Hi all: Cordilleran Flycatcher is a widespread and common breeding species in Colorado's foothills and mountains, a species that is very familiar to many of us, particularly to those hosting breeding birds on their houses or outbuildings. However, I believe that that familiarity and abundance has made for overcertainty in reporting of the species by Colorado's birders. While the proximal impetus for this essay was from my task as a reviewer of Colorado eBird records, it is a subject that I have pondered for quite some time. In fact, I have posted to this venue previously on the topic.
Publications in the 1980s and beyond established the case for splitting what was then known as Western Flycatcher into two component species, Cordilleran and Pacific-slope flycatchers. While the breeding ranges of the two are well established and involve a bit of geographic overlap, nearly all else that we know about the two species' distributions is not supported all that well with hard data. The reason for this is that the two species are VERY difficult to differentiate in the field AND in the hand. Rocky Mountain Bird Observatory has spent years catching migrants on the plains and most such birds have been reported as Western Flycatcher, because it is so very difficult to be sure of what one has in hand. Yes, there are known differences. The Male Position Note differs noticeably between the two species (though with some apparent hybrids or individuals giving both calls or intermediate calls in the overlap zone [and elsewhere?]). However, all other vocalizations overlap significantly, negating their use in definitive species identification. While there are some plumage-color differences, these are subtle and require extensive experience with both species in a variety of lighting conditions to use and, even then, are fraught with difficulty. Thus, identifying any single non-Male Position Note-calling Western Flycatcher to species is, essentially, a case of guessing. In Colorado, I am quite comfortable with identifying all spring-migrant and breeding-season Western Flycatcher to species, specifically Cordilleran Flycatcher, though, if someone came up with data supporting the occurrence of Pacific-slope in the state during that time period, I would gladly reassess that opinion. I believe that such IDs are "safe" because there are no data suggesting the occurrence of the other species in the state then. Granted, that's a bit of circular thinking, but, hey, I'm only human. However, I do object to willy-nilly identifying fall birds as Cordilleran, particularly on the eastern plains. That is because nearly across the board, Rocky Mountain breeding-bird species seem to be able to go north and south on their migrations without drifting off the the cordillera in any significant numbers, particularly in fall. As example, Cassin's Vireo (a northwest-Pacific breeder) is much more common on Colorado's eastern plains in fall than is Plumbeous Vireo (which is rarish there then), which breeds within sight of many well-birded eastern-plains locations. Cassin's Vireo is also not at all rare in the mountains and on the West Slope, though greatly outnumbered by Plumbeous Vireos there. The comparison between the migrant Townsend's Warbler versus the breeder Black-throated Gray Warbler is similar, but on an even larger numerical scale. Likewise, White-throated Swift, Red-naped Sapsucker, Purple Martin, Violet-green Swallow, Western Bluebird, western Veery, Virginia's Warbler, Sage Sparrow, Slate-colored Fox Sparrow, Mountain White-crowned Sparrow, and Gray-headed Junco are all, in fall, fairly rare to exceedingly so in eastern Colorado away from montane or foothill habitats. Yes, there are bird species breeding in montane Colorado that are not rare on the eastern plains in fall, but most of those are more-widespread breeding species in which more-far-flung popluations might be masking the occurrence of actual Colorado breeders on the plains. So, why is it that we willy-nilly report all Western Flycatchers in Colorado in fall as Cordilleran? Because Pacific-slope has not been accepted as occurring in the state? Forgive me, but that is a specious and even-more-circular argument. If we don't figure out how to identify, or even consider, Pacific-slope Flycatcher, how can the species ever be accepted to the state list. Again, consider how relatively common Cassin's Vireo and Townsend's Warbler are in the state in fall, both species that share breeding range with Pacific-slope Flycatcher. Sincerely, Tony Leukering Villas, NJ -- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit https://groups.google.com/groups/opt_out.
