No evidence that endohelminth parasites cause selection against hybrid orioles across the Baltimore–Bullock’s Oriole hybrid zone

Abstract

Abstract The Baltimore–Bullock’s Oriole hybrid zone is one of the best-studied avian hybrid zones in North America, yet our understanding of the causes of selection against hybrids remains poor. We examine if endohelminth parasites may cause selection against hybrid orioles but found no evidence for this hypothesis. Of the 139 male orioles we examined, 43 individuals contained endohelminth parasites from at least 1 of these groups: Cestoda, Acanthocephala, or Nematoda. Across the hybrid zone, Baltimore Orioles (Icterus galbula) and Bullock’s Orioles (I. bullockii) differed in their parasite communities, such that Baltimore Orioles frequently contained both Acanthocephala and Cestoda parasites whereas Bullock’s Orioles primarily contained Cestoda parasites. Despite these differences in parasite communities between parental species, the frequency of hybrid orioles with parasites was similar to parentals, suggesting that hybrids were as susceptible to endohelminth parasites as parentals. Using a subset of 99 adult male orioles, we explored how parasites may be associated with the expression of orange carotenoid-based plumage in hybrids and parentals. Associations between carotenoid-based plumage color and parasites were most strongly expressed in Bullock’s Orioles, but patterns were subtle and counterintuitive because individuals with parasites often had more enhanced color measures compared to individuals without parasites. Taken together, these data suggest that endohelminth parasites impose little fitness costs to male orioles on the breeding grounds and likely do not cause selection against hybrids.