Conspecific Brood Parasitism

In conspecific brood parasitism (CBP), parasitic females lay eggs in the nests of other females of the same species, who care for the offspring. CBP is particularly common in ducks and other waterfowl, which differ from most birds in that females, not males, return and breed in their birth area. The local females may therefore be related, creating scope for cooperation and kin selection among related hosts and parasites. Theory suggests that this may be a contributing reason why CBP is particularly common in waterfowl. Using protein fingerprinting of eggs, a non-destructive technique of albumen sampling combined with sensitive protein electrophoresis (isoelectric focussing), it was found that host-parasite pairs in goldeneyes are more closely related than other females, that individual recognition is probably involved, and that females combining parasitism with normal nesting can greatly increase their reproductive success. Defences against Brood Parasitism Birds parasitized by interspecific brood parasites often adopt defences based on egg recognition, but such behaviors are puzzlingly rare in species parasitized by members of the same species. Conspecific egg recognition is frequent, accurate and used in three defences that reduce the high costs of conspecific brood parasitism in American coots. During a study of American coots, chick starvation was ubiquitous: 98% of all nests (n = 177) lost at least one chick, and on average, each nest lost 52% of its chicks, primarily to starvation. Although parasitism increased the total number of chicks that hatched at host nests, it had no effect on the total number fledged, owing to complete compensatory chick mortality. Together, these patterns indicate that each successful parasitic chick survives at the expense of a host chick—a one-for-one substitution. This high cost per successful chick, combined with the high frequency of parasitism, should result in strong natural selection for effective host defences to reduce the costs of parasitism. Hosts recognized and rejected many parasitic eggs, reducing the fitness costs of parasitism by half. Recognition without rejection also occurred and some hosts banished parasitic eggs to inferior outer incubation positions. Clutch size comparisons revealed that females combine egg recognition and counting to make clutch size decisions--by counting their own eggs, while ignoring distinctive parasitic eggs, females avoid a maladaptive clutch size reduction. This is clear evidence that female birds use visual rather than tactile cues to regulate their clutch sizes, and provides a rare example of the ecological and evolutionary context of counting in animals. Further reading Andersson, M. 2001. Relatedness and the evolution of conspecific brood parasitism. Amer. Natur. 158, 599-614. Åhlund, M. and M. Andersson. 2001. Female ducks can double their reproduction. Nature 414, 600-601. Lyon B E. 2003. Egg recognition and counting reduce costs of avian conspecific brood parasitism. Nature. (Apr 3) 422(6931):495-499.

Adapted from web pages: Malte Andersson, professor of Animal Ecology Egg recognition and counting reduce costs of avian conspecific brood parasitism.