Do Canada Geese Branta canadensis affect the breeding success of Red-throated Diver Gavia stellata and Black-throated Diver G. arctica?

Abstract

During the latest decades, Canada Geese have increased in numbers and expanded over large parts of north-western Europe. Hitherto, the risk for adverse effects on the native bird fauna has been assessed to be negligible, although there are single records of divers (Gavia spp.) having failed or abandoned the breeding site after the establishment of Canada Geese. Therefore, we initiated an inquiry about breeding success and nest locations of divers and geese in Sweden. For Red-throated Diver, we compared the breeding success for pairs nesting at tarns with and without breeding Canada Geese. We found no indication of any negative effects on breeding success: The average production was 0.82 “large” chicks per pair and year for pairs at tarns with breeding Canada Geese, compared to 0.75 at tarns without breeding geese. For Black-throated Diver, the average hatching success for pairs with and without nesting Canada Geese on the same islet was 46% and 47%, respectively. But the average distance to the closest goose nest (on another islet) was lower for failed nests compared to successful ones. We judge, that the expansion of the Canada Goose is not a threat on a national or regional scale, although single diver pairs certainly suffer from having Canada Geese as close neighbours. Among both species of divers there are also indications of adaptations to the geese, e.g. through moving to new nesting sites at the same breeding lake. Finally, potential benefits from the presence of Canada Geese, e.g. if the geese keep other potential predators away, should not be overlooked.