Natural and Experimental Changes in Chick Provisioning in a Long-Lived Seabird, the Antarctic Prion

Abstract

The ability of Antarctic Prions Pachyptila desolata to regulate their provisioning effort was studied at Kerguelen Islands by comparing two seasons when food availability differed, and by experimental manipulation of foraging costs. 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 build 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 success were similar for the two seasons examined, chicks produced in 1996 grew faster and were heavier at the end of the study than in 1995. The difference was due to a higher provisioning rate in 1996 resulting from long trips being 2 days shorter than in 1995, because the Antarctic waters where adults are believed to feed on Antarctic krill during these long trips were 330 km further away in 1995 than in 1996. While returning from long trips, and during short trips, prions mainly feed close to the colonies on the amphipod Themisto gaudichaudii. This prey was less available in 1995, but prions were able to switch to other prey. Cost of foraging was increased experimentally by adding mass to adults. Loaded parents differed from control parents only by increasing the duration of long foraging trips. The results of both the experiment and the study of the natural variation indicate that the ability of Antarctic Prions to increase provisioning is limited. Birds kept their body mass stable, provided food loads of similar mass and had short foraging trips of similar duration. During long trips they spent longer time foraging either to reach more distant feeding grounds or to cover higher 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.