Feeding ecology of short-tailed shearwaters:breeding in Tasmania and foraging in the Antarctic?

Abstract

The food, feedlng and physiological ecology of foraging were studied in the short-tailed shearwater Puffinus tenujrostris of Tasmania, to establish whether this species can rely on Antarctic food to fledge its chick. Parents were found to use a 2-fold foraging strategy, on average performing 2 successive short trips at sea of 1 to 2 d duration followed by 1 long trip of 9 to 17 d. These long foraging tnps are the longest yet recorded for any seabird. During short trips the parents tend to lose mass, feeding the chick with Australian krill and fish larvae caught in coastal and neritic waters around Tasmania. The prey are caught at maximum diving depths of 13 m on average (maximum 30 m). During long trips, adults gain mass and feed their chlcks with a very rich mixture of stomach oil and digested food composed of a high diversity of prey including myctophid fish, sub-Antarctic krill and squids. Prey are probably caught mainly in the Polar Frontal Zone, at least 1000 km south of Tasmania, at maximum depths of 58 m on average (maximum 71 m). Long foraging trips in distant southern waters gave at least twice the yield of trips in close waters but during the former, yield decreased with the time spent foraging, as indicated by the inverse relationship between time spent forag~ng and adult body condition. Decisions whether to forage in close or distant waters appear to be determined by the body condition of :he paieiits iclthei i i ~ a ~ ~ by ihdi oi ihe C ~ ~ C K . Good body condition of parents was associated with high blood prolactin levels: birds in poor body condition invested in the restoration of body reserves and went to the rich area of the Polar Frontal Zone or to Antarctic waters, whereas those in good condition continued to provision the chick with food caught in Tasmanian waters and use their body reserves. The species thus exploits 2 water masses that differ in profitability. Most of the energy budget for rearing the ch~ck 1s balanced by prey caught in rich southern waters where the birds build up their body reserves and find most of the energy needed by the chick. However, nearby shallow waters are important for the breeding strategy as they allow an increased rate of energy flow to the chick. In a species as abundant as the short-tailed shearwater, the 2-fold strategy has the advantage of limiting competition in coastal and shallow waters, as at any one time only 17 % of birds are feeding there. The influence of the manne environment and energetic constra~nts on the evolution of provisioning strategies is discussed.