Flexible effort in breeding seabirds: Adjustment of provisioning according to nestling age and mass

We examined whether rhinoceros auklet (Cerorhinca monocarata) parents are able to adjust their provisioning effort in response to chick demand. A fostering experiment in which nestlings of different ages and masses were exchanged between burrows was employed to examine parental provisioning effort before and after the exchange. Daily mass increments of the nestlings were used to estimate the amount fed on the previous night, using a model based on data from captive chicks raised on a controlled diet. Our results demonstrate that rhinoceros auklet parents somehow assess and respond to the needs of their chicks by delivering more to order and larger fostered chicks and less to younger and smaller fostered chicks. Our results confirm and extend a growing body of information which shows that seabird parents can adjust provisioning effort when feeding young. We highlight how studies to date have differentially manipulated nestling age and hunger, thus complicating a comparative approach. Testing for interspecific differences in parental ability to respond to chick demands will require studies that employ comparable types of manipulations for a variety of seabird species.