Vaux’s Swift

Vaux’s Swift breeds from just south of the Yukon through the western United States to Mexico, Central America, to northern Venezuela. During winter months, northern migrant populations of this species overlap southern residents. Best known for its quick flight and dazzling aerial agility, this bird seldom perches except when nesting or roosting, and it probably mates on the wing. Hollow trees and unlined chimneys are its favored nesting and roosting sites, making the Vaux’s Swift vulnerable to loss of old-growth forest and aging masonry structures. Indeed, recent declines in Vaux’s Swift populations have been documented throughout its range where mature forest is dwindling. Its nest, an open half-circle of loosely woven twigs, is glued together and attached to the inside of a hollow tree or chimney with sticky saliva. In migration, large flocks of this species circle roosts at dusk, feigning entry until the first few birds take the plunge; then the whole flock follows abruptly, literally pouring out of the sky and disappearing into the roost.

Like other swifts, the Vaux’s Swift is almost entirely insectivorous, a consumer of aerial plankton—hawking a variety of ants, bugs, flies, moths, spiders, and aphids from the air. An adult feeding young collects boluses of food in its mouth and carries these back to its nestlings. Each parent makes up to 50 trips per day, delivering more than 5,000 small insects from dawn to dusk.

Field Identification

Smallest swift in North America; 11 cm long; 15–22 g body mass. Aerodynamically designed for fast speeds with long, pointed wings, short and stout humeri, and compact body. Short legs with tiny feet and long claws. Bill small and weak. Small rounded tail, rectrices tipped with spines for support while perching on vertical surfaces at nesting and roosting sites. Plain grayish brown, sometimes with a slight green iridescence; rump and uppertail coverts range from a pale brownish gray to a duller shade like that of the back. Upper breast and throat paler than rest of undersides. Tail sometimes subject to substantial wear depending on nature of roosting and nesting substrate. The faint greenish gloss recorded for some individuals is only present in fresh plumage and wears to bluish or purple gloss in worn plumage. Ages and sexes are similar, differing only in relative feather shapes, wear, and perhaps timing of molt.

General:

An aerialist of western forests, Vaux’s Swift is a dark, tiny-bodied, narrow-winged bird much like the Chimney Swift of the eastern U.S. They spend most of the day in the air, taking small insects and spiders in rapid, twisting flight. They roost and even nest communally in hollow trees in mature evergreen forests (less often in chimneys). Adults construct a nest of twigs, cemented with their saliva to the inside of a tree cavity, and roost near the nest, clinging vertically to the side of the cavity.

Behaviour:

Primary mode of locomotion is flight; flies constantly unless in nest or roost. Fast and erratic flight, much like a bat. Wings give illusion of not always moving in unison; impression of non-synchronous wing beats characteristic of other small swifts (stroboscopic examination showed that wing beats of Chimney Swift (Chaetura pelagica) are always synchronous; illusion of alternate beats is partially due to frequent banking and turning. Fly at all heights; from close to ground or water to high over treetops. Not known to walk or hop; seldom if ever perch on a branch, but do perch on vertical wall in nest and roost. Can climb wall of nest and roost, but more often fly out. Fly into nest or roost head-first, but change to tail-first once inside.

Habitat:

Late stages of coniferous forests and deciduous forests mixed with coniferous trees. Also breeds in chimneys in residential areas. No nests documented at major migratory roost sites (LS). More common in old-growth forests than in younger forests. In southern Washington Cascade Mountains, abundance positively correlated with high density of live trees larger than 100 cm in diameter-at-breast height and with snags; trees primarily Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and western redcedar (Thuja plicata).

In northeastern Oregon, primarily associated with grand fir (Abies grandis) and mixed conifer forest types; less common in ponderosa pine (Pinus ponderosa) stands; seldom seen in high elevation stands of subalpine fir (Abies lasiocarpa) which typically lack suitable nest and roost trees. Large-diameter, hollow trees (live or dead) important for nest and roost sites; chimneys likely used less frequently. Roost trees important during summer for nonbreeding swifts. Other major migratory chimney roosts known to host swifts all summer.

Information:

Vaux’s Swifts forage in flight, eating tiny insects and spiders that they catch via rapid aerial pursuit. They eat flies, hoverflies, booklice, ants, bees, leafhoppers, planthoppers, aphids, spindlebugs, lanternflies, bark beetles, moths, mayflies, and true bugs. They forage in flocks, pairs, or singly, coursing back and forth over productive areas.

Similar species:

Black Swift, Chimney Swift, Northern Rough-winged Swallow​

Conservation Status:

According to the North American Breeding Bird Survey, Vaux’s Swift numbers declined by an estimated 1.8% per year between 1968 and 2015, resulting in a cumulative decline of 58% over that period. Partners in Flight estimates a global breeding population of 870,000 and rates the species a 13 out of 20 on the Continental Concern Score, indicating it is a species of low conservation concern. Modern forestry practices that reduce old-growth forests have a negative impact on populations of this species. Vaux’s Swifts are aerial insectivores, a broad group of birds that are undergoing severe declines, for reasons that have yet to be fully understood.

Capture Rates

The migratory nominate subspecies C. v. vauxi breeds in western North America as far north as the southeastern corner of the Yukon Territory. Vaux’s Swifts only occur uncommonly in the park and usually at high altitudes, and as such avoid mist nets. This is reflected in the capture of a single bird in June 2024.

Molt Summary:

PF: HY partial (Aug-Oct); AHY complete (Apr-Oct); PA absent. PF occurs primarily, on the summer grounds but completes during fall migration or on the winter grounds; the outer pp are the last feathers replaced in AHYs. The juvenal wing coverts apparently are retained during the PF. 

Flight feathers relatively fresh and not actively molting in Aug-Oct.; outer pp and outer ss with thin, white edging when fresh; outer pp worn and brown in Mar-Jul, contrasting with the body plumage; rects narrow, tapered, and relatively abraded in Mar-Jul.

Note: Some intermediates occur that are not reliably aged after completion of the PF.

Unreplaced flight feathers (e.g. the outer pp)relatively worn and/or being replaced in Aug-Nov or later; outer pp and outer ss without thin, white edging; outer pp relatively fresh and blackish in Mar-Jul, contrasting little with the body plumage; rects broad and relatively fresh in Mar-Jul.

Note: See HY/SY. AHYs molting flight feathers in Apr-May may be SYs (see molt) but more study needed.

Vaux’s Swift are part of the genus Chaetura also named “spinetails” referring to the unusual structure of their rectrices. The bare rachides at the tips are stiffened along the whole length a feature which aids the bird in clinging to vertical surfaces. Likewise, the pamprodactyl toe configuration allows all four toes to be turned forward, another adaptation for clinging vertically.