Friday, October 16, 2009

Hawk Watch Hudwit, Higbee Sparrows, and Tern Tips

A Hudsonian Godwit flew over the hawk watch platform today, or so I hear. Just the sort of bird, an over-ocean migrant, you would expect a northeaster to bring.

So were the three Blackpoll Warblers at the Higbee Beach parking lot, though of course we see plenty of these without northeast winds. Higbee was lively enough for a nasty morning, with Baltimore Oriole, lots of sparows including White-crowned, Merlins on a regular basis, towhees, thrashers, Rose-breasted Grosbeaks, and flyover Bobolinks (only two, we're getting towards the end of them).

We received quite a bit of positive feedback on the fall warblers post (see below), so here's another thorny i.d. problem dissected, courtesy of Mike Crewe, who wrote the following post and took the photos that accompany it. Mike, a former leader for Limosa, now works for CMBO and will be leading workshops and walks periodically. Click to enlarge the photos.

"One Good Tern...

"On Monday October 5th, an Arctic Tern was reported to CMBO staff as being present on the beach at Second Avenue jetty. It was a couple of days before I was able to get down to have look during a lunch break - and Wednesday was hideously windy! However, a nice mixed group of terns, gulls and skimmers was resting in the lee of the stone jetty at the end of Beach Drive and gave a great opportunity for some quick photography. A short-legged and seemingly short-billed tern was immediately apparent and certainly looked a good candidate for Arctic Tern. But ended up being a good candidate for demonstrating that one shouldn't rely on a single feature to identify a bird!

I put together the following snapshots as an 'educational tour' of the bird.

[Picture one, above, shows the entire bird. Important features which lead to this bird being a Common Tern are the sloping forehead which gives a rather flat-headed profile (Arctic Tern is more round-headed, not unlike a Black Tern in outline) and the obviously dark - almost black - outer primaries, which contrast strongly with the inner, paler primaries (more of this below). Photo by Mike Crewe.]

[Pictures two, above, and three, below, show the bill. Notice in picture two how it looks relatively short and rather stout-tipped. But in picture three it looks longer and with a more attenuated tip. This is a good example of how one has to be careful that the bird's head is truly at right angles to the observer before assessing shape and size. Whilst picture two looks good for Arctic Tern, picture three is clearly within the range for Common Tern. Photos by Mike Crewe.]

[Picture four, above, shows the bird with what is clearly a juvenile Common Tern (right). The legs on our bird (left) appear noticeably short in comparison with the other bird but don't really seem to be short enough to be typical of Arctic Tern. Arctic Tern legs are so short that the length of the lower half of the leg is about the same as the length of the foot. Leg length is variable in terns and this bird just happens to be at or near the shortest length for a Common Tern. In this picture it is also worth noting that the bills look pretty similar in length and the wing pattern is very similar with obviously dark outer primaries (though a Laughing Gull has stuck his head in the way on the right) Photo by Mike Crewe.]

[The final picture now leads us to what we should be really looking at to clinch the identification of this bird - the wing. Apologies for my autofocus which kicked into the background as soon as the bird took off! However, it is possible to see the coloring of the wing here. What is important is the contrast between the very dark, outer three primaries and the paler (though still smudgy grey) inner primaries. This is a classic molt pattern for Common Tern; the whole wing would be much paler in an Arctic Tern, giving a much more uniform appearance. Photo by Mike Crewe.]

"Finally (though really this should be first!), as with many birds, it is often best to start by trying to age the bird correctly. The scattering of black feathers on the forehead show this bird to be older than a juvenile (ie, not born this year) as juveniles would be white here - or buffy if very young. By the end of their second summer (some 14 months after hatching), most Common Terns can be impossible to tell from older, adult, birds as they molt into non-breeding (basic) plumage. However, the extreme darkness of the outer primaries on this bird and rather obvious dark bar on the shoulder (known as the carpal bar which actually runs along the leading edge of the inner wing) strongly suggest that this is a first-summer bird (a confusing name in same ways as the bird is actually in its second summer - but you know what I mean!)

As a more general comment on molt (and I don't want this to get technical so I'll keep it short) it is well worth remembering that adult Common Terns start molting soon after the breeding season finishes, so by fall they look pretty scruffy, with a mixture of new and old feathers. The molt of primary feathers is started, but then suspended until the birds reach the wintering grounds, resulting in the wing pattern seen in the photos here - a mixture of new inner and old outer primaries as well as new and old feathers elsewhere (note in particular on photo one, the neat row of brand new, pale grey wing coverts in the centre of the wing). In sharp contrast to this, Arctic Terns molt almost no feathers at all until after they migrate south and reach the wintering grounds in the Southern Hemisphere. Thus any molting Common/Arctic Tern in fall really should be a Common.

"Terns, a confusing bunch and perhaps best left alone, unless you want to end up getting into the murky depths of gulls!!! - Mike Crewe"

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