Two recent studies provide insights into the genetics and migration of two hybridizing Vermivora species. One study turns its attention to the north, whereas the other study focuses on the south.
Over the last 150 years, humans have converted large forest areas into agricultural fields in eastern North America. These habitat changes have facilitated contact between some geographically isolated bird species, often resulting in hybridization.
One example of such secondary contact is the story of Golden-winged (Vermivora chrysoptera) and Blue-winged Warblers (V. pinus). These two species were separated by large patches of forest. The conversion of this forest to agriculture enabled the Blue-Winged Warbler to spread north where it occasionally hybridized the Golden-winged Warbler. The combination of habitat loss and hybridization resulted in the decline of the latter species. The Golden-winged Warbler is now one of the most rapidly declining bird species in North America. Recently, two new studies have been added to the growing literature on the interactions of these small passerines (see here for an overview). One study looks at the final frontier in the north, while the other study focuses on migration to the south.
To The North
Previous studies documented extensive exchange of genetic material between these warblers (a process known as introgression). One exception was Manitoba in Canada. Laurel Moulton (University of Manitoba, Canada) and colleagues reassessed the genetic status of the Golden-winged Warbler in this area. Sampling over 200 birds between 2011 and 2014, they found 10 hybrids. It turns out that there is some genetic introgression in Manitoba, but the levels of introgression are the lowest across the range of Golden-winged Warbler. Hence, Manitoba can be seen as an important refuge for Golden-winged Warbler, where it is safe from the expanding Blue-winged Warbler. For now…
To The South
Another recent study focused on a mixed population in the Jefferson and Lewis counties of New York. Ruth Bennett (Cornell University, Ithaca, NY) and colleagues attached geo-locators to 10 Golden-winged Warblers, 10 Blue-winged Warblers and 5 hybrids in order to investigate their migratory strategies. Given the genetic similarity between these species, the researchers expected to uncover similar migration routes.
The next year, data from 7 of the 25 birds were recovered. Two Golden-winged Warblers spend their winter in northern Colombia, while three Blue-winged Warblers flew to the Yucatan peninsula in Mexico and the western tip of Cuba. Two hybrids showed an intermediate choice and wintered in Cuba and Nicaragua. These results confirm the notion that migratory behavior is heritable in passerines with genetic hybrids showing intermediate strategies (see here).
Bennett, Ruth E, Sara Barker Swarthout, Jeffrey S Bolsinger, Amanda Rodewald, Kenneth V Rosenberg, and Ron Rohrbaugh. 2017. ‘Extreme genetic similarity does not predict non‐breeding distribution of two closely related warblers’, Journal of Field Ornithology.
Moulton, Laurel L, Rachel Vallender, Christian Artuso, and Nicola Koper. 2017. ‘The final frontier: early-stage genetic introgression and hybrid habitat use in the northwestern extent of the Golden-winged Warbler breeding range’, Conservation Genetics: 1-7.
The papers have been added to the already quite extensive Parulidae page.