The ecology and evolution of conspecific brood parasitism in American coots (Fulica americana)

I studied the adaptive significance of conspecific brood parasitism in American coots (Fulica americana) breeding in central British Columbia, Canada. Over 40% of all pairs were parasitized during the four year study, and parasitism occurred in two basic ecological contexts. One quarter of the parasitic eggs were attributed to non-nesting females that adopted parasitism as a low-paying alternative to not breeding at all. Three quarters of the parasitism was by nesting females that also laid eggs parasitically. The brood size most pairs raised in their own nest was constrained by post-hatching parental care, not by the egg-laying capacity of females, and parasitism allowed nesting females to bypass this constraint and increase their total production of offspring.;Parasitism was costly to hosts and each parasite chick was raised at the expense of a host offspring. To mitigate these high costs, hosts adopted several defensive tactics, most of which were based on egg recognition. Egg rejection was the most effective host defense and reduced the costs of parasitism by almost half. Parasites also adopted tactics to enhance the success of their parasitic eggs, but none of these appear to be counter-tactics to overcome host defences. Although parasites did not select high quality hosts to parasitize, the pattern of egg dispersion among host nests by individual parasites indicates that sources of offspring mortality have shaped the evolution of parasite tactics to some extent.;To understand why only one quarter of the nesting females were parasitic, I determined the importance of potential ecological and social constraints on parasitism. Host availability and social interactions between hosts and parasites appear to be important. A graphical model was used to illustrate the cost-benefit tradeoffs between parasitism and nesting, and to identify ecological factors that could cause nesting females to differ in the net benefits of parasitism. Parasites tended to be older females and several lines of evidence suggest that this pattern occurs because older females are more fecund. The fact that female coots use several environmental cues to adjust their parasitic behavior indicates that parasitism is a finely-tuned reproductive tactic.