Courtship.—The courtship of the myrtle warbler must be a very pretty performance. Two brief accounts of it have been published: "As summer approaches the males begin their courtship of the females, following them about and displaying their beauties by fluffing out the feathers of their sides, raising their wings and erecting the feathers of the crown, so as to exhibit to the full their beautiful black and yellow markings. After much time spent in courting they mate, and at once look about for a nesting place" (Forbush, 1929). Males seeking mates “made advances to the female contingency, hopping from twig to twig with outspread wings, chipping and fluttering, now repulsed by the fair one, and now accepted by another one to whom advances were made, to finally spend a few days in a favorable spot and begin nest building” (Knight, 1908).
Nesting.—On August 1, 1907, at Clarkes Harbor, Nova Scotia, I found the first and only nest of the myrtle warbler that I have ever seen; it was about 15 feet from the ground on a horizontal branch of a large spruce tree, about 5 feet out from the trunk, and contained three young birds that were nearly fully feathered. Robie W. Tufts says in his Nova Scotia notes: “I have seen these nests built at varying heights from 5 to 50 feet high. One found on June 6, 1919, contained four slightly incubated eggs. It was placed close to the stem of a pine tree, near the top, about 50 feet up. My field experiences tend to support the theory that these birds normally raise two broods a year.” He found one nest built in an apple tree in an orchard, of which he says: “Of the large number of nests of this species I have examined, this is the only one not built in a conifer.”
There are two Nova Scotia nests of the myrtle warbler in the Thayer collection in Cambridge, both taken by H. F. Tufts. They are slightly different in composition and structure, but are probably fairly typical of the species. One, found saddled on a spruce limb 10 feet from the ground, is rather bulky and loosely built; the foundation and sides are made of fine coniferous twigs mixed on the bottom with grasses and rootlets and around the rim firmly interwoven with black horsehair, or perhaps moose hair, and finer rootlets; the cup is smoothly lined with finer hair and feathers. Externally it measures, roughly, 4 by 5 inches in diameter and about 2 inches in height; the cup is about 2 inches in diameter and 13⁄4 inches deep. The other, a very pretty nest found 8 feet up in a small spruce, close to the trunk, is more firmly and compactly built; the base and sides are made up mainly of green mosses and a few gray lichens mixed with fine twigs and a few fine grasses, all firmly interwoven; internally the cup is smoothly lined with fine black and white hairs on top of a few feathers. Externally it measures 21⁄4 inches in height and 3 by 31⁄2 inches in diameter; the cup is 2 inches in diameter and about 11⁄2 inches deep.
Of nestings in Maine, Knight (1908) says: “As soon as nest building begins, the favorite locality selected is a thicket of evergreen trees near the highway, some open pasture containing a few clumps of scattered evergreens, small thickets of evergreens along the banks of some stream or river or about the shore of a pond or lake, or a row of trees about some country dwelling or in an orchard. In the vast majority of cases an evergreen tree is selected as a nesting site, though occasionally some hardwood tree, such as maple, apple or birch, may be taken. A majority of nests seem to be placed in cedar trees, with fir and spruce following as close second choices.”
Forbush (1929) mentions two Massachusetts nests in tall white pines. A nest studied by Mrs. Nice (1930a), at Pelham, Mass., was “six feet up in a small red cedar on a branch next to the trunk. It was a rather shallow affair, composed of cedar twigs and bark, plant fibers, a piece of string and pine needles, and was lined with a few horse hairs and many Ruffed Grouse feathers.”
Dr. Paul Harrington has sent me his notes based on the study of 44 nests of the myrtle warbler in Simcoe County, Ontario. He says that the white pine is generally chosen as a nesting tree, the nests being placed from 6 to 40 feet up, averaging 15 feet; “28 nests were built on horizontal limbs about two-thirds out from the trunk, but none at the outermost end. They were conspicuous from below but not from above, as clumps of needles overhung them in such a way as to afford good protection.” Of the remainder, 2 were built in the top clump of needles in young trees; 5 were in small spruces, the lowest 3 feet, the highest 15, and all on horizontal limbs, 3 near the trunk and 2 halfway out on the limb; 5 were about 15 feet up in crotches of small cedars; 3 were found in red pines, in the outermost clumps of needles 10 to 15 feet from the ground; and 1 nest was 6 feet up in a small balsam. He says that the nest is lined thickly with feathers and a few hairs. “The feathers are so placed that, as well as lining the nest, they form a screen over the inside when the bird is not sitting. This is done by the shafts of the feathers being woven or imbedded into the inside of the nest and the vane lying free.” At Petawa he found these birds nesting in small jack pines.
Dr. F. A. E. Starr, in his notes from northern Ontario, also says that any conifers are suitable nesting sites: “I have found only one exception to the use of a conifer. This nest was built in a hawthorn, and when I collected the nest, the birds moved to a cedar.” A. D. Henderson writes to me: “The myrtle warbler is a fairly numerous summer resident at Belvedere, Alberta, and in the Fort Assiniboine District. It nests mainly in the muskegs in tamarack and spruce trees, but occasionally in deciduous trees close to a muskeg.” The nests are mostly from 10 to 15 feet up. One nest was in a jack pine, “in a bunchy growth at the end of a limb.” Baird, Brewer, and Ridgway (1874) state that MacFarlane found nests on the ground in the Anderson River region.
Eggs.—Most observers agree that four or five eggs form the usual set for the myrtle warbler. Tufts (MS.) says that "five eggs are more commonly found than four.” Dr. Starr says in his notes that “four eggs are rarely laid, two and three being the usual numbers, while sometimes only one is laid, along with those of the cowbird.” This is probably an abnormal situation in which the cowbird fills the nest with its own eggs, leaving little room for those of the warbler.
The eggs are ovate to short ovate and slightly glossy. The ground color is creamy white and is speckled, spotted, or blotched with “auburn,” “argus brown,” “Brussels brown,” “chestnut brown,” or “cinnamon-brown,” with undermarks of “light brownish drab,” “vinaceous gray,” or “purplish gray.” Generally the spots are concentrated at the large end, forming a wreath, but some are marked all over and may also have a few scrawls of blackish brown. I think the handsomest are those having the rich creamy white ground almost immaculate except for a solid wreath, around the large end, of spots and blotches of the browns overlapping and intermingled with the undertones of gray, so that they resemble somewhat the eggs of the wood pewee. On lightly marked eggs the drab or gray spots are the most prominent. The measurements of 50 eggs average 17.5 by 13.3 millimeters; the eggs showing the four extremes measure 20.3 by 13.2, 17.9 by 14.8, 14.8 by 12.9, and 16.0 by 12.4 millimeters (Harris).
Young.—The incubation period for the myrtle warbler is from 12 to 13 days and the young remain in the nest normally from 12 to 14 days. Incubating the eggs and brooding the young is apparently done entirely by the female, but both parents are active in feeding the young and in cleaning the nest. Mrs. Nice (1930a), with the help of Miss Lucille Baker, watched a nest containing young for a total of 19 hours, over a period of 6 days. On the first day the female brooded 25 percent of the time, but less later on; the brooding periods averaged 9 minutes.