Trophic responses to the hatching of offspring in a central-place foraging seabird

We used δ15N and δ13C stable isotope analysis on blood drawn from adult and nestling Rhinoceros Auklets (Cerorhinca monocerata) in 4 years to answer two questions: (1) do adults tend to feed at higher trophic level while provisioning offspring than they did prior to breeding or while incubating eggs across a range of environmental conditions (years), and (2) do adults select prey at a similar trophic level and in similar habitats to complete the temporally overlapping tasks of self feeding and provisioning? Mean δ15N values in adult blood were higher during provisioning than prior to laying or during incubation in all 4 years. Thus, irrespective of environmental conditions, adults increased trophic level after their offspring hatched. Answers to the second question were more equivocal. However, there was support for models in which adult and nestling δ15N and δ13C values did not differ, suggesting that adults did tend to take prey from similar trophic level and habitat to self-feed and load to deliver to offspring. We propose that the two behaviours are related and can be explained as a strategic response by central-place foragers: small, low trophic-level prey (zooplankton) suitable for self-feeding are not as well suited as large, high trophic-level prey (forage fish) for loading in the bill to deliver to offspring, and adults can save time by self feeding and loading in the same habitats.