The winds these two nights were from the WNW and NW and migration was heavy on the NEXRAD radar, but activity was light in terms of birds heard over my backyard just north of Philadelphia.
Considering the large number of Hermit Thrushes that arrived in the morning of these two dates, the fact that only about twenty were heard calling during the night seems to indicate that most migrating Hermit Thrushes must be either silent or call infrequently. Below are several examples, beginning with the typical pure-toned calls, then examples of modulated calls. Fully modulated flight calls are typical of the Wood Thrush but are given at times by all the thrushes and also probably by Rose-breasted Grosbeaks. All calls are presumed as the birds were not seen.
Most Hermit Thrush calls are pure-toned, but some are modulated in the beginning, and others are fully modulated, 5.48am
partially modulated at 2s
pure-toned at 4s
fully modulated at 6s
Lower, fully modulated calls at 1 and 9s, 11.54pm
In this cut there are two odd buzzy calls at the end, perhaps a shortened Hermit Thrush call or perhaps Rose-breasted Grosbeak, 6.00am.
Hermit Thrush and again an anomalous call at 5s, 11.38 pm
Presumed Rose-breasted Grosbeak buzzy call, 11.34pm. Wood Thrush is very similar but has a finer buzz. This call has never been confirmed by a sight recording during the day, and is a call presumed to be made by migrating Rose-breasted Grosbeaks based on its association with other known calls. A confirmed daytime recording of this call would be a great find.
Wood Thrush or Rose-breasted Grosbeak, 11.37pm
Warbler buzz (almost certainly Blackpoll), and Chipping Sparrow, 6.22am
Yellow-rumped Warblers are the latest (except for Orange-crowned) migrating warbler, and also the most abundant, 11.55pm
White-throated Sparrow, a fully modulated variant. Henslow's Sparrow has a very similar call, but with double peaked modulations. As hard as I tried, I couldn't convince myself that this was a Henslow's calling.
Savannah Sparrow, 5.44am
Indigo Bunting, 6.02am
Every night there will be unknown calls. This one could be the rattle of a Lapland Longspur. It also sounds a little like the staccato call of a Winter Wren. Wrens are not known to call during nocturnal migration, 6.10am