Opportunity is not everything: genetic monogamy and limited brood parasitism in a colonial woodpecker

Abstract: Avian social and genetic mating systems are influenced by parental care roles as well as by reproductive opportunities. Alternative reproductive tactics, including conspecific brood parasitism and extra-pair mating, are predicted to be most common when females have access to potential host nests and when adults have access to potential mating partners, respectively. We tested these predictions in the facultatively colonial Hispaniolan woodpecker (Melanerpes striatus), a socially monogamous species with biparental care. Up to 12 pairs may nest concurrently in the same tree or, less frequently, two adjacent trees, potentially facilitating both conspecific brood parasitism and extra-pair mating. Contrary to our predictions, genotyping with single-nucleotide polymorphisms (SNPs) revealed no evidence of extra-pair paternity in either solitary or colonial nests: all 307 nestlings in 101 broods were matched to their social parents. Two instances of apparent conspecific brood parasitism were detected via changes in clutch size, but these could not be confirmed genetically since none of the apparently parasitic eggs survived to hatching. Therefore, if conspecific brood parasitism did occur, it was not a successful route to reproduction: parents fledged only their genetic offspring. These results suggest that reproductive opportunities alone are insufficient to favor alternative reproductive tactics, and that genetic monogamy can persist despite locally high densities of breeding pairs. Other life-history traits, including high levels of nest attendance and male parental care, may constrain parasitism and extra-pair mating in this long-lived tropical species. Significance statement: High breeding density, a feature of colonial nesting, should increase opportunities for infidelity and conspecific brood parasitism because of the close proximity of potential extra-pair mates and parasitic females. Yet, we found that Hispaniolan woodpeckers, which nest both solitarily and colonially (two or more pairs in the same tree) in the same population, were genetically monogamous and lacked successful brood parasitism. Colonial nesting is exceptionally rare in the woodpecker family, which is also characterized by high investment in male parental care, including nocturnal incubation. The essential role of paternal care for successful reproduction in the Hispaniolan woodpecker might have selected against extra-pair mating and conspecific brood parasitism despite the apparent ample opportunities provided by nesting so close to others.