Re: [cobirds] subspecific ID of the Boulder Co. Fork-tailed Flycatcher

In addition to subspecies identification, the pronounced emargination/notching of the 3 outer primaries that Christian pointed out also provides confirmation that the bird is a male. According to both Pyle's Identification Guide to North American Birds, Part 1 and Howell's Rare Birds of North America, females of ssp. savana have tapered/pointed outer primaries that are not strongly notched.  Here's a figure from Howell's book showing the distinction:


On a related note, a relevant research paper on Fork-tailed Flycatcher wing morphology was recently published in the journal Evolutionary Ecology: Provinciato, I.C.C., M.S. Auaujo & A.E. Jahn. 2018. Drivers of wing shape in a widespread Neotropical bird: a duel role of sex-specific and migration-related functions. Evolutionary Ecology 32: 379-393.
In the paper, the authors suggest that wing shape differences between males and females, and between the migratory ssp. savana and the non-migratory ssp. monachus, are the product of a mix of selection pressures, with some (e.g., efficiency in foraging, predator avoidance, and migration) acting on both males and females, while others are sex-specific (e.g., male courtship displays).

David Steingraeber
Fort Collins

On 10/26/2018 7:58 PM, Christian Nunes wrote:

Hi all, 

When Fork-tailed Flycatchers show up in the US or Canada, birders often attempt to ascribe them to one of the two likely populations from which they might come: the widespread South American austral migrant, savana, or the resident monachus of Central America. 

This article summarizes the occurrence of each Fork-tailed Flycatcher subspecies north of their normal range and offers guidance on how to separate them: 

https://www.westernfieldornithologists.org/archive/V25/25(3)%20p0113-p0127.pdf

The most reliable feature is the level of primary emargination. The nominate savana has pronounced emargination on the outer three primaries, while monachus shows this notch on only the outer two primaries. The more likely savana also tends to lack a white collar around the back of the neck, where the gray back color often meets the black nape.  

Here are some photos of the Boulder County bird that elucidate these differences: 

https://ebird.org/view/checklist/S49438796

https://ebird.org/view/checklist/S49381812

https://ebird.org/view/checklist/S49381592

https://ebird.org/view/checklist/S49411141

Based on these shots, the Boulder bird can be identified as being from the nominate South American population, savana. Folks might want to update their notes if they're interested in that level of ID. 

Christian Nunes

Lyons, CO

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