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19 August 2022 No evidence that endohelminth parasites cause selection against hybrid orioles across the Baltimore–Bullock's Oriole hybrid zone
Vanya G. Rohwer, Lea M. Callan, John M. Kinsella, Russell A. Ligon
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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.


  • Baltimore and Bullock's orioles regularly hybridize where their breeding ranges overlap in the Great Plains, USA.

  • Despite years of study, we have a poor understanding of why hybrids perform poorly.

  • We examined if hybrid orioles might suffer from internal parasitic worms, as hybrids may have compromised immune systems making them more susceptible to parasites.

  • We examined for lethal effects of parasites inferred through different ratios of infected vs. uninfected orioles and we examined non-lethal effects inferred through the quality of their orange plumage color.

  • Our findings suggest that while Baltimore and Bullock's orioles differ in their parasite communities, internal parasitic worms present little to no costs to hybrid orioles.

La zona híbrida de Icterus galbula-I. bullockii es una de las zonas híbridas aviares mejor estudiadas en América del Norte, pero nuestra comprensión de las causas de la selección contra los híbridos sigue siendo deficiente. Examinamos si los parásitos endo-helmínticos pueden causar selección contra oropéndolas híbridas, pero no encontramos evidencia para esta hipótesis. De las 139 oropéndolas macho que examinamos, 43 individuos contuvieron parásitos endo-helmínticos de al menos uno de estos grupos: Cestoda, Acanthocephala o Nematoda. A lo largo de la zona híbrida, los individuos de Icterus galbula e I. bullockii difirieron en sus comunidades de parásitos, de modo que Icterus galbula frecuentemente albergó parásitos de Acanthocephala y Cestoda, mientras que I. bullockii albergó principalmente parásitos de Cestoda. A pesar de estas diferencias en las comunidades de parásitos entre especies parentales, la frecuencia de oropéndolas híbridas con parásitos fue similar a la de los parentales, lo que sugiere que los híbridos fueron tan susceptibles a los parásitos endo-helmínticos como los parentales. Usando un subconjunto de 99 oropéndolas macho adultas, exploramos cómo los parásitos pueden estar asociados con la expresión del plumaje basado en carotenoides anaranjados en híbridos y parentales. Las asociaciones entre el color del plumaje basado en carotenoides y los parásitos se expresaron con mayor fuerza en I. bullockii, pero los patrones fueron sutiles y contrarios a la intuición, porque los individuos con parásitos a menudo tuvieron medidas de color más realzadas en comparación con los individuos sin parásitos. Tomados en conjunto, estos datos sugieren que los parásitos endo-helmínticos imponen pequeños costos de adecuación a los machos de oropéndola en las áreas reproductivas y probablemente no causen selección contra los híbridos.

Copyright © American Ornithological Society 2022. All rights reserved. For permissions, e-mail:
Vanya G. Rohwer, Lea M. Callan, John M. Kinsella, and Russell A. Ligon "No evidence that endohelminth parasites cause selection against hybrid orioles across the Baltimore–Bullock's Oriole hybrid zone," Ornithology 139(4), 1-13, (19 August 2022).
Received: 14 June 2022; Accepted: 9 August 2022; Published: 19 August 2022
Baltimore Oriole
Bullock's Oriole
endohelminth parasites
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