A genetic study finds weak population structure in three Scoter species.
Some bird species are difficult to study. Take, for instance, the Scoters (genus Melanitta). These black ducks spend most of their time at sea where they aggregate in big flocks. Several breeding colonies and wintering areas have been found, but ornithologists don’t know how these populations are structured. Do birds switch between breeding areas or do they stay loyal to one location? These questions can be answered using genetic data, as shown by a recent study in the journal Ecology and Evolution.
Five (or three) species
Sarah Sonsthagen and her colleagues focused on three North American: the Black Scoter (M. americana), the Surf Scoter (M. perspicillata) and the White-winged Scoter (M. deglandi). They also analyzed samples from two European species: the Common Scoter (M. nigra, which is sometimes considered conspecific with the Black Scoter) and the Velvet Scoter (M. fusca, which is sometimes seen as conspecific with the White-winged Scoter). So, depending on your taxonomic preference, there are three or five species of Scoter in this study. The researchers used a combination of ddRAD-seq and microsatellites to probe the genetic population structure of these Scoters.
The three North American species showed weak population structure, indicating high levels of gene flow between different locations. In birds, this pattern is mostly attributed to male-biased dispersal. Males choose a partner at the wintering grounds and follow her back to the breeding area. However, female Surf Scoters and White-winged Scoters can occasionally switch between migration routes. For example, the White-winged Scoters that nest in central Canada are a mixture of birds wintering on the Atlantic and the Pacific coast. There is evidence for a female using different wintering areas in different years.
Black Scoters showed some population genetic structure that coincides with their breeding distribution. There appears to be a barrier – behavioral of physical – between birds from Alaska and the Atlantic region.
The three North American species were clearly differentiated from their European cousins. There was no evidence for gene flow between different continents. The pattern is most likely driven by the use of distinct wintering areas.
In addition, the researchers found no signs of gene flow between species. Hybrids between several species (e.g., White-winged Scoter x Surf Scoter) have been observed. However, such hybridization events are probably too rare to influence the genetic structure of these species. The courtship and copulation displays of different Scoter species are quite distinct and could serve as a behavioral barrier.
Sonsthagen, S. A., Wilson, R. E., Lavretsky, P., & Talbot, S. L. (2019). Coast to coast: High genomic connectivity in North American scoters. Ecology and Evolution, 9(12), 7246-7261.
The paper has been added to the Anseriformes page