The Mangrove Swallow is found from Mexico to eastern Panama and in northwestern Peru, and is locally common (Ridgley 1981). It is found mostly in lowlands below 1,000 m (3,300 ft), on both the Pacific and Caribbean slopes (Stiles & Skutch 1989, Garrigues & Dean 2007). Almost always near water, this species can be found in small groups over fairly still, open waters such as ponds, rivers, estuaries, salinas, and mangroves, and is sometimes found over damp fields and meadows. (Ridgley 1981, Stiles & Skutch 1989). It perches low on snags and dead branches and feeds on small insects including flies, flying ants, wasps, and homopterans, which it catches by flying close to the water surface (Stiles & Skutch 1989).
The Mangrove Swallow is 4½-5” (11¾-13 cm) long, and is glossy greenish to steel-blue above (fresh plumage is green, becoming more blue with wear). Color is duller in females; otherwise this species exhibits no sexual dimorphism (Ridgley 1981, Stiles & Skutch 1989). A narrow white stripe from the eye to the bill and a white rump distinguish this species easily from other similar swallows (Garrigues & Dean 2007). This swallow has blackish wing feathers and is white below, tinged with gray on breast and sides. The feet and bill are black. Immature Mangrove Swallows are dingy grayish with faint green gloss above, except for the white rump, and white below, tinged with brownish on sides of chest. The tail is slightly forked (Ridgley 1981). The call is a dzreet, dzreet (Ridgley 1981) or a rolling jeet (Stiles & Skutch 1989), often produced in flight.
Outside of the long breeding season, which occurs during the dry season between January and April (Costa Rica), the Mangrove Swallow is very social, perching in flocks of up to 50 (Stiles & Skutch 1989, Moore et al. 1999). During breeding, socially monogamous pairs are very territorial, and nest sites are spaced several hundred meters apart; males will attack and chase other birds flying close to the nest (Buderman 2010). Cup nests are constructed loosely of grass, horsehair, rootlets, moss, and stems, and can be lined with feathers. Nests are found in cavities in snags, bridges, buildings, or abandoned woodpecker nests, often above water (Stiles & Skutch 1989, Buderman 2010). Mangrove Swallows can nest in the active nests of tyrant fly-catcher species during high water level conditions in which brooding sites become scarce (Drycz 2000). Females lay 3-5 white eggs per clutch, up to twice in a season (Buderman 2010). Low nest success is common, with an average of 1 fledgling produced per nest, which may be due to termite invasion, nest predation, and nest site competitors (Buderman 2010). Extra-pair fertilizations do occur, although less frequently than in other species such as the Tree Swallow (Tachycineta bicolor) (Moore et al. 1999).
The IUCN lists the Mangrove Swallow as a species of Least Concern. Population size is currently upwards of 500,000 individuals, and this species exhibits low sensitivity to disturbance (Buderman 2010).
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