An apparent specialist of forest floor invertebrates (2) (3), Swainson’s warbler tends to feed on ants, beetles, centipedes and spiders (2) (3) (4), which are mostly found beneath the leaf litter (2) (3). This species tends to forage for its prey by probing under the leaf litter and flipping over dead leaves (2) (3) (4), and has even been seen sticking its bill into a curled-up leaf and opening it up to reveal prey (4). It will also glean prey from low bushes and may occasionally catch insects in flight (3). Swainson’s warbler is not known to eat fruit or nectar (2), but it has been recorded eating small lizards (3).
The breeding season for Swainson’s warbler lasts from May through to July (3), with the males arriving at the breeding grounds first to defend large territories (2). Monogamous breeding pairs are formed within a few days of the females arriving. The female Swainson’s warbler constructs the nest alone, building a large, bulky structure which takes between two and five days to complete (2). The nest is formed of leaves (3), sticks and vines (4), and is lined with moss, grass and pine needles (3) (4). The nest is typically placed in understorey vegetation (2) (3), and is either supported by small shrubs and trees or suspended on several thin vines (2).
The female Swainson’s warbler generally lays a clutch of between two and five eggs (4) which are incubated for a period of 13 to 15 days (3). Clutches of up to seven eggs have been observed (2), although three is most common (3). Swainson’s warbler eggs are white and are usually unmarked (2) (4), but can appear pinkish or bluish (2). Both the male and female adult feed the young chicks, which typically leave the nest around 8 to 12 days after hatching. Swainson’s warbler is thought to live for up to ten years (2).
Swainson’s warbler leaves the breeding area around August (2) (3), arriving in its wintering grounds from mid-September. It stays on the wintering grounds until spring migration begins again in March (3).