Social bonds and migratory behavior determine the likelihood of hybridization in birds

What behavioral mechanisms underlie these associations?

Sometimes the title of a paper says it all: “Sociality and migration predict hybridization across birds.” In a recent study in the Proceedings of the Royal Society B, Gavin Leighton and his colleagues correlated the incidence of hybridization across birds with several life history traits. These analyses revealed that bird species with long-lasting social bonds are less likely to hybridize. In addition, a migratory lifestyle increases the likelihood of hybridization. Some interesting findings that require more exploration. What behavioral mechanisms could be driving these associations between sociality, migration and hybridization?

Pair Bonds and Migration

Species with no pair bonds will look for a new partner every year, increasing the opportunities for making the “wrong” choice and hybridize. Species with long-lasting social bonds, on the other hand, will often pair for several mating seasons. This reproductive strategy can lead to substantial fitness costs if an individual mates with the “wrong” species. The resulting hybrids might be sterile or unviable, culminating in a significant waste of reproductive effort. Such maladaptive hybridization can lead to increased selection for proper species recognition. An interesting exception to this result – which might prove the rule – concerns my own study system: geese. Most goose species tend to form long-lasting pair bonds, but still engage in regular hybridization. The production of hybrids has been attributed to several behavioral mechanisms, such as forced extra-pair copulations, interspecific brood parasitism and vagrant individuals (see this paper for an overview).

The last mechanism related to goose hybridization – vagrant individuals – leads into the next main finding of the study: migratory species are more likely to hybridize. To again focus geese, migratory goose species might have a higher chance of reaching new habitats where they interbreed with local species. For example, North American Snow Geese (Anser caerulescens) are occasionally observed in Europe during migration and hybrids between Snow Goose and several European species have indeed been reported. An additional explanation relates to mate choice in sedentary species: these birds might have extended periods of mate searching and pair formation, lowering the probability of choosing a partner from another species.

Phylogenetic distribution of hybridization (grey, inner circle), social bond length (green, middle circle), and migratory status (blue/yellow, outer circle). From: Leighton et al. (2021).

Future Research

Clearly, there are multiple mechanisms to explain the associations uncovered in this study. The details might differ across the avian phylogeny, but the broad-scale importance of sociality and migration seems justified. This macroevolutionary perspective opens new exciting avenues for further research, disentangling how these life history traits (and probably many other factors) interact to determine the probability of hybridization.

A final note on the dataset. The researchers assembled a dataset of bird hybrids using the Handbook of Avian Hybrids of the World, indicating that they took “a liberal approach and included any reported hybridization where an individual was identified by sight as a putative hybrid.” I completely understand this pragmatic choice as it is nearly impossible to assess the reliability of each hybrid combination mentioned in the Handbook of Avian Hybrids of the World. However, I have become more skeptical about the list of bird hybrids that Eugene McCarthy has produced (see for example this blog post). I do not think that the inclusion of less reliable hybrids has affected the analyses in the present study, but I do encourage ornithologists to be careful in using this book in their future work. Always check the original source before including a dubious hybrid record.

References

Leighton, G. M., Lu, L. J., Holop, E., Dobler, J., & Ligon, R. A. (2021). Sociality and migration predict hybridization across birds. Proceedings of the Royal Society B288(1947), 20201946.

Featured image: Greylag Goose (Anser anser) x Canada Goose (Branta canadensis) © Dirk Ottenburghs

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