Sunday August 27, 2017:  25 new birds of 14 species, 17 recaps.  No new species.  Bird of the day was Magnolia Warbler, with 6 new bands.

Today was a bit slower than yesterday, but we didn’t mind since we just had a two-person crew.  Most of our morning was fairly routine – we captured a handful of easy-to-extract birds each run, and the identification, aging and sexing was pretty straightforward as well.  Until . . . we caught this guy:


There are four species of Empidonax flycatchers that we regularly capture at the station:  Yellow-bellied, Alder, Willow, and Least.  There is a fifth species – Acadian – that could wander through, but which we rarely catch.  These five species often closely resemble each other.  In fact, Alder and Willow are so similar that we can rarely distinguish them in the hand, and so we lump them together and call them Traill’s Flycatcher – which was their name when they were formerly considered a single species.

Luckily, we generally have ways to separate most of our Empidonax.  Yellow-bellied Flycatchers are . . . well . . . yellow-bellied.  Least Flycatchers are small with tiny bills, bright white eye-rings, white chins and throats, and an emarginated sixth primary (we’ll explain that below!).  Traill’s have white chins and throats, but have a bigger bill, a less distinct eye-ring, and an un-emarginated primary.

The bird above definitely gave off a Traill’s vibe . . . a mushy eye-ring and a brownish olive complexion with a white chin and throat.  But the bill was tiny!


We took two bill measurements – from the front of the nares to the tip of the bill, and the width of the bill as measured at the front of the nares (both shown in red in the photo).  A Traill’s Flycatcher’s bill typically measures 7.6-10.3mm long and 5.0-6.1mm wide.  This bird had a bill measuring 7.3mm long and 4.6 mm wide . . . which fits much better with Least Flycatcher (although actually the width is even on the low end for Least!).

So, to be sure of our ID, we did a bunch of fussy measurements involving the difference between the lengths of various wing feathers, and we scrutinized the sixth primary to see if there was any hint of emargination.  There wasn’t.


The arrows in the photo above point to emarginations – dips in the leading edges of the feathers – in the 7th, 8th and 9th primaries of this flycatcher. The feather marked P6 has a straight leading edge, without a dip.

So . . . how do we resolve the conflict?  We take a holistic view of all the evidence:


The only “wrong” thing for Traill’s Flycatcher, was the bill size, whereas Least Flycatcher was ruled out in several ways, including by wing morphology and by the eyering.  (The leg color was a little funny for both, but sometimes color is hard to judge under our artificial lights).  The numbers in our cheat-sheet represent 95% of individuals, so in this case we decided we had a Traill’s Flycatcher who was on the extreme small-end of the bill size range, rather than a Least Flycatcher with an unusual wing morphology AND an unusual eyering.

There is a lot of science in what we do, but there’s always a little bit of judgement too!