Eggs.—Swainson’s warbler usually lays three eggs; sets of four are rare and of five very rare. Although there are records of nests containing two incubated eggs or two young birds, these probably represent incomplete sets or cases where an egg or a nestling has been destroyed.
Eggs are quite globular, the two ends sometimes scarcely distinguishable; the shell is thick and has a distinct polish; the ground color is white with a bluish tinge; however, a set of three eggs in the writer’s collection had a faint greenish tinge, while several observers describe sets of pale pink or buffy white.
Rarely, spotted eggs are found. Wayne (1910) says: “Spotted eggs are, however, very rare and I have found only four or five nests containing them.” The only spotted egg the writer has found is in the set referred to above; of these, two are immaculate, while the third is “faintly though distinctly speckled around the larger end with reddish brown” (Dingle, 1926).
Brewster (1885b) describes a set collected by the late Arthur T. Wayne: “One is perfectly plain; another * * * has two or three minute specks which may be genuine shell markings; while the third is unmistakably spotted and blotched with pale lilac. Over most of the surface these markings are fine, faint, and sparsely distributed, but about the larger end they become coarser, thicker, and deeper colored, forming a well-defined ring or wreath.”
Burleigh (1923) writes: “Unlike all the descriptions I had read, and the few eggs I had seen, these were light pink in ground color and dotted distinctly over the entire surface with light brown spots, this almost forming a wreath at the larger end of one egg.” These eggs were found near Augusta, Ga., and the parent was secured.
Wayne (1910) was of the opinion that two broods are raised in a season.
[Author’s Note: The measurements of 50 eggs average 19.5 by 15.0 millimeters; the eggs showing the four extremes measure 21.6 by 14.2, 20.8 by 16.0, 18.0 by 14.1, and 19.5 by 13.5 millimeters (Harris).]
Plumages.—[Author’s Note: Ridgway (1902) describes the juvenal plumage of Swainson’s warbler as follows: “Head, neck, back, rump, upper tail-coverts, chest, sides, and flanks plain brown (varying from broccoli to bister); rest of under parts whitish or dull pale yellowish, more or less clouded with brown; middle and greater wing-coverts indistinctly tipped with cinnamon-brown; otherwise like adults, but no trace of lighter superciliary nor darker postocular stripes.” Specimens that I have seen in this plumage are more nearly “cinnamon-brown” than the colors named above on the back and wing coverts, and the latter show very little evidence of cinnamon tips.]
The postjuvenal molt, which evidently includes only the contour plumage and the wing coverts, occurs early in the summer; I have seen young birds beginning to acquire the first winter plumage as early as June 12, and others that had nearly completed the molt on July 20; these birds were not yet fully grown. Wayne (1910) writes: “I have taken young birds which were as large as the adults and which were acquiring their autumnal plumage as early as June 2, but it must be borne in mind that the season in which these young were taken (1906) was exceptionally advanced.”