Monday, July 29, 2013

Arctic Tern Study

The BTO have been putting together a series of identification videos and now and again I spend some time going through these to remind myself of key ID points on some potentially confusing species. They really are most useful and to date there are 27 videos covering over 60 species. This morning I was taking another look at the one titled: 'Taking a Look at Terns 1: Arctic and Common Terns.' These two species can be a little tricky if you are not sure what to look for. This prompted me to post a few of my Arctic Tern shots. When you have had a good look at these I recommend checking the BTO video by clicking on the following link:
These pictures were taken at the colony on Inner Farne in the Farne Islands, Northumberland. Whilst we were there we had good views of two breeding Roseate Terns but we had left by the time the Bridled Tern turned up!
The main ID features of Arctic Tern are: a blood-red bill without a black tip, which can be seen best on the third photograph. (The bill of a Common Tern tends to have a distinctive black tip and the bill of a Roseate appears to be all dark with a red base.) The throat, breast and belly tend to be washed grey and this forms quite a clear contrast with the white of the cheeks. This feature can be seen on the photos of the bird in flight as well as the perched birds. (This contrast is less obvious on Common Terns and not really present on Roseates.) Arctic Terns have very long tail streamers which extend beyond the wing tips when the bird is perched as you can see in the second photo. (The tails of Common Terns do not extend as far as the primary tips.)

Arctic Tern Sterna paradisaea   Adult
Arctic Tern Sterna paradisaea   Adult showing length of tail and contrast between the grey under-parts and the white cheek.
Arctic Tern Sterna paradisaea   Adult showing blood red bill.

The flight shots show that the flight feathers are translucent and almost pure white when viewed from below and the outer primaries have a neat black trailing edge. Common Terns would show a much broader diffuse black trailing edge to the primaries.
Arctic Tern Sterna paradisaea   Adult. Note the distinctive contrast of the cheek

Arctic Tern Sterna paradisaea   Adult showing the neat black trailing edge to the primaries.

Arctic Tern Sterna paradisaea   Adult - note the white, translucent flight feathers.

Arctic Tern Sterna paradisaea   Adult

Other ID features of Arctic Tern, useful for separating it from Common Tern, are its slightly smaller size, shorter legs, narrower wings, uniform pale grey upper-wings which lack the dark wedge found on Common Terns and its more bouncy flight. None of these features can be seen on these pictures and you really should see the birds side by side to benefit from some of these points so go and take a look at the BTO videos.

No comments:

Post a Comment