Regulation of chick provisioning in the thin-billed prion: an interannual comparison and manipulation of parents

To investigate how thin-billed prions, Pachyptila belcheri, regulate their chick-provisioning behaviour, we compared data from two seasons with differ ent food availability (1992 and 1997) and increased the costs of foraging b y using load bands during 1997. The 1992 breeding season was poorer than 19 97, but during both seasons parents used a two-part foraging strategy, mixi ng long trips, when they built up body reserves, with short trips, when the y used previously stored reserves and foraged to feed the chick. In 1992, p arents alternated long and short trips and spent more time on long trips, w hereas in 1997 they performed several short trips before a long trip. As a consequence, provisioning rates were higher in 1997. Similarly, manipulated birds decreased their provisioning rates compared with control birds. Duri ng the two seasons, both control and experimental parents kept their body m ass constant, and survival rates did not differ between seasons or groups. Using a logistic regression, we examined the factors influencing the decisi on whether, after a first short trip, to start a long trip or a second shor t trip. This decision was not influenced by the body condition of the chick , the meal mass, or the duration of the previous foraging trip, only by adu lt body condition. The results show that parents regulate their provisionin g according to their body mass, which is the main factor influencing decisi ons concerning energy allocation between current reproduction and survival in this small long-lived seabird.