It seems too, well, yellow for an Orange-crowned Warbler. But why does it have a sort-of forked tail? And if it's a Yellow Warbler, why not any red streaks? And why isn't the tail yellow to the tip? Wilson's? What's with that forked tail? What am I missing? Help, please!
Many thanks to Dave Irons for the following excellent answer to my dilemma. The photos were taken April 23, 2012, but it should be noted that we are two weeks behind Salem in spring leaf-out.
Dave Irons answered:
There are several clues here that I think help answer your warbler ID question. Here's what I see.
1. Vegetation -- The deciduous tree in the photo is just starting to bud and leaf out. In the Willamette Valley that typically happens from late March to mid-April with bigger trees. You say this photo was taken in April, but don't give us the exact date. I think that if this image had been taken in the latter days of April the leaf out would be further along. This alone likely takes Yellow Warbler out of the equation. The first northbound Yellow Warblers normally arrive in the Willamette Valley about 25 April or later and they are generally scarce before the 1st of May.
2. Undertail coverts -- Another strike against Yellow Warbler is the length of the undertail coverts (along with the lack of yellow going out to the end of the tail, which you mention). Yellow Warblers have long undertail coverts, which tend to make the tail look short from below, which is clearly not the case with this bird.
3. Bill shape -- Looking at the bill of this bird, the tip seems somewhat blunt. Orange-crowned Warblers and other species in the genus Oreothlypis (formerlyVermivora) have thin bills that come to a very sharp point. On bill shape alone I think we can eliminate Orange-crowned Warbler, which leaves us with only Wilson's Warbler as a likely candidate (the only other all-yellow warbler that one would expect in Oregon during April).
4. Bill color -- This is the clincher. The underside of the bill is very pale and looks quite yellowish. After hatch-year Orange-crowned Warblers have all-dark bills with no yellow whatsoever. Conversely, Wilson's Warblers show yellow mandibles (the lower half of the bill). In spring, Yellow Warblers also have dark bills.
5. Tertial edges -- On the top photo I think that we can see enough of the upper wing to determine that edges of the tertials (the inner most folded feathers on the wings) do not contrast with the rest of those feathers. In all plumages, Yellow Warblers have darker duskier tertials with noticeably pale edges. This is a great field mark if you are ever struggling to sort out dull immature Yellows and Orange-crowneds.
Given the presumed date (based on condition of the vegetation), the mostly yellow coloration, the somewhat blunt-tipped bill that is pale below, and the lack of contrast in the tertials, I think it is pretty safe to conclude that this bird is a Wilson's Warbler. Wilson's typically return to Oregon by the second week of April, not long after the arrival of the first Orange-crowneds. When I first started paying attention to such things (late 1970's), the average arrival dates for Wilson's fell around the 17th of April and Wilson's were exceptionally rare in Oregon during winter. Over recent decades the mean arrival date for Wilson's seems to have moved forward by about a week and this species is now found in Oregon in most winters.