Swainson's Thrush Migration

I taught a fall migration class today at work and wanted to look up some information about migrating Swainson's Thrushes for the class. I stumbled upon this interesting scientific paper from 2002 about the genetic connection between migration patterns in Swainson's Thrushes and the expansion of their range in the late pleistocene. Not as the crow flies: a historical explanation for circuitous migration in Swainson's thrush (Catharus ustulatus) is thankfully available as a free pdf.

Be forewarned, the paper contains bodice ripping passages such as, "To survey the distribution of these clades in migrating and overwintering birds, we amplified the 800 bp fragment of control region using the primers described above and used restriction enzyme Sfc I to assay a variable site in which cleavage is diagnostic of the coastal clade. Five microlitres of the digest reaction were electrophoresed on an 6% polyacrylamide gel and restriction fragments were stained with thidium bromide and visualized under ultraviolet light." Try to control yourselves people, I know this is exciting stuff.

While the section on methodology may not be your cup of tea, the conclusions are fascinating. The authors conclude that in the late pleistocene the Swainson's Thrush population was divided into two refugia as glaciation wiped out habitat in the center of the continent. These two populations remain relatively distinct today as the coastal (western) population and the continental (eastern) population The eastern population eventually spread north-westward as suitable boreal forest habitat emerged post-glaciation. When it comes time to migrate south, the continental population, which has spread as far wet as Alaska, follows their genetic heritage and first flies thousands of miles east to get back to their ancestral grounds before turning and heading south. The birds essentially re-trace their ancestral expansion route. This points to a strong genetic component to migration.

Many similar findings are discussed in Scott Weidensaul's excellent book on migration, Living on the Wind: Across the Hemisphere With Migratory Birds.

~Kirk

(photo credit: US NPS)