Ecological and social constraints on conspecific brood parasitism by nesting female American coots ( Fulica americana )

1. In a population of American coots ( Fulica americana ) breeding in central British Columbia, Canada, some females pursued a reproductive strategy that combined nesting with laying parasitic eggs in the nests of conspecifics. To understand why only one quarter of the nesting females laid parasitically, I examined social and ecological factors that could potentially constrain nesting females from engaging in brood parasitism. 2. Variation among females in each of three prerequisite conditions essential for adaptive parasitism to occur − host availability, access to host nests and a benefit to allocating eggs to parasitism − helps explain why not all females are parasitic. 3. Due to limited spatiotemporal patterns of parasitism, an estimated 23% to 39% of non-parasitic females completely lacked potential hosts to parasitize when they began breeding. In theory, females could avoid host limitation by delaying breeding to wait for hosts, but timing of breeding comparisons did not support this idea. Relative to the vegetation density on their territories, parasites and non-parasites began laying eggs on the same date but, due to the time taken to lay parasitic eggs, parasites initiated their own nests later. 4. Brood parasitism was less likely to occur between dyads of females where the potential host female was substantially larger than the potential parasitic female, which suggests that antagonistic social interactions between hosts and parasites may constrain some females from parasitism. 5. Comparisons of two classes of non-parasitic females, those with and without hosts, revealed smaller clutch sizes in the former, suggesting that limited fecundity may have constrained some of these females from laying parasitic eggs. Additionally, brood parasites were older and laid more total eggs than non-parasitic females and, among parasites, older females laid more parasitic eggs. Together, these patterns suggest that variation in egg-laying capacity may determine whether females benefit from allocating eggs to parasitism. 6. The large number of correlates of parasitism indicates that parasitism by nesting females is a conditional reproductive tactic, not part of a stochastic mixed evolutionary stable strategy.