"There doesn't seem to be much if any cost to the host species, so you wouldn't expect there to be much pressure on the hosts to evolve defenses against this kind of parasitism," said Bruce Lyon, a professor of ecology and evolutionary biology at the University of California, Santa Cruz.
When Lyon and John Eadie of UC Davis set out to study the black-headed ducks, they expected to find a highly successful brood parasite, unopposed by the antagonistic strategies that host species deploy against more costly parasites like cuckoos and cowbirds. Instead, they found that black-headed duck eggs are often rejected from host nests, and it took four years of detailed field research to figure out why. Lyon and Eadie published their findings in the November 18 issue of the journal Nature.
The key breakthrough was their discovery that each of the black-headed duck's two main host species--the red-gartered coot and red-fronted coot--were busy parasitizing the nests of their own species. The black-headed ducks were being thwarted by defenses that had evolved as a result of brood parasitism among the coots themselves.
The researchers carefully investigated the costs of duck parasitism to the hosts and found few costs that could be prevented by rejection of the duck eggs. In contrast, the intraspecific (within the same species) brood parasitism taking place among the coots is very costl
Contact: Tim Stephens
University of California - Santa Cruz