Breeding Bird Atlas 1 Species Accounts
Number of Broods
After initiating a northward migration on the heels of winter, the Yellow Warbler does a remarkably good job of not crossing the latitudes faster than spring advances, rarely arriving in Massachusetts before the last week of April. General arrival is the first week of May, with the largest numbers, many probably transients, coming the second and third weeks.
Yellow Warbler songs are variable but as is the case with many other warbler species can be divided into two main types used in different contexts. The emphatic songs of the first type can be represented as tseet-tseet-tseet-sitta-sitta-see while the second type, which predominates as nesting progresses, can be phrased as sweet-sweet-sweet-sweet-sweeter-sweeter. Both sexes have a loud chip note and a high zeet call given in flight.
Yellow Warbler territories are variable in size and may be as small as two-fifths of an acre, but some pairs in southern Worcester County held territories of at least 2 acres (Meservey). In territories where trees or other landmarks are absent, the males engage in much chasing back and forth, but, when the boundaries are more readily defined, fewer disputes occur.
After a noisy courtship involving much chipping by both sexes and chasing of the female by the male, the former selects a nest site in a low bush or shrub, generally no higher than 3 to 8 feet. The nest may be placed higher in a sapling or tree if an understory growth is absent. Six nests from eastern Massachusetts ranged in height from 1 to 5 feet, with an average of 2.5 feet, and were placed in forsythia, Red Maple, honeysuckle, barberry, Speckled Alder, and spiraea (CNR). In Berkshire County, 1 nest was located at 8 feet in an unidentified bush; 2 others in Box Elder were at 9 and 10 feet (CNR). Five southern Worcester County nests averaged 4 feet in height; 3 of these were in spiraea and 2 in raspberry (Meservey). In the Amherst area, nest construction was noted for 6 nests from May 20 to June 12, with a median date of May 24. The heights of these and other nests in the region ranged from 1 to 8.5 feet and averaged 4 to 5 feet. They were placed generally in the outer portions of the bush or tree, often in an isolated plant. The most commonly used species was Multiflora Rose; other identified substrates included Elderberry, willow, raspberry, honeysuckle, and blueberry (Spector). In Essex County, spiraea and Speckled Alder were favored nest sites.
The female constructs the nest while the male sings as many as 3,240 songs in one day. Nests typically contain a large proportion of plant down and spider silk and are easily spotted because they are so white. Three to six eggs are laid, commonly four or five, and incubation is by the female for about 11 days. Clutch sizes for 28 state nests were two eggs, probably incomplete (1 nest); three eggs (1 nest); four eggs (16 nests); five eggs (10 nests) (DKW, CNR, Spector, Meservey). In an Amherst study, cowbird eggs were present in 4 of the 9 nests found in the egg stage (excluding the clutch of two). The mean clutch size for 6 nests with only warbler eggs (again excluding the two-egg clutch) was 4.5 (Spector). Known state hatch dates were June 5, June 6, June 7, and June 13 (Spector, Meservey).
Both parents feed the young, which fledge in 9 to 12 days. Parental care continues for at least two weeks after fledging. Brood sizes for 17 state nests were one young (2 nests), two young (2 nests), three young (3 nests), four young (9 nests), five young (1 nest) (Spector, Meservey, CNR). In 3 of these nests that contained only one or two warblers, a cowbird nestling was also present, and in 2 additional nests, one and two cowbird nestlings respectively were observed. Cowbird nestlings were found in 3 of 9 nests at the nestling stage in the Amherst study (Spector). The mean brood size for 8 nests with only warbler nestlings was 3.375 (Spector). Nestlings were reported in Massachusetts from June 5 to July 4 and dependent fledglings from June 14 to July 26 (Spector, Meservey, CNR).
Perhaps the most interesting aspect of this species’ nesting behavior is its defense against cowbird predation. While many small songbirds normally tolerate cowbird eggs in their nest, Yellow Warblers often respond by building a floor over the cowbird eggs, elevating the sides of the nest, and starting anew. This is especially true if the natural clutch is still incomplete; the larger the clutch, the greater the chance the alien egg will be accepted. If the cowbird persists and lays an egg in the modified nest, the warbler may cover the egg again with another floor, with a record of six layers.
The Yellow Warbler often begins its southbound migration as soon as the young can fend for themselves. Some adults molt on or near the breeding areas. Many are gone by the end of July, and most local birds depart by mid-August. Subsequent records, which continue throughout September, are likely of more northern birds. After this, only rare stragglers are noted. In its winter range, from Mexico and the Bahamas to northern South America, where it may arrive as early as August, the species prefers habitats similar to those used in its breeding range. It is also territorial on the wintering grounds.