The following were recorded today at the Pennypack Preserve in Montgomery County, Pennsylvania just north of Philadelphia. Migrants were abundant, and among the 21 species of warbler I came across, there was a Wilson's Warbler that sang an extended version of its song (also called flight or attenuated song). The BNA account states that for Wilson's Warbler there is "no evidence for an uncommon extended song, eg a flight song, as occurs in some warbler species" (such as Common Yellowthroat and Ovenbird).
The extended song is heard at 25s, 94s and 134s into the cut. It sounds similar to the extended songs of other warbler species, but could perhaps be subsong, although the bird mostly sang discrete phrases for the rest of the time I observed it.
sonagrams of 94-106s
The next song of interest was a Blackpoll Warbler singing a very fast version of its song. There's recently been some speculation that this version may be a regional dialect by birds breeding in New Hampshire, see Earbirding and Sibley Guides.
Next, a fast-paced song bout by a Wood Thrush that sang only about six feet above my head, after giving a few ventriloquial "hawk alarm" whistles.
Cape May Warblers are uncommon migrants here and it's a treat to find one, let alone listen to it sing. This male sang at the top of one of the few conifers in the area. When I first heard it, I thought for a moment it was a Tufted Titmouse calling.
Finally, the bird of the day was a male Summer Tanager, an overshoot migrant that called but didn't sing.