FORAGING ECONOMICS AND PERFORMANCE OF POLAR AND SUBPOLAR ATLANTIC SEABIRDS

Seabirds of high latitudes in the North and South Atlantic (chiefly penguins, Procellariformes, alcids, shags, Gannet and Kittiwake) are compared (on absolute and energy-, mass- and time-specific scaled bases) in terms of the rate at which they supply energy to their offspring, the rate of offspring growth, and the duration of the dependence (fledging) period. For a smaller suite of species, time and energy budgets during complete foraging cycles (including time ashore) and while at sea arc compared. The broad-scale comparisons show storm petrels to have consistently low provisioning and growth rates, and Kittiwakes, Gannets, shags and some penguins to have consistently high rates. Penguins (except the Gentoo Penguin) and albatrosses spend most of a foraging cycle at sea; murres, shags, gannet and kittiwake spend at least half the time ashore, guarding their offspring. Energy budgets are much more similar, because of thc disproportionate cost of at-sea activities, although the time spent flying, swimming, resting, and diving varies widely between species and is often difficult to interpret in terms of active foraging. Other apparent anomalies include the large amount of time Common Murres spend resting at sea and the high resting and low flight metabolic rates of kittiwakes and gannets. Assessments of foraging performance need to be more broadly based than hitherto and to take account of both physical constraints and ecological contexts. Further development of these approaches, especially critical interspecies comparisons, requires better discrimination of activities at sea, measurement of activity-specific energy costs and more accurate data on provisioning rates to offspring, particularly of North Atlantic species, notably Gannets and shags.