Ashmole's Halo and Population Regulation in Seabirds

Abstract

Ashmole's halo" is the term given to a zone of food depletion thought to surround the concentrations of predatory birds that exist in large seabird colonies.Philip Ashmole developed the hypothesis during studies of tropical terns.However, the idea has been widely applied outside the tropics, and most evidence for the hypothesis has been based on studies of high-latitude seabirds.We modelled some expectations for measurable variables, including foraging range, food availability and offspring development in relation to colony size for four seabird types (based on characteristics of the genera Uria, Fratercula, Puffinus and Pterodroma) and tested the model predictions against currently available data.We conclude that, although there is evidence for the existence of a zone of food depletion around seabird colonies, our model suggests that this zone is unlikely to be detectable for small colonies and especially for colonies of far-ranging species such as petrels and shearwaters.The hypothesis can be tested more effectively by measuring how food, feeding rates and feeding behaviour vary with distance from a colony than by comparing foraging at colonies of various sizes.