Nest Parasitism in Goldeneyes Bucephala clangula: Some Evolutionary Aspects

Abstract

Intraspecific nest parasitism, in which parasitic females lay eggs in the nests of host females of the same species, occurs in many birds and certain insects. Most known cases concern waterfowl (Anatidae). Host responses to experimental parasitism, adding eggs to nests of laying goldeneye ducks was examined. Three different hypotheses on host responses yield mutually exclusive predictions. (1) The host is a determinate layer; her own clutch is unaffected by the number of parasitic eggs. (2) The host is an indeterminate layer and responds to parasitic eggs as to her own. She reduces her clutch by the number of eggs added by the parasite. (3) The host is an indeterminate layer and estimates the number of parasitic eggs. She responds in an optimal fashion and reduces her own clutch with approximately half the number of parasitic eggs, hence maximizing the production of host offspring. With 2 + 2 eggs added on adjacent days early during the host''s laying period, prediction (2) is substantiated. Goldeneyes then respond to parasitic eggs as to their own, and tend to adjust their final clutch size to a given total number of eggs. The host did not lay fewer eggs if experimental eggs were added late during her laying period. Experimental parasitism with 1 or 2 + 2 eggs during the laying period did not raise the frequency of nest desertion. When 7 eggs were added on the same day, no clutch was incubated, indicating that the host female detected the parasitism and abandoned the nest. This might be adaptive, since a large proportion of the clutch would not be her own. When parasitism occurs early during the laying period of the host, the relative reduction in host chick production equals the proportion of parasitic eggs in the nest. The reproductive success of the parasite should increase in proportion to how early she begins laying in the host nest. Intraspecific nest parasitism has evolved mainly among ducks because suitable nest sites are often rare, the nest is easy to locate, female philopatry makes it likely that host and parasite are genetically related and ducks do not defend their nest during the laying period.