Friday, June 07, 2013

Confusing Flycatchers

What is an Empidonax Flycatcher??

Late last week I reported about a pair of Alder Flycatchers on Lookout Hill in Prospect Park. It was the first time I'd ever seen one in Brooklyn, or more accurately, ever positively identified one. The Alder Flycatcher is a member of a group of songbirds known as "empidonax" flycatchers. Many of them are very similar visually and, therefore, extremely difficult to identify. In North America there are fifteen species that comprise this group of small songbirds. They are:

Yellow-bellied Flycatcher (Empidonax flaviventris)
Acadian Flycatcher (Empidonax virescens)
Alder Flycatcher (Empidonax alnorum)
Willow Flycatcher (Empidonax traillii)
White-throated Flycatcher (Empidonax albigularis)
Least Flycatcher (Empidonax minimus)
Hammond's Flycatcher (Empidonax hammondii)
American Gray Flycatcher (Empidonax wrightii)
American Dusky Flycatcher (Empidonax oberholseri)
Pine Flycatcher (Empidonax affinis)
Pacific-slope Flycatcher (Empidonax difficilis)
Cordilleran Flycatcher (Empidonax occidentalis)
Yellowish Flycatcher (Empidonax flavescens)
Buff-breasted Flycatcher (Empidonax fulvifrons)
Black-capped Flycatcher (Empidonax atriceps)

In general, all of these birds are relatively small, with an olive upper body, white and yellowish undersides and varying thickness eyering. They all chase flies. I searched around for the etymology of the word "empidonax" and found two, comparable definitions. The "Scienceblog" website claims that in Latin it means "Mosquito King". "The Federation of Alberta Naturalists Field Guide to Alberta Birds" (by William Bruce McGillivray, Semenchuk, Glen Peter) says it means "King of the Gnats". Either one tells me that we humans owe these birds a huge debt of gratitude.

In some cases, it is easy to narrow down a particular empid's identity just by location and habitat, especially the Mexican specialties from the above list. Around New York City the only species from this group that we would normally encounter (mostly during migration) would be Yellow-bellied Flycatcher, Acadian Flycatcher, Alder Flycatcher, Willow Flycatcher and Least Flycatcher. In the Northeast, the Least Flycatcher is the most common empid, with its small size and large-headed appearance being a fairly quick way to ID them. Two of this group are so similar, the Alder and Willow, that they weren't determined to be separate species until 1973, primarily by studying their vocalizations. Prior to that, they were called the "Traill's Flycatcher". You can read about their vocal repertoires here. One of the best books I've found for beginning to understand the subtle field mark differences of these species is Kenn Kaufman's excellent "A Peterson Field Guide to Advanced Birding: Birding Challenges and How to Approach Them". He has a whole chapter dedicated to the empidonax conundrum. Each specie's field marks are broken down into size and shape, bill shape and color, voice, plumage color, molt and seasonal variation, behavior and comparisons to other species. As thorough and methodical as Mr. Kaufman's approach is in this book, he does point out that some individuals are even difficult to identify in the hand. That is why I rely on knowing the specie's vocalizations and (mostly) ignore them during the Fall migration when they are silent. I've embedded some song samples below:

Acadian Flycatcher: ("peet-sah" or "flee-sick.")

Alder Flycatcher: ("fee-bee-oo")

Yellow-bellied Flycatcher: ("che-lek")

Willow Flycatcher: ("fitz-bew")

Another really good source of identification information is an article that was published in "Birding" magazine. From "Identifying Empidonax Flycatchers: The Ratio Approach":

"Many birders know that Empidonax flycatchers differ significantly with respect to wing and tail measurements ("morphometrics"). But how do these morphometric statistics translate into useful impressions in the field? The Acadian Flycatcher provides a good example of the approach implied by this question. Even if you do not consciously detect the long primary projection and long bill, note how these characters translate into sword-shaped wing tips and a long, sloping forehead with a hint of a crest."

Whew, sounds intimidating. You can download the entire article as a PDF file here.

There are many identification challenges within this group of songbirds, not the least of which is that we only get to see most of them around NYC for a very short period of time. Studying these birds in the field would require traveling to their breeding grounds. The only species which regularly breeds within New York City's 5 boroughs is the Willow Flycatcher. In Brooklyn you can find them at Marine Park and Calvert Vaux Park. Other than that, I recommend buying "Birding by Ear", studying their songs over the Winter, then, come next Spring, when everyone else is scratching their head at the sight of an olive colored empid, you'll by confidently picking them out by vocalization.

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