Obligate brood parasites depend entirely on other species to raise their offspring. Most avian obligate brood parasites have altricial offspring that require enormous amounts of post-hatching parental care, and the large fecundity boost that comes with complete emancipation from parental care likely played a role in the independent evolution of obligate parasitism in several altricial lineages. The evolution of obligate parasitism in the black-headed duck, however, is puzzling because its self-feeding precocial offspring should not constrain parental fecundity of a potential brood parasite in the way that altricial offspring do. We used an experimental nest predation study to test the idea that high nest predation rates played a role in the evolution of brood parasitism in this enigmatic duck. Experimental duck eggs in untended nests suffered massive rapid predation, while eggs in tended nests of the three main hosts, all aggressive nest-defenders, had very high success, illustrating the benefits of parasitizing these 'bodyguard' hosts.

Painted hen eggs were added to (1) artificial nests placed in a wetland in Argentina and (2) to active nests of three host species placed in a nearby wetland, in the same year (1994). We checked nests regularly to determine whether the nest (real or fake) still contained any eggs or had been depredated. We then recorded for each nest either the number of days until the nest was depredated (start of experiment is day 0) or for nests that survived, the date of the last nest check. Nests that survived to the end of the experiment were all censored for the survival notes.

The data file is an Excel data file. To analyze the data we imported the data into JMP and used Cox proportional hazard survival analyses to compare survival rates of the different treatments. We considered nests no longer surviving when all eggs had disappeared. We also examined patterns of partial predation (not all eggs taken) in terms of whether complete vs partial predation differed among treatments.