Similar to other Tachycineta swallows in being blue above and white below, Tumbes Swallow is the smallest member of the genus, with a stubby bill and a distinct white supraloral stripe. A white rump breaks up the back and easily distinguishes it from Blue-and-white Swallow (Pygochelidon cyanoleuca), with also occurs in the same region. The breast is cream colored, with light bran colored streaking.
Blue-and-white Swallow (Pygochelidon cyanoleuca) also occur in southwestern Ecuador and northwestern Peru, but lack the white rump of Tumbes Swallow, and are more azure on the upperparts. They nest in exposed cliffs and eroding clay banks, and in this region primarily occur in the highlands; they are uncommon visitors to the coastal plain, where Tumbes Swallow occurs. Other swallows that occur in this region include the northern migrant Barn Swallow (Hirundo rustica) and the resident Gray-breasted Martin (Progne chalybea), both of which lack a white breast and rump, and the resident Chestnut-collared Swallow (Petrochelidon ruficollaris), which has a rusty rump.
Adult: Sexes similar. Upperparts dark blue-brown to blue-green, with white supraloral stripe extending from base of bill to above eye. Bill stubby. Underparts white, with narrow bran-colored streaks in breast. Rump white. Tail slightly forked.
Juvenile: Largely buffy brown above, with individual feathers of blue-green as birds age.
Bill: slate gray, black
Tarsi and toes: black
Bare parts color data from personal observations (Taylor and colleagues).
Total length: 11.5-12 cm (Ridgely and Greenfield 2001b), 12 cm (Schulenberg et al. 2007).
Linear measurements (mm):
wing (chord), specimens, male: mean 93.8 ± 1.7 (n = 5; Robbins et al. 1997)
female: mean 88.5 (88.1, 89.0) (n = 2; Robbins et al. 1997)
tail (longest rectrix), specimens, male: mean 48.5 ± 2.5 (n = 5; Robbins et al. 1997)
female: mean 44.9 (44.4-45.5) (n = 2; Robbins et al. 1997)
culmen (from base), specimens, male: mean 8.8 ± 0.2 (n = 5; Robbins et al. 1997)
female: mean 8.6 (8.6, 8.7) (n = 2; Robbins et al. 1997)
Robbins et al. (1997) note that Tumbes Swallow has shorter wings and bill, but a longer tail, than Mangrove Swallow (Tachycineta albilinea).
wing length, live, male: mean 95.40 ± 0.85 (n = 15)
female: mean 93.44 ± 0.52 (n = 18)
head length (nape to bill), live, male: mean 25.00 ± 0.07 (n = 15)
female: 24.78 ± 0.11 (n = 18)
Mass: male, mean: 10.72 ± 0.14 g (n = 15)
female, mean: 11.02 ± 0.16 g (n = 18)