Wood Thrush (Hylocichla mustelina) is a well-known forest species, especially so in the eastern United States where its melodic song rings through eastern forests from May to July. It migrates to Central America for the winter, where it also prefers dark, close-canopy broadleaf forests, very similar to its breeding grounds. During the winter it is much less conspicuous, largely detected by its chattering call notes and occasionally flushed from forest trails.
Wood Thrushes really depend on the dense tropical broadleaf forests of the Yucatan Peninsula, eastern Guatemala, and Belize: the Maya Forest. The winter map (e.g,, 4 January) shows a hotspot of highest abundance right in the central southern Yucatan Peninsula and northern Guatemala. Lower abundance extends into Veracruz and south to Costa Rica and Panama, primarily on the wetter Atlantic Slope. Wood Thrushes maintain territory in winter, and are not known to move around at all during this period.
Once late March arrives, one can clearly see the first Wood Thrushes making landfall on the Gulf Coast. Wood Thrushes are trans-Gulf migrants, jumping directly across the Gulf of Mexico with a suite of other migrants that risk a long flight over open water instead of taking a longer route around the Gulf. Once in the US, Wood Thrushes rapidly fill in eastern forests east of the Great Plains.
On their breeding grounds Wood Thrushes have been imperiled, with factory emissions from the Great Lakes and Midwest of the US being implicated in their declines. As those emissions drift eastward with the wind, precipitates affect forest floor invertebrates, with several studies tying acid rain to reduced food resources for Wood Thrushes and their young. More recently, heavy metals have been a concern.
Like many thrushes, Wood Thrushes get very secretive and difficult to detect during the late summer. They largely stop singing, get very secretive, and their detection rate on eBird lists drops drastically. This is evident on the maps as the relative abundance drops continually from early August to mid-September without any indication of southward movement. These birds are not migrating during this period, and it is not until the 19 September map that migration is evident. Just a week later the first signal becomes evident in the Yucatan.
This is a pattern that is shown by many species and it is important to understand that this does not represent a drop true abundance, but rather a drop in the detection rate. Accounting for changing detectability is one of the holy grails of relative abundance modeling and is something Cornell Lab researchers are trying to account for better in these models.
As the fall migration wraps up in November, this species makes its return to Middle America, with most heading to the Maya Forest of the Yucatan Peninsula. Watch the sequence from July to November to see just how many Wood Thrushes pack into a relatively small region. Clearly the food resources of the super-productive tropical forest here allow for a denser wintering population, since Wood Thrushes from across the entire eastern U.S. and southern Canada all end up packing into a region of Middle America that is a tiny fraction of the size of their breeding range. Consider the conservation implications: loss of wintering habitat will affect many more Wood Thrushes in Central America. The Maya Forest here is under pressure from deforestation, so the future of Wood Thrushes and other Maya Forest species is far from secure.
To learn more about the life history of Wood Thrush, please consider a subscription to Birds of North America (just $42/year), where experts in the species have written a full-length species account covering all aspects of the species biology, from migration to diet to conservation status.