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Gordon Sick

Common or Arctic Tern at Point Pelee, Ontario, Canada

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I always struggle to distinguish a Common Tern from an Arctic Tern. I realize that a Common Tern is much more likely, but Point Pelee is at a major migration point for birds to cross Lake Erie. Here are some pictures I took of a Tern at the southern tip of Point Pelee on May 10, 2019. iBird PRO gives me identifications (sometimes with very high confidence) of Common Tern, Arctic Tern and Roseate Tern. I think I'm really out of range for a Roseate.

The first two pictures are declared as Arctic Terns by iBird Pro. The third gets ratings of Common and Arctic Tern, the fourth gets Roseate and Arctic Tern and the fifth is rated as Roseate Tern. The pictures are all of the same bird. All show a red bill with a black tip and black upper and lower trailing edges on the primary feathers. The last three photos show the feet, which are red.

Suggestions are welcome.

–Gordon

Arctic Tern ??? (1).jpg

Arctic Tern ???.jpg

Common or Arctic Tern ???.jpg

Roseate or Arctic Tern ??? (1).jpg

Roseate Tern ???.jpg

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Here are two pictures (of one Tern) that I took 2 minutes after the previous sequence. The first and second sets of Terns are likely different birds, but of the same species. iBird calls the first picture a Common or Roseate, but gives a 93% confidence to the second of being an Arctic. These photos show a dark outer edge to the tail feathers and a greater ability to estimate the length of the V of the tail. We also see a dark leading and trailing edge of the underside of the whole wing, which means this bird is probably different from the first.

–Gordon

Common or Roseate Tern ???.jpg

Arctic Tern with 93% confidence.jpg

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These birds suggest Common Tern to me for several reasons.

First, range and timing. Arctic Tern would be very rare at Point Pelee. They migrate almost exclusively along the Atlantic and Pacific coasts, and even there, primarily offshore. Arctic Terns are also fairly late migrants in spring in the northeast, for example in Massachusetts they are seen in the largest numbers as spring migrants in late May and early June, so May 10 would seem rather early to me. The location and date do not necessarily mean that Arctic is not at all possible, just that it is very unlikely.

The second thing that is getting me is the bill coloration. During breeding season, Arctics tend to have totally red bills. The birds in the pictures have a black tip the bill which is typical of breeding Common Tern. That being said, in the nonbreeding season, Arctic has a black bill, but I would expect the bill to be totally red during spring migration.

The last thing, and in my mind the most conclusive, is the shape of the bird. The terns in the pictures are rather flat-headed, with long bills and some neck in front of the wings, and not super long tails. This is typical of Common Tern and combines to give the impression of a more evenly proportioned bird that is slender throughout with both a long front end and a fairly long tail. Arctic Terns have longer tails but less neck, a short bill, and a rounder head. This gives the impression of a shorter and more bulbous front end, and a long, tapered back end with the proportionately longer tail.

754336731_arctictern.jpg.5fbd95ae4c2b478da3c79fda5f3c8ecc.jpg

Here is a photo of an Arctic Tern. Note the smaller, all-red bill, and the rounder head and shorter neck when compared with the birds in your photos, and overall shorter front end in front of the wings.

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Arctic Terns have been observed in Ottawa, which is more landlocked than Point Pelee. Point Pelee is along the Saint Lawrence and Great Lakes waterway.

I am hoping that someone with expertise in observing Arctic Terns and their characteristics will shed some light on the differences.

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Common Tern:

Very wide dark trailing edge to the primaries from below

Outer primaries notably darker (on top side) than inner primaries (this feature caused by a very different wing-molt strategy in Common vs. Arctic)

 

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Thanks Tony. This is very helpful.

Often, we get a more definitive view of feather shading than we get of bill colours or body shapes, so I think I'll be using this information a lot in the future.

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