Nesting.—The earlier ornithologists knew nothing about the nesting habits of the cerulean warbler; Audubon’s description of its nest was entirely erroneous, and it was about 50 years after the bird was discovered that its nest was reported. This is not strange, as the nest is not easy to find and still more difficult to secure. Rathbun (MS.) writes in his notes: “During our stay in New York State, we found only three of its nests, because they were rather difficult to locate. We found the first at a height of 55 feet in a little cluster of small, twig-like branches growing on the side of a feathered elm; these clusters were close enough together to be of great use in climbing the tree, which was at least 3 feet in diameter. The nest was discovered by seeing the bird fly into the cluster. Within the next week a second nest was found by watching the female bird; it was at a height of 45 feet in a very small, flat crotch of a soft maple. The third nest was at a height of about 30 feet.
“The nests were identical in all respects except as to shape, which varied because of its situation. Each was nicely made but not unusual in appearance. The material used was almost wholly the fine strips of the grayish bark of small weed stalks, neatly interwoven. Each was smoothly and beautifully lined with the fresh stems of ground mosses of a brownish red color, which contrasted nicely with the gray outer material. Of great interest was the smoothness with which the material was woven in.”
Burtch wrote to Dr. Chapman (1907) that near Branchport, N. Y., where the bird is locally common, “the nest is usually placed on a horizontal branch or drooping branch of an elm, ranging from twenty-five to sixty feet from the ground, and from four, to fifteen, or eighteen feet from the body of the tree over an opening.”
W. E. Saunders (1900) reports eight nests found in southern Ontario; two of these were in oaks, 20 and 23 feet up, two in maples, 30 and 35 feet from the ground, and four in basswoods (lindens), from 17 to 50 feet above ground. He gives the measurements of three nests; they measured externally from 13⁄4 to 2 inches in height and 23⁄4 inches in diameter; internally they varied from 7⁄8 to 1 inch in depth and from 17⁄8 to 13⁄4 inches in diameter. He remarks: “A feature that interested me very much was the extreme shallowness of the nests; all the other warblers with which I am acquainted building a comparatively deep nest, and the query arises, Does the bird build a shallow nest because it places it on a substantial limb, or does it place it on a substantial limb because its nests are shallow? The attachment of the nest, also, is exceedingly frail, and I am inclined to think that few of these nests would remain in position long after the young had left.”
A nest found by Kirkwood (1901) in Baltimore County, Md., is described as follows: “The nest is made of brown bark fibre, with some fine grass stems among it, and is finished inside with a few black horse-hairs. Outside it is finished with gray shreds of bark, spider web, and a few small fragments of newspaper that had been water-soaked. * * * As the branch sloped, one part of the rim is within 3⁄4 of an inch of it, while the opposite part is 13⁄4 inches above it, the material comes down on one side of branch to 21⁄4 inches below the rim. On this side a tiny twig arches out from branch and extending to the rim is embedded in the nest, and the leaves which grew from its top shaded the nest.” The nest was 48 feet and 6 inches up from the ground and 15 feet out from the trunk of a tulip tree, with no other limb between it and the ground.
A neat little nest before me is made of materials similar to those mentioned. It is lined with the reddish brown flowering stems of mosses smoothly woven with other very fine brownish fibres into a compact rim, and it is decorated externally with various brown and gray lichens and mosses. Other nests have been reported in sycamores, beeches, rock maples, sugar maples, and white oaks.
Eggs.—The cerulean warbler lays from 3 to 5 eggs, usually 4. They are ovate to short ovate and have a slight luster. The ground color is grayish white, creamy white, or even very pale greenish white, and they are speckled, spotted or blotched with “bay,” “chestnut,” or “auburn,” intermingled with spots of “light brownish drab,” or “brownish drab.” Some eggs have spots scattered all over the surface, but usually they are concentrated at the large end, where a loose wreath is formed. Generally the eggs are finely marked, but occasionally are quite heavily blotched. The measurements of 50 eggs average 17.0 by 13.0 millimeters; the eggs showing the four extremes measure 17.9 by 13.0, 17.0 by 13.7, 16.0 by 12.4, and 17.2 by 12.0 millimeters (Harris).
Young.—The period of incubation seems to be unknown, and we have no information on the care and development of the young. Incubation is said to be performed by the female alone, but both parents assist in feeding the young. After the young are out of the nest, they may be seen travelling through the woods in family parties with their parents. There seems to be no evidence that more than one brood is raised in a season.
Plumages.—Ridgway (1902) describes the young cerulean warbler in nestling (juvenal) plumage as “above uniform brownish gray (deep drab gray), the pileum divided longitudinally by a broad median stripe of grayish white; sides of head (including a broad superciliary stripe) and entire under parts white; a narrow postocular stripe of deep drab gray; wings as in adults, but edgings greenish rather than bluish.”