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

We examined temporal variation in food delivery to nestling Gory's shearwat ers, Calonectris diomedea, by repeated periodic weighings during the night. We tested whether the magnitude and frequency of meals were influenced by the rendition of chicks. In contrast to previous studies of chick provision ing in petrels and shearwaters, the evidence of feeding derived from chick weight gains was complemented by data provided by an electronic system, whi ch 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 obtaine d with the logging system combined with chick weighing also showed that, to some extent, food provisioning was regulated, chicks left in poorer condit ion being more likely to receive food the next night than those left in bet ter condition. The methods based on chick weight gains alone did not detect this regulation effect. Our findings suggest that resolving parental visit s to the nest is crucial to obtain accurate parameter estimates, and to add ress 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 fo raging success at sea. (C) 1999 The Association for the Study of Animal Beh aviour.