Date: 10/26/18 6:58 pm From: Christian Nunes <pajaroboy...> Subject: [cobirds] subspecific ID of the Boulder Co. Fork-tailed Flycatcher
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:
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: