posted October 11th, 2010; last edited November 18th, 2010 –– David Sibley

Vaux’s Swift     Chaetura vauxi

A small, dark gray swift, essentially the western counterpart of Chimney Swift.
Identification summary: Distinguished from other western swifts – White-throated and Black – by smaller size, shorter wings and tail. Extremely similar to Chimney Swift and reliably distinguished only by voice. Identifying a lone bird of these two species in flight, by visual characteristics alone, cannot be 100% certain on current knowledge.

Identification challenge: Vaux’s vs. Chimney Swift

These two species are extremely similar and closely-related. Most are safely identified by range (Vaux’s in the Rocky Mountains and west, Chimney east) and there is no place where both species occur together regularly. Chimney Swift enters the range of Vaux’s as a rare visitor throughout the west in spring, summer, and fall, and Vaux’s is an extremely rare visitor eastwards to Texas, the Gulf Coast, and Florida, mainly in winter. Vaux’s is distinguished from Chimney Swift by the following:

  • calls higher-pitched, quieter, more insect-like (vs. lower-pitched, louder, stronger in Chimney)
Voice is the most reliable means of identification. Unfortunately, calls are rarely heard away from breeding areas.
  • wings relatively straight-edged and evenly-tapered on Vaux’s (vs. wings with bulge on inner primaries)

    Vaux's (left) and Chimney Swift, click for more info on wing shape differences.

Differences are discussed and illustrated here. Proportions can be very hard to judge, as swifts turn so quickly in flight and present different appearances at different angles, but studying series of photographs and/or long study of flying birds might reveal differences that an experienced observer could use to identify these species.
  • wings relatively broad (vs. slightly narrower)
This difference is small and wing width varies a lot with wing position on any individual bird, this is a difference that comes from an overall impression gained from watching a bird over a period of time. See more details here.
  • overall size slightly smaller (vs. slightly larger)
Wing chord average 112 mm (vs. 129 mm on Chimney; Pyle 1997); tail 36 mm (vs. 44 mm on Chimney). Size is useful only in direct comparison, as it is extremely easy to misjudge the size of a lone flying bird, particularly when one is already primed (by location or weather, etc.) to expect something different.
  • pale band across forehead (vs. dark forehead)
Both species have a narrow and faint pale eyebrow. In most or all Vaux’s Swifts this continues as a narrow pale band across the forehead, just above the base of the bill. In Chimney Swift the pale eyebrow ends at the front of the eye and does not continue across the forehead. Unlikely to be visible in the field, but under exceptional conditions or with extremely good photographs this might be visible. Needs further testing.
  • Smaller bill (vs. larger bill)
Bill depth is about 2 mm on Vaux’s (vs. 2.6 mm on Chimney; Pyle 1997) and culmen more gradually curved (vs. more abruptly angled down to bill tip on Chimney). This is apparent when examining birds in hand, and may be visible in extremely good photos of birds in flight, but is unlikely to be useful under normal field observations.

Unreliable or untested features

  • slightly paler underparts and rump (vs. darker underparts and rump)
This is an average difference that shows when a series of specimens are laid out, but individual birds of both species overlap with the other. Throat color is very similar, with many Chimney Swifts just as white-throated as most Vaux’s, but Vaux’s tends to have a paler gray belly. Aside from the issue of overlapping color, it is so difficult to judge shades of gray under normal field conditions, with a rapidly moving bird against a bright sky, that this feature is of little or no practical use.
  • quicker wingbeats with more “twinkling” flight
Similar caveats as on size (above) apply to wingbeats and flight style. There are real differences between the species on average, but it is too easy for the suggestion that a bird is “different” to guide our perception, especially when we don’t have the benefit of direct comparison. Flight characteristics may be a helpful clue to indicate the presence of a potential Vaux’s Swift, but the final identification must be based on other features.
  • Glides less and for shorter periods (vs. more gliding by Chimney Swift)
Some observers have commented on the impression that Vaux’s Swift flaps more continuously, while Chimney Swift flaps in intermittent bursts with long glides between. This is likely to vary depending on the weather and wind conditions, the motivation of the bird, and other factors, but may be helpful to pick out a potential Vaux’s Swift. More study is needed.
  • possibly shorter uppertail coverts
Pyle (1997) reports that on Vaux’s Swift the tips of the longest uppertail coverts fall 2-4 mm short of the beginning of the spiny tips of the tail feathers, while on Chimney Swift the coverts often reach the base of the spines. This would never be visible in the field, but deserves further investigation.

Geographic Variation

Two subspecies recorded in North America:

C. v. vauxi – the widespread breeding subspecies in US and Canada

C. v. tamaulipensis – breeding in northeast Mexico, recorded once in southern Arizona (specimen from Fort Huachuca, 14 May 1950, Phillips et al. 1964. The Birds of Arizona)

The East Mexican subspecies tamaulipensis differs significantly from the temperate northern breeder vauxi, and breeding range is widely separated. The crown and back of tamaulipensis are glossy blackish, darker than northern Vaux’s (and darker than Chimney) so the pale gray rump contrasts more strongly. The forehead is dark blackish, lacking the pale band of northern Vaux’s Swifts.

Voice is similar in North American and Central American populations, but there are very few available recordings, and slight differences may exist. Compare the display calls in a recording from Panama, with those in a recording from Oregon. Further study is needed to determine the full extent of the differences between these two populations.

Five other named subspecies are found in Central and South America.

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