BIRD OF THE WEEK: May 8, 2020
SCIENTIFIC NAME: Setophaga petechia
POPULATION: 92 million
HABITAT: Breeds in wet woods, thickets, and riparian areas; winters in open woodlands, on farms and gardens with scattered trees, and in mangrove forests
The Yellow Warbler is the most widespread American wood-warbler. It nests from Alaska to northern South America (including the Galapágos Islands), and in parts of the Caribbean as well, and winters as far south as Peru.
One of the Yellow Warbler's calls, a repeated seet, serves specifically as a Brown-headed Cowbird alert. When a female hears another bird make this call, she rushes back to her nest to prevent the cowbird, a notorious nest parasite, from laying eggs there.
The Yellow Warbler's song is a sweet-sounding series of whistled notes often characterized as "sweet, sweet, sweet, I'm so sweet," repeated as often as ten times in a minute.
(Audio of Yellow Warbler song by Dominic Garcia-Hall, XC394040. Accessible at www.xeno-canto.org/394040.)
Another superlative associated with the Yellow Warbler is the species' incredible diversity: 37 subspecies are recognized, divided among four groups. Subspecies vary mostly in plumage color and pattern.
The Yellow Warbler nests throughout most of Canada, Alaska, and at least two-thirds of the area covered by the lower 48 U.S. states. Long-distance migrants, few if any of these birds remain north of the Mexican border in winter.
Several resident, or nonmigratory, groups are found in Mexico, Central America, South America, and the Caribbean. A resident subspecies even inhabits the Galápagos Islands off Ecuador's Pacific coast. Males in nonmigratory populations have varying amounts of chestnut color on their heads, from just the cap to most of the bird's head, as is the case with "Mangrove" Yellow Warblers.
For years, some ornithologists split the migratory and resident populations into two species, Setophaga aestiva and S. petechia. All now fall under the latter name.
The Yellow Warbler feeds mainly on insects and spiders, gleaning them from shrubs and tree branches or sallying out from a perch to grab winged insects mid-air. This diminutive hunter sometimes hovers while seeking prey that might be hiding on the undersides of leaves. Like many other migratory songbirds, the Yellow Warbler adds fruit to its diet in winter.
Winter or summer, this warbler provides valuable pest control: One study, conducted on Costa Rican wintering grounds, showed that the Yellow Warbler and other insectivorous birds ate large quantities of invasive coffee berry borer beetles, helping reduce infestations on coffee plantations in that country by 50 percent.
A male Yellow Warbler quickly claims a territory on the breeding grounds, chasing off intruding males. He courts prospective mates through incessant singing. In fact, one Yellow Warbler may sing more than 3,000 times in a day to attract a female! Once paired, the male attends his mate closely as she builds her nest, wary for other males, which often invade established territories and attempt to mate with resident females.
Like many other birds such as the Kirtland's Warbler and Wood Thrush, the Yellow Warbler is frequently parasitized by Brown-headed Cowbirds. This problem is especially common in fragmented habitats, which provide easier access for female cowbirds to lay their eggs in host birds' nests. But the Yellow Warbler fights back. It seems to recognize the foreign eggs, and often builds a new nest, covering over a cowbird-parasitized clutch with new nesting material. If the cowbird returns and re-lays, the warbler covers them again — sometimes resulting in nests with up to six tiers!
Females build and maintain the cup-shaped nests, incubate the eggs, and brood the hatchlings. Meanwhile, male Yellow Warblers aggressively guard nest sites and bring food to females sitting on eggs or young. Both sexes share chick-rearing duties: After the nestlings fledge, some may follow the mother, while the rest remain with the father.
Although still numerous, Yellow Warblers are threatened by habitat loss, chiefly destruction of riparian habitats, and the overuse of pesticides. One subspecies, the Barbados Yellow Warbler, has been listed under the Endangered Species Act since 1970.
ABC's work helps to conserve the Yellow Warbler and other migratory birds across their full annual life-cycle through its BirdScapes approach to conservation. Several BirdScapes in the southwestern United States protect riparian areas for the Endangered western subspecies of the Yellow-billed Cuckoo that also shelter the Yellow Warbler and other birds.
ABC also has several initiatives in place that tackle threats, including our Cats Indoors program, which encourages pet owners to keep cats and birds safe.