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

Abstract

To investigate how thin-billed prions, Pachyptila belcheri, regulate their chick-provisioning behaviour, we compared data from two seasons with different food availability (1992 and 1997) and increased the costs of foraging by using load bands during 1997. The 1992 breeding season was poorer than 1997, but during both seasons parents used a two-part foraging strategy, mixing long trips, when they built up body reserves, with short trips, when they used previously stored reserves and foraged to feed the chick. In 1992, parents alternated long and short trips and spent more time on long trips, whereas 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. During the two seasons, both control and experimental parents kept their body mass constant, and survival rates did not differ between seasons or groups. Using a logistic regression, we examined the factors influencing the decision whether, after a first short trip, to start a long trip or a second short 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 adult body condition. The results show that parents regulate their provisioning according to their body mass, which is the main factor influencing decisions concerning energy allocation between current reproduction and survival in this small long-lived seabird.