Kenn Kaufman’s column “ID Tips” appears in every issue of BirdWatching. In our June 2017 issue, he described the differences between Swainson’s, Hermit, and Gray-cheeked Thrushes, and he wrote about the two populations of Swainson’s Thrush and how they differ from one another:

Birders in general stopped referring to Olive-backed and Russet-backed Thrushes decades ago, but recently scientists have been taking a closer look at the birds represented by the names. The two populations, currently classified under Swainson’s Thrush, may merit treatment as full species.

The Olive-backed group breeds from western Alaska to Maine, and far south in the Rockies, while the Russet-backed group breeds from southeastern Alaska south to California. Potential contact in their breeding ranges occurs mostly in British Columbia, and research there has highlighted differences between the groups.

Although Russet-backed and Olive-backed types do interbreed, the hybrid zone between them is narrow — only about 50 miles wide in the best-studied spot. In addition to plumage differences, the songs also tend to differ, with Russet-backed songs averaging longer and lower-pitched. Russet-backed birds also migrate earlier in spring, reaching the breeding grounds sooner.

Recently, studies have looked at the genetic basis for the different migrations. Olive-backed birds from British Columbia migrate east and then south in fall, heading for South America. Russet-backed birds move south, ending up in Mexico and Central America. Hybrids apparently take an intermediate route through more hostile terrain, making it less likely that they will survive the journey. If hybrids are at a disadvantage, that leads to more isolation between the two groups, helping to drive their evolution toward distinct species status. – Kenn Kaufman

Kenn Kaufman’s “ID Tips,” featuring the photographs of Brian E. Small, appears in every issue of BirdWatching. The article above is an excerpt of a column that ran in our June 2017 issue.

 

Swainson’s Thrushes confound expectations, change altitude while migrating at night

Earbirding: Read more about the differences between the thrushes

Read other articles by and about Kenn Kaufman

 

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Originally Published October 25, 2017