Conspecific brood parasitism (CBP) is a reproductive strategy in which females lay eggs in the nests of other conspecific females. This behavior occurs in many species of birds, fishes, amphibians, and insects. CBP is intriguing because females laying eggs parasitically do not incur the costs associated with parental care; instead hosts (recipients of parasitic eggs) incur the costs of raising these parasitic offspring. The factors that influence females to lay eggs parasitically are unclear, and few studies have examined the role of maternal effects in CBP (parasitic eggs may contain substances increasing offspring survival). Here I investigate CBP in ruddy ducks (Oxyura jamaicensis), an over-water nesting species, where females lay large expensive eggs to produce highly precocial offspring (requiring minimal parental care).

First I identify parasitic offspring and the females that produce them using a combination of two molecular techniques—egg albumin protein fingerprinting (a maternal marker) and a large number of nuclear microsatellite loci. CBP was common in ruddy ducks (64% of nests contain parasitic offspring) and females used a mixed reproductive strategy (laying eggs parasitically in addition to nesting).

Next, I investigate whether kin selection might play a role in the evolution of CBP in ruddy ducks, given hosts and parasites may be genetic relatives (via high female natal philopatry). I use molecular markers to estimate relatedness among all females and then specifically between hosts and parasites. I found that female ruddy ducks showed no relatedness structure (neighbors were unrelated). Relatedness among host-parasite pairs was relatively low and not significantly different than background levels of relatedness in the population, thus kin selection is not a likely explanation for the evolution of CBP in ruddy ducks.

Third, I investigate possible parasite adaptations (via maternal effects), realized through egg characteristics. Steroid hormone content and egg size between parasitic and non-parasitic eggs did not differ; however, parasitic eggs were more likely to be male.

Finally, I discuss broad conclusions for the patterns of CBP in ruddy ducks with respect to other systems. In addition I suggest possible questions for further investigation of CBP as reproductive strategy in populations.