Differences in prey selection and behaviour during self-feeding and chick provisioning in rhinoceros auklets.

We determined whether a marine diving bird, the rhinoceros auklet, Cerorhinca monocerata, used different foraging behaviour and collected different prey items for its young than when feeding itself. Foraging behaviour was determined by conducting visual scans, and prey items were sampled by collecting fish delivered to chicks and by collecting fish where auklets were self-feeding, which was verified by two other sources of information. Adult auklets ate small fish (59.1+/-0.5 mm, N=547), including juvenile Pacific sand lance, Ammodytes hexapterus, and Pacific herring, Clupea harengus, but collected larger fish to feed their chicks (95.2+/-1.3 mm, N=321), including primarily Pacific sand lance, Pacific herring, Pacific salmon species, Oncorhynchus spp., and surf smelt, Hypomesus pretiosus. In addition, auklets collected fish for themselves primarily by diving in mixed-species feeding flocks before 1600 hours, whereas they collected fish to feed their chicks by diving solitarily after 1600 hours. This suggests that auklets switched both foraging behaviour and prey selection when collecting fish for self-feeding, compared with when collecting fish for chick provisioning. Several avian studies have documented different diets of adults and chicks, but this is the first research to observe directly and document different foraging behaviour used in adult and chick provisioning. We emphasize the importance of distinguishing between self-feeding and chick provisioning in foraging and life history studies. Copyright 1999 The Association for the Study of Animal Behaviour.