Breeding Bird Atlas 1 Species Accounts
Number of Broods
By 1930, the species was considered a rare summer resident in the eastern part of the state and was accidental or absent from the western portions. The invading population became established in a band extending from southern Worcester County east to the coast and reaching north to Essex County and south to Cape Cod. During the 1930s, the Connecticut River valley became another population center. Blue-winged Warbler numbers climbed slowly until about 1955 and then rose markedly, until there was a veritable population explosion during the 1970s. The Berkshires and other western areas were the last regions to be colonized.
This range expansion received a great impetus resulting from the large patches of suitable habitat available after the widespread abandonment of farmland. Massachusetts traditionally was the breeding ground of the Golden-winged Warbler, a generally uncommon species with a few pockets of local abundance. Blue-winged and Golden-winged warblers are close relatives with very similar behaviors and requirements for nesting, and they interbreed freely where their ranges overlap. Crossbreeding and the subsequent backcrossing of hybrids produces a range of genetically mixed individuals that may look like either the parent species or may be either of two distinctive forms, the “Brewster’s” and “Lawrence’s” warblers. For reasons that are not yet completely understood, the Blue-winged Warbler seems to have replaced the Golden-winged Warbler in approximately 50 years after initial contact (see Golden-winged Warbler account).
During the Atlas period, the Blue-winged Warbler was a common breeder throughout the south-central and southeastern sections of the state and in portions of Essex County. It was less common in the Berkshires and sparsely distributed on upper Cape Cod and Martha’s Vineyard. Although the species is now regularly reported from areas of northern New England, Massachusetts still basically marks the limit of the northeastern breeding range, and the population drops off very rapidly toward the northern borders of the state.
Favored breeding sites are old, brushy fields with scattered clumps of saplings and trees, wood clearings, border areas with low undergrowth, power-line cuts, and the edges of wooded swamps. Breeders arrive from late April through the first two weeks of May, when males carve out territories with much singing, chasing, and fighting. Unmated males sing tirelessly from elevated, and often bare, song perches and generally actively pursue females when they appear. These same song perches are also used later by mated males patrolling their territories. Males sing two types of songs that are used in different contexts. The more familiar pattern is the bee-buzz or swee-zee song, which occurs in many variations. The second song is longer and also variable and can be represented as wee-chi-chi-chi-chur-chee-chur. Both sexes have several tzip or buzz call notes and a thin seet note, and the young utter a long series of buzz notes as a begging call.
Nests, built mainly if not entirely by the female, are placed on or very close to the ground, often on a foundation of dry leaves, and are usually among or attached to the upright stems of weeds or grass clumps. They are composed of grass and pieces of leaves and are lined with finer material. A 1909 Sudbury nest was in mixed woods between the exposed roots of a decayed stump and partially concealed by ferns (ACB). Six recent nests from Worcester County were all located on the ground as follows: 1 in grass beneath saplings at the edge of a narrow opening next to dense brush, 2 in clumps of goldenrod, 1 in a grass clump in a field 5 or 6 feet out from the surrounding trees, 1 in a grass clump in a clearing with many tree seedlings, and 1 in ferns in a small overgrown orchard near woods. One of these nests opened to the side with a small dome over the entrance. Four, five, or sometimes six eggs are laid. Clutch sizes for 2 Massachusetts nests were five and six eggs respectively (Kroodsma). Females incubate for 10 to 12 days, and both parents feed the young, which leave the nest in another 10 to 11 days. Brood sizes for 8 state nests were two young (1 nest), three young (1 nest), four young (3 nests), six young (3 nests) (Kroodsma, EHF). A ninth nest contained two Blue-winged Warbler nestlings and a young cowbird. Nestlings have been recorded in the Commonwealth from June 3 to July 8 (Kroodsma, EHF). Adults feeding fledglings have been observed in the state from June 11 to August 16, with most records from June 27 to July 18.
One brood is raised each season, and a paucity of late nesting dates seems to indicate that second attempts are rare. Females are occasionally seen carrying nest materials in mid-June, but even unmated males cease singing by the end of the month or during the first few days of July.
Observations on color-marked individuals in Worcester County indicated that rarely extra helpers assisted pairs in feeding the young. At times, birds were observed caring for young that were not their own, as exemplified by a female “Lawrence’s” hybrid feeding one large and one very young fledgling on June 27 and a female Blue-winged Warbler feeding three large and one small fledgling on July 8.
Adults undergo a complete molt in late July and early August, during which time the young attain their yellowish winter plumage. Throughout August, Blue-winged Warblers forage in mixed-species flocks, which they accompany into wooded areas as well as border zones. Most depart on their southward flight at the end of August and during the first week of September. Only a few stragglers are encountered after that time. The wintering grounds include Mexico, Central America, and northern South America.