Saturday, September 4, 2010

Empids At Higbee's

A magnificent rush of birds at Higbee's Beach WMA this morning included a veritable 'fall-out' of Empidonax flycatchers which was bound to result in folks deciding either to get stuck in to a thorny identification challenge, or to go home and have a second breakfast! I did both, so here's some photos from the former (no pictures of the latter sadly!).

Though taken from behind and with the head slightly turned away, this is clearly a Least Flycatcher, the most common Empidonax encountered at Higbee's Beach. Note the rather large-headed look (relative to the size of the body), the obvious eye-ring (especially behind the eye) and the rather small bill. Importantly, photographing it from behind allows us to see the very short primary projection of this species. The 'primary projection' refers to the length of primaries that can be seen extending beyond the tertials (the tertials are the three innermost - ie nearest the body - secondaries which tend to close over most of the primaries and secondaries when the wing is closed). [Photo by Mike Crewe]

I put up the picture above first so that it could be compared with this picture. This is the same bird, on the same perch. However, I think that if I was faced with just this photo, I may well struggle to identify this bird as a Least Flycatcher! Least is typically rather upright in stance, yet here it is almost horizontal. Also, with the wing tips moved out from the body, they are casting a shadow on the side of the rump, making the bird appear to have a longer primary projection than it actually does. A flattening of the crown feathers has also given the bird a smaller-headed appearance. Lesson 1: It pays to watch a bird for a while before you try to give it a name. Lesson 2: It's not always a good thing to name a bird from a single photograph! [Photo by Mike Crewe]

Much scarcer than Least Flycatchers at Higbee's Beach, Yellow-bellied Flycatchers can look structurally rather similar, being small and with a relatively large, rounded head. Yellow-bellies have a longer primary projection than Least, but are more easily told by their overall yellow wash to the underparts which extends right across the face on this individual. Essentially, a Yellow-bellied Flycatcher should show a throat that is the same color as the breast - Least has a yellowish (or white) breast and clean white throat. Note also here the slight crest. Yellow-bellied Flycatchers prefer to hunt unobtrusively within woodland and thicker cover, so are perhaps more regular at Cape May than records suggest as they will be less readily found than Least Flycatchers. [Photo by Mike Crewe]

And so to a really thorny issue! Whatever happened to Traill's Flycatcher? Well in case you didn't know, it got split up into Willow and Alder Flycatchers and the two species remain a real problem to separate, being best told in spring when singing, or in the hand using a complicated biometric formula based on the wing and bill. There are general trends though, and although there is much overlap, a bird showing pretty much the full suite of Alder characteristics - like this one - probably is one. Note the relatively small bill (foreshortened here but even so, it's not particularly long), rounded head, fairly distinct eyering (paler and more obvious than the lores) and slightly olive tone to the upper parts of this bird. All features that are more typical of Alder than Willow (although none of which are diagnostic I should add). Four birds like this were together along one of the Higbee's hedgelines this morning. [Photo by Mike Crewe]

But the reason why I feel quietly confident about the bird above being an Alder Flycatcher is because of this picture. The outer part of a bird's wing - its 'hand' consists of the longest flight feathers, the primaries. The relative shapes and lengths of these feathers varies from species to species and this - at least in part - makes up the birds wing formula. Typically for songbirds, Empids have a 10th (outermost) primary that is much shorter than the ninth primary. In Willow/Alder flycatchers, the 9th, 8th and 7th are of similar length and thus form a slightly rounded 'wing point'. The following primaries (6th to 1st numbered inwardly) are then progressivley shorter. Here, the bird has ruffled its wing and the short 10th primary can be seen. Most importantly, its length relative to the other primaries can be judged. So, the 9th primary can't be seen here as it is slightly shorter than - and thus hidden by - the 8th. Thus the 8th primary is the longest we can see here, with the tip of the 7th just next to it. Carrying on from there, we can see that the 10th is noticeably longer than the 5th, coming closer to the tip of the sixth in length - though still shorter than the sixth. This difference in length - which if the bird were held in the hand could be taken as a measurement in millimetres - coupled with the bill length (which certainly looked relatively short on this bird) gives us a formula which sorts out the 'Traill's' pair pretty well - making this an Alder in my book! [Photo by Mike Crewe]

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