This Mourning Warbler was on our deck on Sunday. It's rare to get such a good look, as it's a skulking bird that usually stays low to the ground in dense vegetation. Mourning Warblers breed across much of the upper Northeast and upper Midwest and across Canada. This warbler migrates in the fall starting in August, with many arrivals in the Northeast and upper Midwest by the third week in August, with migration peaking at the end of August and dribbling off in the beginning of Oct. So it was right on time for our area of NH. This bird had hit our window, not hard (perhaps it was chased?) and it was foggy out. It stayed a few minutes then flew off and was OK. I was outside and took the photos with a telephoto lens.
Telling Mourning Warblers from the similar looking western species, the MacGillivray's Warbler is a tough challenge, especially with young birds in the fall. When IDing these fall warblers (or all birds for that matter) you must look closely at everything you can see. Look at the overall shape of the bird, the facial pattern and eye-rings, the color of the throat and the breast and whether the throat color extends onto the breast, and the length the tail extends past the undertail coverts.
Young Mourning Warblers have thin eye-rings that are whitish or yellowish white and can be broken or almost complete, a brownish-olive hood, throat color varies from bright yellow to grayish buff. Throat color breaks through onto yellow of underparts giving a broken bib effect. Some 1st-winter males may show some mottled dark feathers on the breast, as this bird does. It is not always possible to tell imm. male from imm. female. Note the long yellow undertail coverts and the fact the tail does not extend that much past the undertail coverts. The MacGilllivray's tail extension would be longer. A bird bander, holding this bird in the hand, would also be looking closely at the primary coverts and shape of tail feathers to age this bird. Even in the photo you can see that the tail feathers seem very pointed, which is a sign of a young bird.
Young MacGillivray's Warblers would have thick, white eye crescents which may, in rare instances, form a nearly complete eye-ring. The olive brown hood becomes grayish and extends solidly across the upper breast with no extension of the throat color onto breast. The throat is usually pale gray or gray-buff and rarely, in imm. female is yellowish.
Another similar species in the East is the Connecticut Warbler. Young Connecticuts have buffy complete eye-rings (which on rare occasions can be thinly broken in the rear), the male with varying amounts of gray on forehead and breast, whitish or grayish throats. Some imm. females have very brownish hoods and buffy or yellow-buff throats. Connecticuts have very long undertail coverts and a short extension of the tail past the coverts.
Yes, we said these warblers were a challenging ID issue, but that's the fun of it also. Hope this helps you look very, very closely the next time you see one. (Much more on this with lots of photos will be included in our new The Stokes Field Guide to the Birds of North America, coming out very soon.)