Vaux's Swifts in Oregon, like most swift species, are characterized by minimal to no sexual dimorphism. This means that males and females look remarkably similar, making it extremely difficult, if not impossible, to distinguish them by sight alone in the field.

Here's what we know about their appearance and any subtle behavioral cues:

General Appearance of Adult Vaux's Swifts (both sexes):
Size and Shape: They are small, cigar-shaped birds, typically around 4.3 inches (11 cm) long, with a body designed for speed. Their wings are long, narrow, and swept back, often appearing sickle-shaped in flight. Their tail is short and squared, sometimes with a slight notch.
Coloration: Their plumage is a rather uniform sooty gray or brownish-gray overall. Their throat and upper breast are usually a slightly paler brownish-gray. There might be a very subtle, almost imperceptible, green iridescence in some lighting.
Legs and Feet: They have very short legs and small feet adapted for clinging to vertical surfaces like tree trunks or chimney walls, rather than perching on branches.
Bill: Short and stout.

Plumage: Both male and female Vaux's Swifts have virtually identical plumage. There are no distinct color patterns, markings, or size differences that reliably indicate sex.
Size: While there might be slight individual variations in size or weight, these are not consistent enough to differentiate sexes. The average measurements (length, weight, wingspan) are similar for both genders.

Subtle Behavioral Cues (Not always definitive for individual sexing):
While direct visual identification of sex is impractical, some behaviors might offer clues, especially during the breeding season:
Courtship Displays: Males and females engage in aerial courtship displays, often involving rapid gliding and synchronized flight patterns. They might chase each other and glide with wings held in a V shape. Mating even occurs in mid-air.
Vocalizations: Vaux's Swifts are highly vocal, known for their high-pitched, rapid, chattering calls, which are often described as insect-like. While both sexes vocalize, there's no widely reported difference in their calls that would allow for sex determination.
Nest Building and Incubation: Both male and female Vaux's Swifts participate in constructing their semi-circular nests (made of twigs glued together with saliva) inside hollow trees or chimneys. Both parents also share the duties of incubating the 3-7 white eggs and feeding the young by regurgitating insects.
Communal Roosting: Outside of the breeding season, particularly during migration, Vaux's Swifts are famous for forming massive communal roosts in large hollow trees or chimneys (like the one at Chapman Elementary School in Portland, Oregon). In these large aggregations, it's impossible to distinguish individuals by sex.