From 2.50 am to sunrise on August 23rd 2011 I recorded migrants flying over my house near Philadelphia, PA. The overnight winds were light and northwesterly, good for southbound migration. After analyzing the recordings, I was fairly confident in identifying 12 species of warbler, as well as a few Veery and Bobolink. A surprise was a Dickcissel, an uncommon bird in the east, and an Eastern Kingbird, rarely heard at night. Among the warblers, Chestnut-sided was the most frequently heard (at least 35 separate individuals), and 12 Canada Warblers were tallied. A number of calls could not be assigned to a species or species group because of their atypical appearance on the sonagram.
One of the easiest flight calls to recognise is the sharp tik of the Black-throated Blue warbler. It's a unique call among the warblers, and sounds more like the alarm call of a Northern Cardinal.
Another distinctive call is the low electric buzz of the Common Yellowthroat. It's a good deal lower than the buzzy calls of other warblers, but not as low as the Dickcissel (see below).
The next three species also have quite distinctive calls:
Canada Warblers have a slurred call with a unique appearance on the sonagram, but if not heard well it could be confused with American Redstart.
The American Redstart has a slurred, doubled flight call.
The Ovenbird has a piercing flight call that is a little similar to the American Redstart but is higher, clearer and more distinctly rising.
Ovenbirds often call repeatedly in rapid succession.
The most frequently heard warbler species was the Chestnut-sided Warbler. Most other buzzy warbler flight calls are higher in frequency or less modulated.
Several other warbler species have short buzzy flight calls, and cannot generally be separated because they are so similar to one another. Additionally, there is considerable variability in the calls. The following calls could be by Yellow or Blackburnian Warblers, it being a little early for Blackpoll, and a good deal early for Connecticut, but who knows?
Worm-eating Warblers have a high, dry buzz that is often doubled. As with many of these calls, the identification is not certain, and this a presumed Worm-eating Warbler calling, but the sonagram is typical for this species.
Magnolia Warblers have a slightly longer, more modulated buzz. If it was rising, it could have been a waterthrush flying over.
Rising double-banded notes are made by several species, including Black-throated Green, Tennessee and Nashville Warblers. Only five of these calls were recorded this night.
A possible Prairie Warbler; the flight call tends to be level.
Presumed Pine Warbler. Northern Parula also has a descending call but it is steeper. Surprisingly, no parulas were recorded this night.
The unique low buzz of a Dickcissel heard at 3.10am.
Eastern Kingbird call at 3.40am.