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Posted by Toe on 08:04:54 05/24/10
In Reply to: Re: Artic Tern at posted by Scott S.
Best way to tell them apart is to look at the primaries. The Common Tern will show distinctive black on the first primaries, while the Arctic Tern will show almost no black on the upper wings except for a very thin line on the leading edge of the first primary. The Common will have black on 5-6 primaries. The underwing of the Common also shows more black on the trailing edge of the primary, while the Arctic has again a very thin line. Bottom line, the Arctic Terns wing at this time of year will appear very light, all the same color.
Also, the bill on the Arctic is noticably smaller, legs are shorter. Tail length is not a good indicator since this could vary depending on molt. On the Ponce pelagic trip, we had flocks of Arctics that showed different tail lengths. Folks were trying hard to make some with shorter tails into Commons (just to add to the list), but in the end, I don't think we saw any Commons in the mix. They were all Arctics.
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