Hello Everyone,
I would like to start a conversation about Chaetura swift identification in light of the recent Vaux's Swift reports from Valco Ponds. I find that this is an extremely difficult and underrated identification question, and I would love to hear comments from the community to help clarify my own thoughts. I also hope this discussion can be an opportunity for everyone to dig into an ID topic that is rarely studied in colorado!
I recently spent some time observing the swift(s) present at Valco Ponds, and found myself unable to confidently place the bird in the Vaux's category. There are a myriad of factors that go into this difficult ID, and I think that many of these traits are quite variable. I have listed my thoughts on each identification trait generally used in Chaetura swift ID below.
First, per eBird bar charts, Vaux's Swift should not be considered the expected swift during the third week of April. Chimney Swift has been observed in Colorado as early as April 9th, and I have personally heard them calling on the plains as early as April 23rd. I think it's important to note this so that we don't go into this ID with a potentially biased judgment.
Secondly, in terms of physical structure, I have read that a consistent physical difference is that Chimney Swifts typically have longer, pointier wings, and bulging inner primaries. This creates a pinched look where the wing meets the body, which is less present in Vaux's Swift. However, after digging through many Chaetura photos on eBird, I've found that this almost entirely depends on the angle of the photos. I feel that I can find photos of both species having both wing shapes depending on what I want to see. If a bird is pursuing aerial prey, they constantly change their wing shape, and that distorts with the wing proportions. Additionally, there are also documented differences in both physical size and flight pattern. I personally was not able to observe the bird next to any other birds, but a Vaux's Swift should be about the size of a swallow, if not slightly smaller. A chimney Swift should be slightly larger than the surrounding swallows. If anyone was able to observe the bird in question with swallows, I would love to hear about it! As for the flight style, Vaux's are considerably more bat-like in their flight, while Chimney Swifts have longer slower strokes. I was not able to personally differentiate those traits while watching the bird at Valco, but again, I would love to hear if anyone saw something that they feel strongly indicates one way or the other.
Next, in terms of plumage, I noticed a lot of field notes stating that the bird was IDd by a paler throat and a paler rump. This has been a purported field mark for a while, but it is not an absolute trait, but rather a gradient. Both Chimney Swifts and Vaux's Swifts have pale throats, and it is therefore important to look at this field mark in the context of the entire underparts. Vaux's swifts have low contrast throats because their breasts are lighter, while Chimney Swifts actually seem to be the Chaetura with higher contrast throats. This variability is also present in the lighter rump trait, where many chimney swifts also show a light rump. These traits seem to be highly dependent on the light conditions during observation. I have included some photos of the Valco bird that show potentially contradictory characteristics (potentially due to the lighting conditions)
Low Contrast throat: https://ebird.org/checklist/S169344097
Higher Contrast Throat:
https://ebird.org/checklist/S169701997
https://ebird.org/checklist/S169742187
https://ebird.org/checklist/S169493238
Finally, I think that this ID problem is complicated by the possibility of multiple birds currently residing at Valco Ponds. Some observers have noted multiple birds on their checklists. I also believe that there are multiple birds at Valco Ponds, as many of the characteristics I listed above vary greatly between photos, differing more than I would expect for a single individual.
If anyone has any additional photos that are not on eBird, I would also love to see them! Fantastic work by Chris and Brandon for calling attention to this bird, and great work by all those who took photographs!
I look forward to hearing your opinions and learning more about this fascinating ID conundrum!
Best,
Luke Pheneger, Boulder
William Anderson, Erie
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