Tony,
You raise excellent questions about the range of ravens in SE Colorado. I haven't spotted many ravens in Prowers County, but I've spent some time carefully studying a few in Kiowa, Bent, and Lincoln Counties in the past two years, and in other SE counties before that. In general I think your ID article in Colorado Birds is spot on. Identifying the ravens in the field is extremely difficult. After my careful studies I usually end up entering them into eBird as "raven sp.".
I tend to be a Chihuahuan Raven skeptic. Whenever I encounter a raven in Colorado, including on the SE plains, I need to be convinced that it is a Chihuahuan. In my experience, Common is the expected raven species in the canyons south of the Arkansas river, and I have come to expect it at John Martin Dam, which is at least a potential nesting site. This may represent a fairly recent range expansion for Common Raven.
Fifty years ago, Bailey and Niedrach wrote that "White-necked [i.e. Chihuahuan] Ravens once occurred on the Colorado plains in thousands, but they have disappeared from all except a small area, with nesting now localized from Cheyenne, Kiowa, Kit Carson, and Lincoln Counties to the eastern border of the state. [...] All of our observations of the species have been in the four counties mentioned above, especially south of Kit Carson. In this area in the 1920s and 1930s there was a nest on almost every windmill, and each of the dwarfed trees had one or more. [...] During the past twenty years we have seen a few ravens on practically every trip through that region, but the birds have gradually decreased, until now, in 1964, they are no longer common."
Bailey and Niedrach gave zero information about either raven species east of the mountains and south of the Arkansas River. They say of Common Raven, "a fairly common bird of the mountains and western areas of the state, [...] reported from all counties west of the prairies." It seems that fifty years ago, there were few ravens of either species south of the Arkansas. The area was surely undersampled, but Niedrach mentions several trips to Baca County and Bradbury led museum field trips to Baca County in 1921-22, so it seems clear that ravens must have been much less common there then than they are now.
A couple years ago, Andrew Spencer and I decided to start measuring the nasal bristles of the Common and Chihuahan Ravens in museum collections, because it turned out nobody had ever done a quantitative study of nasal bristle length, which is often touted as a key field mark to separate the two species. I'll be presenting the results of our study at this year's CFO convention in Montrose. For now, I'll say that identifying raven specimens on a table in a museum is straightforward. The size distance is striking when the birds are dead and in hand, and the color difference in the feather bases is pretty easy to see when you part the neck feathers. But a live bird in the field is a totally different matter.
Personally, I'd recommend the conservative approach, asking all birders to supply comments and media on all ravens they encounter in SE Colorado, with "raven sp." the default unless the bird is carefully studied.
Nathan Pieplow
Boulder
On Sun, May 26, 2019 at 10:37 AM coloradodipper via Colorado Birds <cobirds@googlegroups.com> wrote:
All:--
I write to request help with discerning the occurrence pattern of Common Raven in Prowers County. In my experience, the only ravens that I've seen/heard in Prowers (away from the very southernmost bit by Two Buttes SWA) that I was convinced were Commons -- and not the expected Chihuahuan Raven -- were all in the fall/winter period, a period in which Commons seem to spread out/wander. Additionally, all of my very few definite Common Raven records in the county are from my previous sojourn in the state, so the subsequent 11+ years might have seen a change in the species' occurrence pattern.
Because Prowers and Bent Counties are currently covered by the same eBird filter, and because Common Raven is an expected breeding species in, at least, southern Bent, the filter allows Commons all year in the filter region, thus in Prowers County. As I did not record Common Raven in Prowers in summer in my previous occurrence in the state and I have yet to record it at all in my current sojourn here, I am confused by the eBird record for the species in the county:
https://ebird.org/barchart?r=US-CO-099&bmo=1&emo=12&byr=1900&eyr=2019&spp=comrav
Relevant to the discussion is my essay on raven ID in the In The Scope column in Colorado Birds:
So, I'd appreciate the thoughts of those that have spent time in the county AND paid careful attention to ravens there. I could use other opinions, because I am in the process of splitting Prowers Co. out of the current filter, and I need to figure out what to do with Common Raven.
Thanks,
Tony
Tony LeukeringWiley, CO
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