Environmental constraints on the foraging behaviour, spatial usage and population sizes of albatrosses

Abstract

Satellite-tracking of wide-ranging, apex marine predators, combined with remote-sensing, can be used to test ecological hypotheses and to estimate spatial abundance. I used this approach to quantify the habitat usage of central place foraging black-browed albatrosses (BBA) from nine colonies, modelling population-level distribution as a function of habitat accessibility, habitat preference and conspecific competition. Throughout breeding, BBA preferred neritic waters, steeper bathymetry, and, during incubation, warmer sea surface temperatures. BBA from South Georgia also preferred highly dynamic oceanic waters. Foraging areas were partially spatially segregated with respect to colony and region, presumably to reduce intraspecific competition. Although such competition is often invoked to explain observed colony sizes, by accounting for travel costs, I demonstrate a strong relationship between the sizes of regional populations and the availability, accessibility and productivity of neritic waters, supporting the hypothesis that seabird populations are constrained by breeding season food availability. In response to this constraint, albatrosses have evolved to exploit energetically efficient gliding flight, allowing them to access prey 100-1000s of km from their colonies. Hence, I used satellite tracking and activity data to quantify the effects of relative wind speed on the flight speed of four albatross species. Groundspeed was linearly related to the wind speed in the direction of flight, its effect being greatest on wandering albatrosses, followed by BBA, light-mantled and grey- headed albatrosses, and airspeeds were higher in males than females. Commuting birds tended to encounter headwinds during outward trips and tailwinds on their return, such that return trips were faster. This supports the hypothesis that foraging upwind of the colony is more efficient but could also result from wind climate and the relative location of prey. The ability to use tracking data to estimate spatial usage is timely given the acute threat currently posed to albatrosses by incidental fisheries mortality.