The black-headed duck (Heteronetta atricapila) of South America is an obligate brood parasite -- it is duck with the lifestyle of a cuckoo. Our four-year study in Argentina (with John Eadie at UC Davis) focused on host-parasite interactions in this precocial brood parasite to contrast with the large body of literature on altricial brood parasites like cuckoos and cowbirds.

We made several discoveries:

1) The ducks are dependent on surprisingly few host species. The ducks lay egg in a diversity of hosts but only a couple of these turn out to be important in terms of contributing to the population. Two species of coots are particularly important red-gartered coots (Fulica armillata) and red-fronted coot (F. rufifrons); brown-headed gulls (Larus maculipennis) somewhat less so. In our population, 85% of ducklings hatch from eggs laid in coot nests. Chinese Birds paper

2) Apparent signatures of interspecific co-evolution actually result from within host antagonism. We were surprised to observe apparent signatures of antagonistic co-evolution between species, such as rejection of duck eggs by the two coots. We found no evidence that duck parasitism is costly to coots. Paradoxically, egg mimicry experiments showed that the ducks would not benefit from having more mimetic eggs. We suggest that within species brood parasitism in the coots caused the natural selection for recognition and rejection of parasitic coot eggs. The rejection of the duck eggs appears to be an incidental byproduct of the strife within the hosts themselves—the ducks appear to be trapped in their hosts’ arm race. We recently provided additional support for this interpretation with experiments that show that a species parasitized solely by conspecific parasites (American coots in Canada) show identical rejection patterns to the Argentina coots.  Nature paper, BC paper.

3) Being trapped in somebody else’s arms race can be risky. The story has an unexpected conservation twist: rejection rates of duck eggs by the coots soars under some conditions, like flooding and vegetation loss. This in turn leads to a catastrophic decline in the reproductive success of the duck. However, because selection from within the hosts drives egg rejection behavior, the ducks seem to lack any feasible evolutionary responses that could reduce these negative effects (Eadie & Lyon MS).