Re: [Maine-birds] Tern ID?

Your bird is a Common Tern--a 1-year-old called 1st summer in many guides. These are seen at breeding colonies and roost sites but are uncommon here during summer. They are routine, however, and an identification challenge if one is unfamiliar with them.

This is a plumage that baffled ornithologists for a long time and has interesting connections to Maine. This plumage is also known as "portlandica" because a new species with that name was described from a specimen taken near Portland, Maine. That specimen was soon determined to be an Arctic Tern. Latter, Common and Roseate, as well as other terns, were found to show this winter plumage appearance in summer. Our understanding of this plumage, the so-called "white-faced" terns, was led by none other than Maine's own Ralph Palmer, who studied their occurrence at Popham Beach.

The identification is covered in modern field guides, but it is still confusing. The white forehead is your first tip that this is not a typical breeding tern. The all black bill and dark legs are shown by these 1st summer birds, and potentially confusing. Roseate has an all dark bill (some show orange or red at the base now, however), but then breeding Roseates also would have all black crowns and a different wing pattern. Their bill is also less robust, being thinner and spikier. The wing pattern is the clincher. The dusky outer primaries with the broad and blurry dark trailing edge below, well shown on the closer up-raised wing, is classic Common Tern. Roseate is much paler gray above and has only the outer two primaries with a dark outer (leading) margin and none to only a slight dark the trailing edge below. The dark inner secondaries showing at the base of the far up-raised wing is another good clue for Common Tern. Roseate has all white secondaries. Not shown by your image because it is hidden is the darker carpel bar.

Here is a link to the article by Ralph Palmer published in a 1941 issue of The Auk. It is rather long, but does describe this bird fairly well with accompanying plates showing specimens:
http://sora.unm.edu/sites/default/files/journals/auk/v058n02/p0164-p0178.pdf

For birders, the ground-breaking articles by Bob Scott and Peter Grant published during the late 1960s in the journal British Birds are classics on this topic and examples of careful observation, note-taking, and analysis. You can read their study of similarly plumaged Common Terns to yours here:
http://www.britishbirds.co.uk/search?model=pdf&id=4227
The accompanying plate is here:
http://www.britishbirds.co.uk/search?id=4228
(click on thumbnail of black-and-white sketch with terns)

Also helpful is their article on juvenile Sterna terns here:
http://www.britishbirds.co.uk/search?model=pdf&id=4272

Louis Bevier
Fairfield

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