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Open Access Publications from the University of California

Parasites, Immune Response, and Sexual Selection in Western Bluebirds

  • Author(s): Jacobs, Anne Cassiope
  • Advisor(s): Zuk, Marlene
  • et al.

The Hamilton-Zuk hypothesis proposes that females choose elaborately ornamented males as mates because such ornaments indicate parasite resistance. Females benefit from selecting resistant males by passing resistance genes on to the offspring. Many studies have tested this hypothesis using immunity as a proxy for parasite resistance; however, the relationship between the strength of the immune response and the quality of the sexual signal remains controversial, and such studies rarely consider whether immunity provides a good measure of parasite resistance. Here I examined the relationship between immune response, parasite load, and sexual signaling, as well as what factors affect mating and reproductive success in western bluebirds (Sialia mexicana). Coloration appeared to act as a signal in social mate choice, and social pairs mate assortatively by UV-blue coloration. Males with brighter UV-blue coloration also sired more of the offspring within their own nests. However, coloration played no role in extra-pair mate choice. Likewise, immune response did not predict coloration (sexual signal quality) or male reproductive success. Immune response did relate to parasite loads, however, and individuals that had a stronger immune response tended to have lower levels of infection with avian malaria. Parasite infection and age both predicted male success at siring extra-pair offspring; extra-pair males tended to be older and uninfected. This suggests that females select extra-pair males based on their ability to resist parasites, consistent with the Hamilton-Zuk hypothesis. However, uninfected males did not have higher reproductive success than infected males, despite the advantage of siring extra-pair offspring. Double brooding, another method by which birds can increase their reproductive success in a season, occurred rarely in our population, with only 5% of pairs producing second broods. Pairs that produced second broods tended to breed earlier and the females in these pairs tended to be heavier. Environment also played a role in double brooding, with more pairs producing second broods during years in which the breeding season was long, warm, and dry. This highlights the importance of parasites and environmental factors in determining mate choice and reproductive success.

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