Aggression towards egg-removing cowbird elicits clutch abandonment in parasitized yellow warblers, Dendroica petechia

The proximate causes triggering nest abandonment have important implications in the host–parasite arms race. Cowbird parasitism and rejection of parasitism are costly to some hosts; therefore, cues affecting their responses have important evolutionary implications. Experimental addition of a cowbird egg to the nest of a yellow warbler, a host that abandons nests parasitized by cowbirds via burial or desertion, elicited a rejection frequency similar to that of naturally laid cowbird eggs. Eggs experimentally added before sunrise (time of natural parasitism) were rejected at the same frequency as eggs added after sunrise; therefore, timing of parasitism did not influence rejection. Interaction with a robotic egg-removing cowbird increased the probability of abandonment, and the most aggressive individuals were likely to bury the model cowbird egg. Individual behaviours, therefore, were correlated across contexts. Host–parasite interactions and aggression are the two most important components in host defence. Future work should focus on the physiological aspects of individual variation, such as differences in hormone levels between individuals that accept and reject parasitized clutches.