Foraging and provisioning strategies of black-browed albatrosses in relation to the requirements of the chick: natural variation and experimental study

We studied regulation of the food supply to black-browed albatrosschicks at Kerguelen by simultaneously recording the provisioningrates achieved by individual parents and satellite trackingforaging birds during two seasons, by studying changes in adultmass, and by experimentally manipulating the food requirementof chicks. In 1994 black-browed albatrosses had a higher breedingsuccess and produced heavier chicks that grew faster than in1995. They spent a similar time foraging but brought heaviermeals to their chick in 1994. Satellite tracking indicated thatin both seasons birds foraged in the same oceanographic area,250 km from the colony. Travel times to and from this area remainedunchanged, and similar times were spent foraging there. In ourstudy area, black-browed albatrosses appear to rely on a foodresource that is predictable in location, but whose availabilityvaries from one year to the next. The principal difference betweenyears of differing food availability was that birds broughtlarger meals when food was more abundant Costs of commutingto nearby feeding areas are probably low and allow the deliveryof energy to the chick at a high rate. A study carried out in1991 indicated that there was no relationship between the changesin adult mass from one trip to the next and the duration offoraging trips or feed mass, suggesting that adult body conditionhad little influence on the provisioning strategy of this species.An experiment whereby some chicks were deprived of food andothers received supplementary food showed that parents of underfedchicks spent the same time foraging and brought slightly largeramounts of food to their chicks as control parents. We suggestthat parents are searching for food to the maximum limits oftheir ability and thus cannot reduce further foraging time,but underfed chicks can swallow more food. Parents of overfedchicks delivered less food and increased the time between feeds.The reduction in provisioning frequency is interpreted as thecapacity of parents to modify their foraging behavior accordingto the nutritional status of the chick, but the reduction offeed mass is probably the result of chicks being close to theirmaximum assimilatory capacity. Comparison between Procellariiformspecies indicates extensive differences in the degree to whichparents can regulate the supply of food to their chicks. Neriticspecies like black-browed albatrosses appear to have a reducedability to regulate, and especially to increase provisioningrates, whereas more pelagic species may have a greater regulationability