Trophic Interactions in South West Atlantic Seabird Colonies

Abstract

Thesis (Ph.D.)--University of Washington, 2012Seabirds act as both predators and as prey. In the marine environment, seabird foraging patterns can reflect ocean conditions. On land, their breeding success reflects constraints placed on seabirds by central-place foraging, as well as aspects of the evolution of coloniality. In Chapter 1, I investigate whether differences in marine productivity influence foraging and reproductive success at two Magellanic Penguin (Spheniscus magellanicus) colonies--New Island (Falkland Islands / Islas Malvinas) and Punta Tombo (Argentina). Magellanic Penguins reflected ocean productivity in both their foraging and breeding patterns. During the late chick stage, ocean productivity was higher at New Island, and foraging areas were smaller. Accordingly, New Island penguins had shorter foraging trips and higher reproductive output. Plasticity in their natural history allows seabirds to balance self-maintenance, foraging and reproductive success under variable ocean conditions. In Chapter 2, I explore the causes and consequences of albatross chicks off their nests at Steeple Jason Island, Falkland Islands. In Black-browed Albatross (Thalassarche melanophrys), chicks that prematurely end up outside their nests typically die. The number of chicks off their nests increased with time, and correlated negatively with adult nest attendance. Off-nest chicks on the colony edge were more likely to be found adjacent to other off-nest chicks. Off-nest chicks with injuries or feathers matted with stomach oil regurgitate occurred primarily on the colony edge and in clusters. These patterns were most consistent with predation or attempted predation by Southern Giant Petrels (Macronectes giganteus). In Chapter 3, I examine resource use overlap between two avian predators at a mixed seabird colony in the Falkland Islands. Striated Caracaras (Phalcoboenus australis) and Falkland Skuas (Stercorarius antarctica antarctica) are locally on Steeple Jason Island. Both predators ate the same prey types--eggs and chicks of albatross and of penguins. Food was not limiting during the breeding season, and interspecific fights were rare. Both species foraged in similar colony locations, times of day, and stages of the season. Any proportional preferences in their prey preferences appear to derive from behavioral or morphological differences between the two predators