INTRASPECIFIC NEST PARASITISM IN THE BARNACLE GOOSE - BEHAVIORAL TACTICS OF PARASITES AND HOSTS

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. Parasit ic egg-laying attempts were observed both in mornings and evenings, an d lasted on average at least 20 min. Parasitic females approached host nests very fast and immediately sat down on or close to the nest. Hos t females attacked parasitic females intensively, but host males were much less aggressive. Males paired to the parasitic females were somet imes seen, but they never took any active part in the parasitic egg-la ying attempts. Parasitic females probably successfully laid an egg mos t of the times, as the clutch size in host nests was on average 0.9 eg gs larger than in nests where parasitic egg-laying attempts were not o bserved. Host females were observed to retrieve eggs laid outside the nest cup. Of 27 known cases, parasitic females made their egg-laying a ttempts before or at the host's start of incubation on 12 occasions, a nd after the start of incubation 15 times. It is suggested that parasi tic females exploited features in the behaviour of potential hosts, su ch as egg retrieval and low aggressiveness in host males, to succeed i n their egg-laying attempts. Nest parasitism seems to be a facultative , 'best-of-a-bad-job' tactic in barnacle geese, as parasitic females w ere observed to have nests of their own before or after the year they behaved parasitically, but never in that particular year. (C) 1995 The Association for the Study of Animal Behaviour