Some 100 species of birds are what scientists call "obligate brood parasites"--instead of building nests and raising their own young, they lay their eggs in the nests of other species and let those birds do the hard work of parenting for them. The black-headed duck of South America is one of these, but it stands out from all the others in a striking way. Black-headed ducks dont need any parental care other than incubation for their eggs--the ducklings leave the nest one day after hatching and paddle off into the reeds to fend for themselves.
"There doesnt seem to be much if any cost to the host species, so you wouldnt 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 ducks 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.
Tim Stephens | EurekAlert!
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