothesis that accumulation of fat is just a by-product of chronic overfeeding arising from stochastic variation in foraging success at sea. Copyright 1999 The Association for the Study of Animal Behaviour.