Perhaps counter-intuitively, most of the positively identifiable "Trail's" Flycatchers in Cape May during fall migration are Alders. That's counterintuitive because Willows are more commonly encountered in most birders' general experience, and because they breed locally, e.g. sometimes at the South Cape May Meadows (though apparently not this year) or along the brushy edges of salt marsh.
A look at the range maps explains the situation. Simply put, there are more Alders north of Cape May than there are Willows, hence more Alders to come south. Alder Flycatcher breeds from the Appalachians up and across Canada to Alaska; Willow's range barely crosses the Canadian border.
Once on our northern NJ Birding by Ear workshop, Mark Garland remarked that he knows he's in a good place if he's hearing Alder Flycatchers (we were at the time, in the Delaware Water Gap NRA). Alders breed in northern swamp and bog margins, home to many other marvelous things, from winnowing Wilson's Snipe to pitcher plants.
Alder Flycatchers normally begin appearing in Cape May as southbound migrants in mid-late August, peaking in early September, and continuing through late September.
[Different Alder Flycatcher, also Vermont 2 weeks ago. Okay, they're not always in Alders, so what do you do? Hear it sing or pip, that's what! Alders look subtly darker and/or more olive on the back than Willows, sometimes, maybe, yeah, sure, and there are other characters to fool with. Listen to the field guides; listen to the bird.]
[A different view of a different empid, a female Acadian Flycatcher banded in Bear Swamp, Cumberland County last Saturday. The breast and belly feathers are being gently blown aside to reveal the brood patch. (Don't worry, the bird is being held very gently as well.) In almost all North American passerine birds, the females do all or most of the incubating, and develop a complete brood patch, an area bare of feathers (seldom noticeable in the field) that fills with fluid and facilitates heat transfer to the eggs. This brood patch has begun to recede, indicating this bird's young have left the nest. The feathers will be replaced during the bird's pre-basic molt. Note, by the way, how the Acadian's breast feathers have dark bases - out of place dark, or sometimes light, patches on any bird can sometimes be explained by disheveled or missing feathers, revealing parts of other feathers not normally visible.]