Determining the body condition of short-tailed shearwaters: implications for migratory flight ranges and starvation events

Short-tailed shearwaters Puffinus tenuirostris migrate annually from breedi ng areas in southeast Australia and Tasmania to the Bering Sea to feed on a bundant prey aggregations, mainly euphausiids, Occasionally thousands of sh earwaters die of starvation en route, within, or on return from the Bering Sea, Collection of live and dead shearwaters in the southeastern Bering Sea in 1997, 1998, and 1999 allowed us to measure seasonal changes in energy r eserves during a major mortality event, As birds lost body mass, lipid mass initially decreased faster than that of pectoralis muscle, but loss of pec toralis mass increased markedly at a body mass around 500 g when lipids wer e almost depleted (similar to 33 g remaining). Death occurred as body mass approached 426 g. Individuals near this body mass had lipid values permitti ng estimated flight ranges of 140 to 400 km, a range less than that potenti ally covered in 1 d by shearwaters searching for prey (440 to 1124 km d(-1) ). Seasonal differences in body composition were most striking among body a nd bone marrow lipid contents, with the lowest values occurring during the die-off in fall 1997 and in fall 1998. The lack of shearwater mortality in fall 1998 may have resulted from more consistent winds that decreased fligh t costs and from greater availability of alternative fish prey. Our data al low estimates of usable energy stores and flight ranges based on lipid rese rves in short-tailed shearwaters. Estimated flight ranges suggest that if f eeding conditions are poor near Japan or near other termination points of t he transequatorial migration routes shearwaters may have few reserves avail able to support foraging for food and starvation events may occur, Our find ings suggest how their energetic strategies and migration are shaped by sea sonal and annual variability of prey during transglobal movements of short- tailed shearwaters between oceanic regions.