Tuesday, May 18, 2010

Time to refresh ID key on "wood-thrushes"

With the recent influx of migrant thrushes across the Mid-Atlantic region it is time to revisit the ID key to distinguish these similar and skulky birds. As such, the photos below are not chosen for their wow-factor, but because they show important identifying features and reflect typical conditions under which these mostly groud-dwelling birds are often encountered and ID'd.I will examine each species in the order at which they arrive on spring migration. The earliest arriving Catharus thrush is the Hermit Thrush (Catharus guttatus):First, the tail and rump are the most most brightly colored parts of the bird contrasting against a rather dull gray-brown back.Next, they have a narrow white eye-ring surrounded by washed-out gray cheeks and heavily spotted breast.
The second species to arrive is the Wood thrush (Hylocicla mustelina):In addition to its much larger and bulkier body, the most striking feature of a Wood Thrush is that the neck is the most brightly colored body part. The golden neck stands out against the otherwise uniform rufous brown crown, back, wings and tail.
This photo shows the boldly spotted bright white underside contrasting against the rufous brown upper parts.
Third species to arrive, usually overlapping with lingering Hermit Thrushes is the Veery (Catharus fuscescens):With a bright rufous back it superficially resembles a Wood thrush and when seen from behind it could also be confused with a Hermit thrush. However, there is no contrast in color between crown, back, rump and tail and when viewed from the side or front (as shown here) you'll notice the nearly complete lack of spots on the breast. Typically, only a few brown streaks or spots can be discerned on the buffish or rufous colored upper breast.
Next to arrive are the three most northern species, the most numerous of which is the Swainson's Thrush (Catharus ustulatus):
The Swainson's thrush is best identified by its bold, buffish eye-ring on a warm buffish cheek and olive-brown upper parts.
Like the Hermit Thrush they have bold spots on the upper breast, but the spots do not stand out as contrasting against the more buff throat and upper breast.
Mixed in with the flocks of Swainson's Thrushes and typically occurring in a 1:10 ratio is the Gray-cheeked Thrush (Catharus minimus):
The best distinguishing feature of this species is an overall featureless appearance, lacking bold eye-ring and any warm rufous coloration. Seen from below they are inseparable from a Swainson's Thrush, but from the side the lack of eye-ring and gray cheek is diagnostic. They also have colder gray upper parts compared to Swainson's thrush and the last species in the group; the rare Bicknell's Thrush (Catharus bicknelli). Unfortunately, I have not acquired any photos of this species yet. It is supposed to display a warmer more rufous coloration to the wings, rump and flanks compared to Gray-cheeked Thrush. However, there is much overlap in plumage between these two species and they can only be safely distinguished by song and call vocalizations. They are passing through the Mid-Atlantic region as I write so pay close attention to the calls of any Gray-cheeked Thrushes. Many birdwatchers have successfully confirmed their identity by testing their response to limited playback of both Gray-cheeked and Bicknell's vocalizations.

No comments:

Post a Comment