A genetic study detected only one hybrid individual.
Estimating the incidence of hybridization on an individual level is extremely challenging. Recently, Nicholas Justyn and his colleagues used data from the citizen science database eBird to investigate how often birds hybridize in North America. They found that 0.064% of the reported sightings were hybrids. This estimate can probably be regarded as a lower bound, because birdwatchers tend to under-report common hybrids (as I argued together with David Slager in a response to this study, and see also this paper by Hannah Justen and her colleagues). These papers highlight the difficulty of estimating hybridization rates in wild birds, even if you are focusing on just two species. In some cases, hybrids might be difficult to identify morphologically or the study species live in remote areas. Here, genetic data can be a valuable asset (see for example this blog post on penguin hybrids).
A recent study in the Journal of Avian Biology attempted to estimate the incidence of hybridization between Barrow’s Goldeneye (Bucephala islandica) and Common Goldeneye (B. clangula). Field observations suggest that these sea ducks occasionally interbreed (see the Anseriformes page for an overview), but the exact proportion of hybrids in their populations remains unknown.
Joshua Brown and his colleagues followed a genetic approach and took a closer look at the DNA of 61 individuals. Using two different genetic markers (microsatellites and ddRAD-seq), they found evidence for one hybrid individual. Additional demographic analyses pointed to an evolutionary model of allopatric speciation with secondary contact. The migration rate, however, amounted to less than one migrant per generation in both directions. In other words, an extremely low estimate of gene flow due to hybridization. These findings indicate that hybrid goldeneyes are a rare sighting. The authors attribute this low occurrence of hybridization to “assortative mating, differences in habitat preferences and territorial behaviors exhibited during mate pairing.”
In addition to quantifying hybridization, the researchers also investigated population structure in both species. Previous work reported clear population structure in terms of mitochondrial DNA, suggesting that females rarely disperse between breeding grounds (mtDNA is inherited through the female line). The same study also found no overlap in winter band recoveries among individuals marked in Alaska and British Columbia. Based on these patterns, the researchers expected to find some population structure in the nuclear DNA as well.
Surprisingly, there was no discernable population structure in the microsatellites or the ddRAD-seq data. This lack of nuclear population structure might be explained by dispersal of males between colonies. However, Barrow’s Goldeneye shows a high level of breeding site fidelity in both sexes, with the average yearly return rate of males (67%) roughly identical to that of females (63%). The situation in Common Goldeneyes is unknown due to lack of data. But not all males are equal. It is known that subadult males return to natal nesting grounds significantly less often than subadult females and are thus much more likely to disperse between colonies. Hence, the authors argue “that homogeneity across the nuclear genome most likely results from high levels of juvenile male dispersal despite high mtDNA structure.”
Brown et al. (2020). High site fidelity does not equate to population genetic structure for common goldeneye and Barrow’s goldeneye in North America. Journal of Avian Biology, 51(12).
Featured image: Common Goldeneye (Bucephala clangula) © Becky Matsubara | Wikimedia Commons
This paper has been added to the Anseriformes page.