Intraspecific nest parasitism in the barnacle goose: behavioural tactics of parasites and hosts

Abstract

Intraspecific nest parasitism in the barnacle goose,Branta leucopsis, was recorded by direct observations of females trying to lay eggs in the nests of other females. This was observed on 36 occasions. Parasitic egg-laying attempts were observed both in mornings and evenings, and lasted on average at least 20min. Parasitic females approached host nests very fast and immediately sat down on or close to the nest. Host females attacked parasitic females intensively, but host males were much less aggressive. Males paired to the parasitic females were sometimes seen, but they never took any active part in the parasitic egg-laying attempts. Parasitic females probably successfully laid an egg most of the times, as the clutch size in host nests was on average 0·9 eggs larger than in nests where parasitic egg-laying attempts were not observed. Host females were observed to retrieve eggs laid outside the nest cup. Of 27 known cases, parasitic females made their egg-laying attempts before or at the host's start of incubation on 12 occasions, and after the start of incubation 15 times. It is suggested that parasitic females exploited features in the behaviour of potential hosts, such as egg retrieval and low aggressiveness in host males, to succeed in their egg-laying attempts. Nest parasitism seems to be a facultative, ‘best-of-a-bad-job’ tactic in barnacle geese, as parasitic females were observed to have nests of their own before or after the year they behaved parasitically, but never in that particular year.