Behavioral ecology of juvenile brown-headed cowbirds: implications for the evolution and maintenance of avian brood parasitism

[摘要] Avian interspecific brood parasites are species that forego parental care and rely on heterospecifics to raise parasitic offspring. Host-parasite coevolutionary arms races and the constraints associated with a non-parasitic bird transitioning to a parasite likely contribute to the rarity of this reproductive strategy (ca. 1% of species). For my dissertation, I conducted research on juvenile brown-headed cowbirds (Molothrus ater) to elucidate the factors that contribute to the evolution and persistence of avian brood parasitism. To avoid adopting the behaviors and mate choice preferences (sexual imprinting) of the foster species, juvenile cowbirds could theoretically follow adult female cowbirds away from the host’s care to avoid mis-imprinting on their host’s phenotype. Using an automated radio telemetry system to identify the presence-absence (every 1-2 minutes) within a forest for juvenile and adult female cowbirds, I concluded that juveniles typically depart the host’s care solitarily. Therefore, innate segregation behaviors likely facilitate the avoidance of mis-imprinting on the host’s phenotype. Next, I used a prothonotary warbler (Protonotaria citrea) nest box dataset, comprising 21 years (1994-2014), to demonstrate that fledgling cowbirds provide reliable information indicating host quality that is used in future egg-laying decisions of adult female cowbirds. Both site-wide cowbird productivity and experimental egg removal indicated that reproductive performance information of cowbirds affects the probability of parasitism during the following nesting attempts within and between years. This result suggests that by avoiding poor hosts (e.g. egg rejecters), cowbirds could increase their reproductive success and delay the development of anti-parasite strategies in hosts. Finally, I demonstrated that fledging a cowbird in the first brood increases the likelihood for female warblers to initiate a second brood within the same season (i.e. double-brooding). I established that non-parasitized warblers respond to natural fecundity reduction (i.e. hatching failure and nestling death) with increased frequency of double-brooding. Then, I showed that raising a brood parasite induces compensatory double-brooding for female warblers by parasite-induced fecundity reduction. Consequently, hosts can recoup some fitness lost to infection (i.e. compensatory double-brooding) and parasites can enhance transmission by gaining additional parasitism opportunities, further delaying the development of anti-parasite strategies and avoiding the initiation of host-parasite arms races.

[效力级别] Virulence

[关键词] Brood parasitism;Virulence;Host manipulation;Coevolution;Host resistance;Host tolerance;Learning