Odor preferences in hybrid chickadees: implications for reproductive isolation and asymmetric introgression

Preferences for mating cues can have important effects on speciation in natural hybrid zones. While mating preferences of parental species are well studied, hybrid preferences for cues of parental species have received less attention. Nevertheless, hybrid preferences have potentially important consequences for reproductive isolation and patterns of introgression. Here, we test preferences in wild-caught hybrid chickadees for odor cues from the parental species, black-capped and Carolina chickadees. Olfaction has been historically understudied in avian species, especially passerines, but much recent work has begun to show how olfactory cues play important roles in songbird ecology. In contrast to the strong conspecific odor preferences we previously found in pure-species chickadees, male hybrids as a group show no preference for the odor of either parental species, while still exhibiting individual instances of preference. Female hybrids show preferences for black-capped odor, but this signature is driven by individuals with elevated proportions of black-capped ancestry while Carolina-like female hybrids show no preferences. We discuss the implications of a lack of some female hybrid preferences on reproductive isolation, and the potential for asymmetric preferences in male and black-capped-like female hybrids to contribute to directional introgression and northward movement of the hybrid zone. Significance statement Mate preference has important implications for the fate of natural hybrid zones and the speciation process. Mating discrimination by pure species individuals against hybrids is commonly observed in hybrid zones. However, mate preferences of hybrid individuals for the parental species can be just as important. We previously found that hybrid zone black-capped and Carolina chickadees produce distinct odor profiles and show clear preferences for conspecific odor cues. Here, we assessed preferences of wild-caught hybrid chickadees for the odors of both parental species. In contrast to pure-species birds, hybrid males overall show no preference for either parental species odor as a group. Still, individual males do show clear preferences. Hybrid females show a preference for black-capped odor that is dependent on their own degree of black-capped ancestry. Carolina-like hybrid females show no preferences. Our results suggest the potential for hybrid preference to contribute to reproductive isolation and northward movement of the chickadee hybrid zone.

Automated summary

Preferences for mating cues, including odor signals, can impact speciation and reproductive isolation in natural hybrid zones. Hybrid individuals show varying preferences for parental species cues, with male hybrids showing overall no preference and female hybrids showing a preference for black-capped chickadee odor dependent on their black-capped ancestry. These preferences may play a role in reproductive isolation and the movement of hybrid zones.