Yellow Warbler by John Sill ©Massachusetts Audubon Society. Courtesy of the Museum of American Bird Art.
John Sill is a freelance wildlife artist living in the mountains of North Carolina. He was the illustrator for the Bird Identification Calendar for Mass Audubon for many years. His work has appeared in Birds In Art at the Leigh-Yawkey Woodson Art Museum, Wausau, Wisconsin, and in Art of the Animal Kingdom at the Bennington Center for the Arts in Vermont. He continues to illustrate the “About” and “About Habitats” series of natural history books for children written by his wife Cathryn.
For many people, the Yellow Warbler (Setophaga petechia), with its late April arrival, is a harbinger of spring. Aptly named, the male in breeding plumage is bright yellow, with greenish-yellow upperparts, and heavy red streaks below. The dark eyes and bill are prominent in the all-yellow head. Females are duller and the red streaking is either reduced or lacking. Winter-plumaged birds are duller colored. Immatures resemble drab females, with the immature females appearing nearly gray.
Yellow Warbler taxonomy is complex and somewhat controversial, with 33 recognized subspecies that are divided into three groups: the Yellow Warbler, or aestiva group; the Golden Warbler, or petechia group; and the Mangrove Warbler, or erithachorides group. All three groups bear the names of what were originally described as separate species. The aestiva group is divided into six subspecies, the other two into 16 and 11 subspecies respectively. The northern limit of the aestiva group’s breeding range extends from the Aleutian Islands and most of Alaska through northern Canada to the shrubby edges of the tundra, then dips south of Hudson Bay and east to southern Labrador and all of Newfoundland. The range extends south to northern Georgia and west through Oklahoma to California—comprising about half of the United States—and extends south along the west coast to Baja California, and from Arizona to southern Mexico. Aestiva birds are migratory. This group winters from southern Baja and western Mexico south through Central America and in South America east of the Andes south to Bolivia. They spend the winter in scrubby wooded habitats, and frequently in mangroves. In Massachusetts, the Yellow Warbler is a widespread common migrant and breeder. Yellow Warblers arrive in late April and depart from late July to early August.
The other groups of Yellow Warblers are sedentary. The Golden Warbler group ranges from southern Florida through the West Indies and the Caribbean south to Venezuela; males have a chestnut-colored patch on the crown. The Mangrove Warbler group ranges from Baja California south through Central America and in South America in the west through central Peru; males have a chestnut-colored head. The Yellow Warbler is certainly one of the most diverse warbler species, but with DNA studies now available it would not be a surprise if ultimately it once again gets split into two or more species.
Yellow Warblers are usually monogamous, but sometimes also may be polygynous. They tend to be site faithful when breeding, and may mate with the same partners in successive years. Males sing from perches in shrubs or trees. The song is a series of short units that have been described mnemonically, as sweet, sweet, sweet, sweeter than sweet. Males deliver two basic patterns of song: one pattern is used for male-female communications, including mate attraction. The other pattern is for male-male communication, for example in territorial advertisement. A number of behaviors are involved in establishing a territory and attracting a mate. A male may fly in a circular pattern toward another warbler at its territorial boundary, may glide with wings and tail spread, or fly slowly with exaggerated wing beats holding its head up over its back. Chases are frequent and sometimes end in fights. A stationary display involves a spread tail and lifted wings.
In our area, Yellow Warblers prefer to nest in wet thickets, especially thickets containing willows, and their habitat includes most disturbed successional habitats. The nest is a deep cup built by the female in a fork in a shrub or tree. It is constructed of bark and grasses and covered in fine gray fibers. The clutch is variable but usually consists of three to five eggs that can vary in color: grayish white to pale green or blue and spotted and blotched with brown or olive around the large end of the egg. Only the female develops a brood patch and she alone incubates the eggs for the 10 to 12 days until hatching. The altricial chicks are helpless upon hatching, and are covered with natal down and their eyes are closed. The female does the all brooding for the eight to ten days until fledging. The female may give a distraction display if the nest is approached. Both parents feed the young, which may remain with the parents as long as three weeks after fledging.
Yellow Warblers feed upon a wide variety of insects and insect larvae. They primarily forage by gleaning leaves, but also hawk flying insects and snatch prey from leaves by hovering. They occasionally eat fruit.
Yellow Warbler nest predators include snakes, mammals such as squirrels, weasels, and raccoons, and birds such as crows and jays. In some areas, they suffer from cowbird nest parasitism. However, if cowbirds lay eggs early in the nesting cycle, Yellow Warblers may cover the clutch and start afresh, producing a multi-tiered nest effect. One nest had six tiers and 11 cowbird eggs! They also may desert their nest and presumably rebuild elsewhere.
As a primarily long-distance nocturnal migrant, many are killed in collisions with buildings or towers. Habitat alteration, including cattle grazing, is a problem, but the Breeding Bird Survey data indicate that their overall population is stable, even though there are different regional trends. For example, there has been a decline in the Pacific rain forest area, and increases in southern New England and the Great Lakes area. Thus it appears that with its vast breeding area, this delightful little warbler is reasonably secure.
William E. Davis, Jr.