FAS Top Menu

Vaux’s Swift

By Lisa Bate

The Vaux’s Swift (Chaetura vauxi) is the smallest swift in North America, just slightly smaller than its eastern counterpart, the Chimney Swift (Chaetura pelagica). Best described as a “flying cigar,” this species is easily recognized by its small, cigar-shaped body with long, pointed wings, and short stubby tail. Typically, they can be seen flying just above the forest canopy or over water. Their flight often appears erratic, as if their wings do not work in unison, yet they are aerodynamic specialists. This visual illusion is thought to occur because of their incredible flight speed (> 200 mph [>300 kph] at times, but more typically between 35 and 80 mph [56 to 128 kph]), and frequent turning.

Compared to Chimney Swifts which are darker below, Vaux’s are plain, grayish-brown in color with the upper breast and throat lighter than the rest of their under parts. Weighing less than a penny, their length averages 4.8 inches (11 cm) with a wing span of 12 inches (30 cm). While it may be visually difficult to differentiate between the two species, Vaux’s are most easily distinguished from Chimney Swifts by their calls and flight behavior. They have a high pitched, rapid, insect-like twitter, whereas the Chimney Swift has a much louder chattering call. Also Vaux’s are less likely to soar. They never perch, as they have small weak feet. Instead they cling vertically at nest and roost sites. They are named for William S. Vaux (1811-1882), a member of Academy of Natural Sciences of Philadelphia, by his friend John Townsend, who first described this species. Since the name is English, not French, the ‘x’ is sounded.

The northern populations of Vaux’s are migratory, breeding in western North America from southeast Alaska, east into Montana and south into central California. The southern populations are year-round residents ranging from Mexico to Venezuela. In winter, northern migratory populations overlap with southern residents.

Perhaps the most fascinating aspect of Vaux’s Swifts, is their dependency on hollow trees and snags as their primary choice for nesting and roosting (occasionally they use chimneys). Because hollow trees are essential to them for reproduction and survival, they are known as “hollow-tree obligates” in the world of biology. “Obligate” refers to something that is essential or critical to their survival.

Recent declines in Vaux’s populations in the Pacific Northwest have been attributed to the loss of mature and/or old growth forests where the number of hollow trees has diminished. Here in northwest Montana, where we still retain many hollow trees, Vaux’s are considered common.

The hollowing process starts when a live tree sustains an injury, such as a broken branch or top, and exposes the heartwood to “heart rot” fungal spores in the air. Once heart rot, or the decay of the heartwood, develops, it spreads until the entire column of heartwood decays, creating a hollow tree. The heartrot fungi do not kill the tree by feeding on sapwood, so trees retain a hard, protective shell as they continue to grow, providing both thermal and protective cover for the swifts. It takes a long time to hollow a tree and Vaux’s select the largest (average 27 in diameter [68 cm]) hollow trees for roosting and nesting. Therefore, it usually takes hundreds of years for a hollow tree to be made suitable for their nesting and roosting needs.

Courtship for Vaux’s begins upon their arrival at their breeding grounds. Pairs mate on the wing, coming together for a few seconds, dropping in the air, and then separating before reaching the ground. Both adults build the nest and incubate the eggs. Sometimes one or two nonbreeding birds will also share in the incubation and feeding of the young. Trees are commonly entered by an entrance hole excavated by a Pileated Woodpecker or its broken top. Usually there is only one nest per tree. They build an open cup, half-circle nest out of loosely woven twigs on the inside surface of the hollow tree, holding the nest in place with sticky saliva. The nest, often situated above a branch stub or protrusion, may have 3 to 7 eggs. Young stay in the nest the first 18 days, and then perch vertically inside the tree until day 30 when they fledge. Like other swifts, Vaux’s are insectivores. During the nestling season, each parent delivers more than 5,000 insects per day to the young.

Vaux’s roost communally by the hundreds or thousands in hollow trees, and sometimes in chimneys. This is especially true prior to and during migration. Just before roosting, the swifts circle in a vertical fashion above the roost as other birds join in. Once a few birds finally take the plunge, the whole flock abruptly follows, giving the impression of birds falling out of the sky.

So this summer, don’t forget to watch above the large, broken top grand fir, hemlock, larch and cottonwood trees in your area. Or perhaps join in on a Flathead Audubon Society field trip in the late summer to observe Vaux’s flying into chimney roosts. You may just get lucky and witness some of the dazzling aerial displays and unique behaviors of our smallest swift.

, ,

Comments are closed.

Copyright 2016 Flathead Audubon Society

%d bloggers like this: