Wednesday, August 11, 2010

Arctic Tern at Maumee Bay State Park

Wednesday, August 11, 2010: Maumee Bay State Park, on the Lake Erie shoreline in Lucas County just east of Toledo, Ohio, is a superb place to study terns in late summer and early fall. The concentration of Common and Forster's terns there often reaches 200-300 individuals, offering a fine opportunity to compare their plumages, shapes, and flight patterns.

I went to the park today mostly to look at juvenile Ring-billed Gulls, but while I was there I scanned through the flock and was surprised to find an adult Arctic Tern resting with the Common and Forster's terns. This species is generally quite rare in the interior of eastern North America; Ohio had only a few previous records. The Arctic Tern was present for at least half an hour, long enough for me to shoot some photos and call some other birders, but by the time anyone else arrived, the bird had disappeared. Fortunately it came back in mid-afternoon, and many other birders got to see it late in the day.
Field guide treatments may emphasize bill colors: usually solid red on adult Arctic Tern, red with a black tip on Common Tern. However, many adult Commons in late summer become entirely red-billed. The Arctic Tern present today, however, had a very dark bill, which is an uncommon phenomenon for so early in the season but more expected later on in fall.

Here are some photos and some discussion of how today's bird was identified.
While standing, this bird was noticeable by its overall dark gray look and its short legs, but these are both tricky points. Both Common and Arctic terns vary in their overall shade of gray, and Common Tern can look shorter-legged depending on its posture and the position of its feathers. To confirm the I.D., it's very helpful to see flight pattern.
Here, with the bird taking off, we can see that all of its primaries are about the same shade of gray on the upperside, rather than having the outer primaries much darker than the inner ones. And the trailing edge of the primaries (outer part of the wing) is traced by a narrow black line. On Common Tern, there is a broader, smeary dark trailing edge but it involves the tips of fewer feathers.
Here's another angle showing the relatively uniform color on the upperside of the wing and the narrow black trailing edge on the whitish underside of the wingtip, as well as the strong wash of gray on the underparts. Notice also that the wings appear to be set far forward on the body, an illusion enhanced by the small head and long tail.
This photo compares the Arctic Tern (above) with a Common Tern, perhaps a two-year-old bird rather than a full adult, with a different shape and different wing pattern.
With practice, the wing pattern of Arctic Tern can be seen from a number of different angles.

In this photo, the Arctic Tern is seen flying with several Common and Forster's terns. The differences in shape and wing pattern are evident here.


Glen said...

Incredible. Thanks for the awesome documentation and write up! Excellent photos.

Jerry Jourdan said...

Terrific post, Kenn. Those flight pics really show the black wing tips so diagnostic. I will say that its a feature so hard to see on flying birds, since wing angle / lighting can cause the black to disappear.

John Freeland said...

Great photos, especially the flight shots. Much appreciated

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