Sex differences in provisioning rules: responses of Manx shearwaters to supplementary chick feeding

Sex differences in food provisioning have been found in a number of socially monogamous birds with biparental care, but the reasons remain unclear. In Manx shearwaters, males provide 40--50% more food for chicks than do females, and previous empirical data have suggested that this difference could arise because females are able to regulate food delivery by reducing the provisioning of well-nourished chicks, whereas males are not (hypothesis 1). Alternatively, however, males may be as capable as females of assessing and responding to the variation in the nutritional requirements of their chick but have a higher threshold for reducing food delivery to well-nourished chicks (hypothesis 2). To test these two hypotheses, we used supplementary feeding to manipulate the nutritional status of chicks and then examined the responses of male and female parents and their offspring. Supplementary feeding significantly reduced both the begging behavior of chicks and the frequency and sizes of meals delivered by parents. Males and females reduced their overall provisioning rates to a similar extent (males by 38%, females by 42%), so maintaining the same difference in contributions to provisioning in the control group (males 58%, females 42%) and the experimental treatment (males 59%, females 41%). These data strongly support hypothesis 2. Supplementary feeding of chicks resulted in fewer visits by parents and a higher proportion of long trips in both sexes (4 days for males, 5--7 days for females). However, maximum trip durations were unchanged, suggesting that supplementary feeding of chicks had no effect on the foraging ranges or overall food-provisioning strategies of parents. Copyright 2006.