Social network analyses might provide the answer to this question.
Why do some birds choose a partner from a different species? Couldn’t they find a member of their own species? Did they want to try something new? Or did they just make a mistake? These questions come to mind when you observe a hybrid individual in the wild. A recent study, published in The American Naturalist, tries to solve this conundrum for a pair of quail species (genus Callipepla).
California Quail (C. californica) and Gambel’s (C. gambelii) Quail are sister species that hybridize along a contact zone in Southern California. Previous work showed that these species can discriminate between each other in captivity. In the wild, however, mating seems random, resulting in mixed pairs and consequently hybrids. These patterns raise the question how quails choose a partner.
To investigate the mating patterns of these quails, David Zonana and his colleagues turned to social network analyses. They equipped several birds with automated radio-frequency identification tags that generate detailed data on which individuals interact with one another. These interactions can be visualized in social networks in which the individuals represent dots and the connecting lines (or edges) indicate the strength of interaction. The figure below shows the social network for the quails.
Analyses of these social networks can tell us more about the way quails select their partner. Surprisingly, they do not use species-specific plumage during mate choice. These plumage patterns did not correlate with the structure and strength of the associations in the network. Instead, the birds focus on two other characteristics: body mass and monomorphic plumage (i.e. the same in both sexes).
Body Mass and Shared Plumage Patterns
Pairing up by body mass is consistent with a previous study on Gambel’s Quail. This study found that larger males are more likely to win competitions with other males. Consequently, these males have first pick with the females and they settle down with large females that are more experienced. This choice increases the chances of a successful brood.
The second cue for mate choice is monomorphic plumage. Given that quails can discriminate between species in captivity, it is surprising that they pair up with individuals that look similar to them. This pattern suggests that quails are sexually imprinted in particular plumage traits that are shared by males and females. Which specific traits are used by the quails remains to be determined.
Mate Choice in the Contact Zone
These finding can explain the occurrence of hybridization in the contact zone. Quails pair up with individuals that share particular plumage patterns with them. Outside the contact zone, this strategy works perfectly because they only encounter members of their own species. In the contact zone, however, they run into quails from another species. But instead of focusing in the species-specific differences, they keep using the shared plumage traits to pick their partner. And voila, hybrids!
Gee, J. M. (2003). How a hybrid zone is maintained: behavioral mechanisms of interbreeding between California and Gambel’s quail (Callipepla californica and C. gambelii). Evolution, 57(10), 2407-2415.
Hagelin, J. C. (2003). A field study of ornaments, body size, and mating behavior of the Gambel’s Quail. The Wilson Bulletin, 246-257.
Zonana, D. M., Gee, J. M., Bridge, E. S., Breed, M. D., & Doak, D. F. (2019). Assessing Behavioral Associations in a Hybrid Zone through Social Network Analysis: Complex Assortative Behaviors Structure Associations in a Hybrid Quail Population. The American Naturalist, 193(6), 852-865.
This paper has been added to the Galliformes page.