The Black-headed Duck (Heteronetta atricapila) of South America is an obligate brood parasite — it is a 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. The ducklings are among the most independent baby birds in the world— they leave the host nest upon hatching and feed themselves, so the hosts only provide incubation. The photo above shows a pair of ducks framing their most important host, a Red-gartered Coot.

Photos below shows a coot nest with two white duck eggs that stick out like a sore thumb and a day old duckling ready to head off on its own.


We made several discoveries:

1) The ducks are dependent on surprisingly few host species. Although the ducks lay eggs in a diversity of host species nests, only a couple of these turn out to be important. Two species of coots are particularly important hosts (red-gartered and red-fronted coots) while the brown-hooded gull is somewhat important. In our population, 85% of ducklings hatch from eggs laid in coot nests (Lyon & Eadie 2004)

Photo below shows several hosts and potential hosts. The two coots are the most important hosts; gulls somewhat less so. Snail kites are heavily parasitized but unimportant hosts because they are rare. Southern Screamers are abundant but rarely parasitized, and Snowy-crowned Terns are fairly abundant but never parasitized, despite nesting in mixed colonies with Brown-hooded Gulls, a moderately parasitized species. We interpret the latter pattern as evidence that the ducks are sophisticated birdwatchers! (Lyon and Eadie 2013)


2) Apparent signatures of interspecific co-evolution actually result from within host antagonism. We were surprised to observe apparent signatures of antagonism between species—the two coot species reject many duck eggs. 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 even though the coots reject their conspicuous eggs. We suggest that within species brood parasitism in the coots themselves 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 (Lyon & Eadie 2004). 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 (Lyon et al. 2015).

The photo below shows the egg series we used in our mimicry experiments.


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 soar 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 2020).

The photo below shows a typical bulrush dominated wetland at our study site.