American yellow warbler

Did You Know?

• The Californian yellow warbler (S. a. brewsteri) and Sonoran yellow warbler (S. a. sonorana) are listed as species of concern by the ESA.
• The North American populations are legally protected by the Migratory Bird Treaty Act.

Taxonomy

The American yellow warbler was first formally named in 1789 by German naturalist Johann Friedrich Gmelin, who worked from a hand-colored engraving by François-Nicolas Martinet. Gmelin assigned it to the genus Motacilla with the scientific name Motacilla aestiva. The species name aestiva comes from the Latin word for "summery." In 1957, the type location was set to Quebec, Canada. Today, this warbler is one of 35 species in the genus Setophaga, a name derived from Greek words meaning "moth-eating." It was once thought to be the same species as the mangrove warbler, but scientists split them based on differences in mitochondrial DNA, plumage, and calls. Nine subspecies are recognized: S. a. rubiginosa, S. a. banksi, S. a. parkesi, S. a. amnicola, S. a. aestiva, S. a. morcomi, S. a. brewsteri, S. a. sonorana, and S. a. dugesi.

Description

Other than in male breeding plumage and body size, all subspecies are very similar. Winter, female, and immature birds have greenish-yellow uppersides and duller yellow below; young males soon acquire breast and head coloration. Females are somewhat duller, especially on the head. The remiges and rectrices are blackish olive with yellow edges, sometimes forming an indistinct wing-band. The eyes and short thin beak are dark; the feet are olive-buff. The 35 subspecies of S. petechia are divided into three groups based on male head color in breeding season; the aestiva group is currently accepted as a separate species. Depending on subspecies, length ranges from 10 to 18 cm, wingspan from 16 to 22 cm, and weight from 7 to 25 g, averaging about 16 g globally but only 9–10 g in most U.S. breeding adults. Wing chord measures 5.5–7 cm, tail 3.9–5.6 cm, bill 0.8–1.3 cm, and tarsus 1.7–2.2 cm. Summer males are the yellowest warblers wherever they occur, brilliant yellow below and greenish-golden above, with rusty-red streaks on breast and flanks. The song is a musical strophe rendered sweet sweet sweet, I'm so sweet, varying between populations. The call is a soft or hard chip or ship, often given by females after a male's song. Territorial defense includes hissing calls; seet appears to be a specialized cowbird alert.

Behaviour and ecology

American yellow warblers breed in most of North America from the tundra southwards, except the far Southwest and Gulf of Mexico coast. They winter from southern California to the Amazon region, Bolivia, and Peru. They arrive on breeding grounds in late spring, around April or May, and begin moving south as early as July after young fledge; most have left by the end of August. Breeding habitat is typically riparian or moist land with small trees, especially willows. Wintering birds chiefly inhabit mangrove swamps and dense woody growth. Less preferred habitats include shrubland, farmlands, and forest edges; they will also use suburban areas, orchards, and parks. Outside breeding season they are usually in small groups, but while breeding they are fiercely territorial. Roughly 60% of their diet is caterpillars; they also eat wasps, mayflies, moths, mosquitoes, beetles, damselflies, treehoppers, other insects, larvae, and spiders. They glean prey from shrubs and branches and hawk flying insects. They also consume other invertebrates, berries, and small juicy fruits, especially in winter quarters. The yellow warbler reduces coffee berry borer beetles in Costa Rica coffee plantations by 50%.

Breeding

Yellow warblers nest in trees, building a small but sturdy cup nest. Females and males rear young about equally but emphasize different tasks: females build and maintain the nest, incubate, and brood; males guard the nest site and procure food, bringing it to the nest and passing it to the female. As young approach fledging, the male's workload increases. The American yellow warbler starts breeding in May or June. Males court females with songs, singing 3,200 or more per day. They are generally serially monogamous; about 5% of males are bigamous. Very few breed more than once per year. If a breeding attempt fails, either parent usually tries to raise a second brood. The clutch is 3–6 eggs, typically 4–5. Incubation usually takes 11 days, sometimes up to 14. Nestlings average 1.3 g at hatching, are brooded for 8–9 days, and leave the nest the following day or the one after. Almost half of parents attend fledglings for two weeks or more after they leave the nest; sometimes adults separate early, each accompanied by one to three young.