Eggs.—Three or four distinctive eggs seem to constitute the full set for the northern olive warbler. These are ovate to short ovate and have a very slight lustre. They are grayish or bluish white, or even very pale blue, liberally speckled, spotted and blotched with “dark olive-gray,” “dark grayish olive,” “drab,” “olive-brown,” or “dark brownish drab.” These are interspersed with undertones of “mouse gray,” “deep mouse gray,” or “Quaker drab.” On some eggs the spots are sharp and distinct, while on others the olive, brown, and drab markings are clouded into the undertones. The spottings are usually well scattered over the entire surface, but tend to become heavier at the large end. The measurements of 28 eggs average 17.1 by 12.8 millimeters; the eggs showing the four extremes measure 19.0 by 16.0, 16.0 by 12.2, and 18.1 by 12.0 millimeters (Harris).
Young.—Information is lacking on incubation and care of the young.
Plumages.—The plumages and molts of the olive warbler are as distinctive as its nest and eggs. The sexes are not quite alike in juvenal plumage. Ridgway (1902) describes the young male as “pileum, hindneck, back, scapulars, rump, and upper tail-coverts plain dull olive or brownish olive; supra-auricular region and sides of neck dull yellowish buffy, the latter tinged with olive; chin, throat, and chest dull yellowish buffy; otherwise like adult female.” And of the young female, “similar to the young male but paler and grayer above; supra-auricular and post-auricular regions pale brownish buffy; chin, throat, and chest still paler buffy, the chin and upper throat dull buffy whitish.” The white tips of the greater wing coverts are tinged with yellowish.
I have not been able to trace the postjuvenal molt in the series I have examined but it apparently occurs in July and produces very little change, young birds of both sexes in their first winter plumage closely resembling the adult female at that season, though the crown and nape are grayer and the throat and breast are paler. I can find no evidence of a prenuptial molt. Young males evidently breed in this plumage and do not acquire the fully adult plumage until their second fall, or perhaps later. In Brewster’s series, collected in March, three males are in this condition, of which he (1882) says that “two of them, although in unworn dress, are absolutely undistinguishable from adults of the opposite sex; the third (No. 77), however, has the throat appreciably tinged with the brownish-saffron of the adult male.” This last may be a bird that is one year older, for Ridgway (1902) describes the “second year” male as “identical in coloration with the adult female.” Judging from the series that I have examined, including all of Brewster’s birds, I am inclined to think that the adult winter plumage is acquired at the first postnuptial molt, or when the bird is a little over one year old.
There can be no doubt, however, that young males breed in this immature plumage, for Price (1895) secured a pair that were feeding a brood of young, and the “male was not in fully adult plumage and was very similar in coloration to the female.” Swarth (1904) writes: “The male bird breeds in the immature plumage, for on June 21, 1902, I assisted O. W. Howard in securing a nest, containing four eggs, the parents of which were indistinguishable in color and markings. * * * I was surprised at the large proportion of birds in this immature plumage that were seen. At a very liberal estimate I should say that the males in adult plumage comprised barely a third of the birds seen in the spring.”
Adults have a complete postnuptial molt, mainly in July; all June birds that I have seen are in worn plumage, and August and September birds are in new, fresh feathers. The fall plumage of the male is similar to that of the spring male, but the colors of the head, neck, and chest are duller, more clay color, the back is more olive and the sides are browner. In the female at this season the crown is tipped with grayish and the throat and breast with buffy, while the sides are browner than in the spring; the white tips of the greater wing coverts are tinged with yellowish. The nuptial plumage is acquired mainly, if not entirely, by wear, the edgings wearing away and the colors becoming brighter.
Food.—Nothing definite seems to have been recorded on the food of the olive warbler, but its habit of creeping over the branches and twigs of the pines, much after the manner of the pine warbler, would seem to indicate that it was foraging for the many small insects that infest these trees. It is evidently one of the protectors of the pine forests. Brewster (1882b) says: “In their actions these Warblers reminded Mr. Stephens of Dendroeca occidentalis. They spent much of their time at the extremities of the pine branches where they searched among the bunches of needles for insects, with which their stomachs were usually well filled. Occasionally one was seen to pursue a falling insect to the ground, where it would alight for a moment before returning to the tree above.”
Behavior.—One of the members of Henshaw’s (1875) party brought in a specimen of this warbler, on September 20, “which he stated he had shot from among a flock of Audubon’s Warblers and Snowbirds, which he had started from the ground while walking the pine woods. With the rest, it had apparently been feeding upon the ground, and had flown up to a low branch of a pine, where it sat and began to give forth a very beautiful song, which he described as consisting of detached, melodious, whistling notes.”
W. E. D. Scott (1885), writing of his field work in the Santa Catalina Mountains in late November, says:
Associated with flocks of the Mexican Bluebird (Sialia, mexicana), which was, by the way, the only kind of Bluebird observed, was always to be found one and sometimes two representatives of the Olive Warbler (Peucedramus olivaceus). The Bluebirds were generally feeding on some insects in the tall pines, in flocks of from six to ten individuals. The Olive Warblers were on the best of terms with their blue friends, and as the Bluebirds were shy and restless they made it difficult to obtain or observe very closely their smaller allies. I did not in these pine woods see the two species apart, and became at length so well aware of the intimacy that existed between them, that I would fire at any small bird passing high overhead in company with Bluebirds. They were chance shots, certainly, but the only two small birds obtained flying in this way with the Bluebirds were Olive Warblers. * * * Generally they preferred the largest branches of the pines when they alighted, though I took one not more than three feet from the ground in a small bush. Their movements while feeding or searching for food are very deliberate, though I noticed now and again certain motions when at the extremity of a bough that reminded me of a Kinglet or a Titmouse.