Cases of foreign egg adoption by Brown Skuas Stercorarius antarcticus on Marion and Gough Islands

Ground-nesting birds, such as large gulls and skuas, are known to include a variety of items in their nests as pseudo-eggs, as well as to adopt the eggs of conspecifics or other species. Three hypotheses have been proposed to explain this phenomenon: incubation stimulus, mistaken food, and mistaken egg hypotheses. Of 657 Brown Skua Stercorarius antarcticus nests surveyed on Marion Island during the 2017/18 breeding season, two instances of egg adoption were recorded. Another case of egg adoption was found in a Brown Skua nest on Gough Island in the 2018/19 breeding season. It is likely that all three were cases of mistaken food. This study adds further evidence to the assumption that avian egg predators such as skuas and gulls which are able to steal and carry an egg unbroken in flight, are at risk of this maladaptive behaviour.