The following are recordings made just north of Philadelphia the night of September 15-16th which following the passage of a cold front had a heavy flight of migrants. Swainson's Thrushes predominated during the night, with Rose-breasted Grosbeak a distant second. A few early Gray-cheeked Thrushes were heard but very few Veery, which were by then past their peak. No Scarlet Tanagers were heard; this species and Wood Thrush less commonly call in nocturnal migration. In the predawn descent, when calls increase dramatically, I counted 1396 Swainson's Thrush calls, 64 Gray-cheeked Thrush calls, 13 Veery and 18 Rose-breasted Grosbeak calls in the 25 minutes from 5.49am to 6.14am. Rose-breasted Grosbeaks, which called throughout the night, as expected did not increase calling during the predawn descent. Identifications are presumed with nocturnal migrants, as no birds were seen.
Swainson's Thrushes calling shortly before sunrise (6.08am)
There are two common Swainson's Thrush flight calls, a rising spring peeper-like call and this more modulated call (6.07am).
In mid-September Gray-cheeked Thrushes begin to be heard, generally the call is distinctive and one of the easiest to identify (5.05am)
It is well past the peak for Veery, whose calls are low and burry and often strongly bisyllabic (5.59am).
Rose-breasted Grosbeaks have variable calls.
A few examples:
Clear whistled notes (2.30am)
hoarse notes (3.18am)
buzzy notes (with Black-and-white Warbler, 3.08 am). These calls are extremely similar to Wood Thrush calls, but shorter and coarser according to Evans and O'Brien. I'm having a hard time figuring them out, so these recordings may actually be Wood Thrush calls
various off-key or bisyllabic calls (3.23am)
with Green Heron (2.10am)
Double-banded upsweep could be Black-throated Green, Nashville, Tennessee or even Orange-crowned (1.20am).
Short buzz typical of Blackpoll, Yellow, Blackburnian or Connecticut (5.06am).
The distinctive bisyllabic call of an American Redstart (5.45am).
The Black-and-white Warbler has a similar but higher call with a hissing quality (3.08am).
The piercing rising call of an Ovenbird (3.24am).
Northern Parulas have a distinctive descending call (5.42am).
Black-throated Blue Warblers have a cardinal-like tick flight call (4.14am).