On cool fall mornings with light north-westerly winds in the northeastern US the sky can be filled with the calls of migrating thrushes. Although they each have distinctive calls, there are going to be plenty of calls that remain unidentified.
Veeries in flight before sunrise (with a few Swainson's Thrush calls in background, Sep, PA). Veeries are the earliest of the migrant thrushes in fall. The calls are usually lower and burrier than those of Swainson's Thrushes.
(before dawn, Sep, PA)
(6.00am, Sep, PA)
Nocturnal calls are often bisyllabic, and may rise, be level, or descend a little, but occasionally a distinctly descending call is heard (Sep, PA).
A perched Veery (May, PA, Red-bellied Woodpecker and Hooded Warbler calling). Veeries often give their flight calls during the day or in between song phrases.
Swainson's Thrushes in flight, 65 minutes before sunrise, over my backyard in Elkins Park, PA, in early October. The calls often sound like a Spring Peeper.
In this case some of the calls sound doubled (Oct, PA).
This is a call variant which is more completely modulated than the common call (Oct, PA).
In the above example, analysis of sonagrams reveals a whole spectrum of intermediate calls between these variants and the more common call on the left:
A good example of this common call (6.27am, Oct, PA)
Swainson's and Gray-cheeked Thrushes also occasionally give their flight calls during the day (especially right after they have landed early in the morning). Perched Swainson's Thrush shortly before dawn (Oct, PA).
Modulated call, perched bird (Sep, NJ)
Individual birds often call a number of times as they pass overhead, making it difficult to estimate the number of birds that are calling. In this case, a presumed single bird is heard approaching, calling every 4-6 seconds, and getting louder as it nears the microphone (Sep, PA).
Some calls are best left unidentified. This call appears to be intermediate between Swainson's Thrush and Veery, perhaps leaning towards Swainson's. Note the completely modulated second call (Sep, PA).
Gray-cheeked Thrushes in flight 90 mins before sunrise (Oct, PA). The call descends and is higher than the Swainson's Thrush call, which rises.
(with Swainson's Thrushes, 60 mins before sunrise, Oct, PA)
Fully modulated call at 23s, presumably Gray-cheeked Thrush
Buzzy version (5.10am, Oct, PA)
An odd purer-toned call, presumably Gray-cheeked Thrush (6.09am Oct, PA)
A perched Gray-cheeked Thrush with two slightly different versions of the "pee-uu" flight call. These may also represent alarm calls (breeding grounds, June, AK).
The call of Bicknell's Thrush is higher than that of the Gray-cheeked, with a straighter appearance on the sonagram, and the highest point is at the peak of the initial ascent. The high point averaged 5.3khz vs 4.1khz for Gray-cheeked, and the end point 3.7khz vs 2.8khz in Bill Evans's paper (Evans, W. R. 1994. Nocturnal flight call of Bicknell’s Thrush. Wilson Bull. 106:55-61).
The following call matches those characteristics, and was recorded on a night with moderate northeasterly winds, which might be expected to produce more Bicknell's in the Philadelphia area than northwesterly winds, given the breeding distribution of Bicknell's Thrush (4.57am, early Oct, PA).
This example of a probable Bicknell's Thrush has a peak at 5.75 khz, high even for Bicknell's (5.49am, early Oct, PA)
Another example of a probable Bicknell's Thrush, based on shape and high point, although the end frequency is relatively low at 3.3khz. Bicknell's calls tend to sound purer-toned than Gray-cheeked (11.53pm, early October)
The Hermit Thrush has a clear, descending flight call. It is the latest Catharus thrush to migrate south in fall.
(breeding grounds, July, ME)
(breeding grounds, May, RI)
Calls by perched birds shortly before dawn (Oct, PA)
Hermit Thrush calls in nocturnal migration (5.28am, Oct, PA)
The Wood Thrush has a level, buzzy call (perched male, May, NJ).
Perched Wood Thrush alternating flight call with alarm note (May, NJ).
Presumed Wood Thrush in nocturnal migration (3.30am, Sep, PA).
Some presumed Rose-breasted Grosbeak calls sound very similar, but with about half the degree of modulation of the Wood Thrush call (Oct, PA).
Calls are given by American Robins in flight during the day, but robins are not generally heard during nocturnal migration, except along the coast (Jan, PA).
Another, more sibilant example (Feb, NJ).
The six common thrushes have calls that can be fairly easily separated, but note that the Rose-breasted Grosbeak has a variety of flight calls, some of which can sound similar to Swainson's, Wood and Hermit Thrush (see Tanagers, Buntings and Grosbeaks:flight calls).
Here's two calls of presumed Rose-breasted Grosbeak, one a distinctive call and a second call similar to Swainson's Thrush (in flight 55 minutes before sunrise, with presumed Swainson's Thrush at 3 secs, end of Sep, PA)
The Scarlet Tanager is another species that calls during nocturnal migration, and could be confused with thrushes. The most common flight call is a clear high chu-wee; buzzier versions sound similar to Veery but are more bisyllabic (perched bird with White-breasted Nuthatch and Golden-crowned Kinglet, Oct, PA).
Presumed Scarlet Tanager in flight with Swainson's Thrushes, fifth call at 12s (before dawn, Sep, PA).
Unlike the other thrushes the Eastern Bluebird is a daytime migrant. It often calls from perches, as with this male (Feb, PA).