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Identification of birds has two parts, the first being fitting visible field marks (whether of plumage, structure, or behavior) of the bird to a possible identification. This part of the equation nearly all do at least reasonably well. However, the second part of ID is often overlooked or given short shrift: determining field marks present that are not consistent with a proposed identification. In my experience, most ID mistakes made happen here.
A bird, dark above and light below, flying low over water sent many among us into the alcids for a solution. In fact, at least seven species of alcids were provided as answers for the January quiz with all, except for Pigeon Guillemot, being small species. The quiz bird does, indeed, look small and chunky, and many alcid species fit that description. Some species suggested as answers can be ruled out by looking at the feet of the quiz bird. Or, more precisely, noting that the feet are not apparent. This feature fits the small alcids quite well, but the larger alcids, including guillemots, have longer legs that are held in a trailing position in flight (rather than tucked up inside the feather coat) and many of those have brightly-colored (orange or red) legs. Additionally, as alcids have short tails, the feet of most of these larger alcids extend a bit beyond the tail tip. So, we can rule out those species. Regardless of which small alcid one picked for the answer to this quiz, the bird would have to be in basic plumage (though most of the Synthliboramphus murrelets do not have significant seasonal plumage change).
Now we need to use that second part of the ID process. In my opinion, the critical aspects of this bird's field characters lie on the upperparts, as the underparts are nearly unrelieved white. The uppertail coverts (visible at the base of the tail) and the tertials (visible at the base of the wing) all seem to sport white fringes, a feature at odds with any ID as an alcid. Additionally, the back and rump seem to be some odd metallic greenish color, a character also at odds with an alcid ID. These features send us back to the drawing board, or field guide.
Nearly all birds can be seen, at some time or other, flying over water, as evidenced by our respondents' unconcern with proposing species such as Pygmy Nuthatch and Black-throated Blue Warbler; note that our quiz bird's distinctly black mask fits well with both of those species. However, the quiz bird's uppertail coverts and tertials rule out those options, as does the back coloration. In fact, the back coloration should take us straight to the swallows without "passing 'Go' or collecting $200;" there just aren't that many ABA-area bird species that exhibit such color. Once there, we can quickly eliminate the brown swallows from consideration, as well as the red-rumped species and Barn Swallow. At this point, we're left with just one genus to consider: Tachycineta. Once here, we can quickly rule out Violet-green and Mangrove due to the lack of white extending up onto the rump and Bahama is eliminated due to our quiz bird's not-obviously-forked tail. I took this picture of a Tree Swallow at Cape May Point S.P., Cape May Co., NJ, in November 2008.
The following people (listed by submission date beginning with the earliest) submitted correct answers for the January Bird Photo Quiz—Tree Swallow:
The following list shows the number of submissions for each species guessed.
The photo and answer for this quiz were supplied by Tony Leukering.