Natural and experimental changes in chick provisioning in a long-lived seabird, the Antarctic Prion

ability of Antarctic Prions Pachyptila desolata to regulate their provision ing effort was studied at Kerguelen Islands by comparing two seasons when f ood availability differed, and by experimental manipulation of foraging cos ts. Antarctic Prions provision their chick by a two-fold foraging strategy: on average, they alternate a long foraging trip 7-9 days long when they bu ild up body reserves and then feed the chick, with 2 or 3 successive short trips lasting 1 or 2 days when they use previously stored body reserves to forage for provisioning the chick. Although hatching success and fledging s uccess were similar for the two seasons examined, chicks produced in 1996 g rew faster and were heavier at the end of the study than in 1995. The diffe rence was due to a higher provisioning rate in 1996 resulting from long tri ps being 2 days shorter than in 1995, because the Antarctic waters where ad ults are believed to feed on Antarctic krill during these long trips were 3 30 km further away in 1995 than in 1996. While returning from long trips, a nd during short trips, prions mainly feed close to the colonies on the amph ipod Themisto gaudichaudii. This prey was less available in 1995, but prion s were able to switch to other prey. Cost of foraging was increased experim entally by adding mass to adults. Loaded parents differed from control pare nts only by increasing the duration of long foraging trips. The results of both the experiment and the study of the natural variation indicate that th e ability of Antarctic Prions to increase provisioning is limited. Birds ke pt their body mass stable, provided food loads of similar mass and had shor t foraging trips of similar duration. During long trips they spent longer t ime foraging either to reach more distant feeding grounds or to cover highe r foraging costs. The main constraint on chick provisioning is probably the necessity for birds to maintain,a threshold body mass to limit the risk of increased mortality due to breeding effort; the higher yields during long foraging trips permit them to do so.