Luke,
Thanks for bringing this topic up as you are right, the Chaetura are a VERY tough group to identify with many birds in potential crossover situations, like migrants in Colorado, best being left at the genus level. I do believe you can make a definitive call on an individual basis with birds that "tick all the boxes", but we have to be very careful if the specific bird requires a little squinting to make the call. I did not run down to chase the Valco swifts, so my commentary here is for general consumption.
The group may recall I split time between Denver and North Idaho so am fortunate enough to see both species in good numbers every year. The main take away I've had with this experience is if Chimneys are "flying cigars", then Vaux's are "flying stogies". This points to the bird's structure and size as the best clues in separating the two – Vaux's always appear stumpy while Chimneys give you a sleeker vibe. Tough to admit as a dude, but size does matter and while relative size comparisons are not always possible, Vaux's appear small even when on their own while Chimneys don't – think Bank Swallows vs Tree Swallows. The small, stumpy look of a Vaux's seems to be accentuated with shorter, somewhat broader wings, which in turn must lead to their more fluttery flight behavior than Chimneys which also seem to glide much more frequently than the Vaux's. In Idaho, Vaux's will often migrate with Violet-green Swallows and can actually be tough to notice at first as their flight patterns can be very similar.
With respect to plumage, I believe these metrics should be the last box to tick. It is true Vaux's generally seem paler overall with more extensive pale throats/upper breasts, and certainly paler rumps, though they are variable. I've encountered a few Vaux's that approached Chimney in being on the dark end (gets me very excited), but to be honest don't recall the opposite with any of the Chimneys I've seen. One plumage factor that I don't recall reading about, but have noticed in the field, is that Chimney Swifts seem to have a browner tone to their appearance while Vaux's always seem greyer – again not sure this is real, but worth considering.
What does seem to be a real characteristic is the "primary bulge" shown by Chimney Swifts, but as you pointed out Luke, this can be very subjective and not likely that discernable in field conditions, but rather from multiple photographs of banking birds (I know this via the records we review on the IBRC on occasion). You have to be careful with this mark, however, as Vaux's can also seem to show this feature if the wing is slightly cocked, and Chimneys may not show this if their wings are not at the right angle – long story short, though to rely on this alone.
Last comment from my peanut gallery pulpit relates to Chaetura in Idaho I thought folks might find interesting. Like here in Colorado with birders picking up Vaux's Swifts mixed in with migrating swallow flocks, we do the same thing up there but for Chimney Swifts as most State records are of individual birds in with mixed flocks of swallows – basically a reverse parallel Universe.
Luke, thanks again for bringing this up, this is a great forum for local birders to air these identification issues out (much more approachable then ID Frontiers).
Good Swifting (the bird kind, not Taylor),
Doug
Denver
From: cobirds@googlegroups.com <cobirds@googlegroups.com> On Behalf Of Luke Pheneger
Sent: Tuesday, April 23, 2024 12:24 PM
To: Colorado Birds <Cobirds@googlegroups.com>
Subject: [cobirds] Chaetura Swift Help
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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