Herbert Friedmann (1929) says: “This aberrant warbler is a rather uncommon victim of the Cowbird, only a couple dozen definite instances having come to my notice. * * * The largest number of Cowbirds’ eggs found in a single nest of this Warbler is five, together with three eggs of the owner.” George W. Byers (1950) reports a nest of this warbler, in Michigan, that held two eggs of the warbler and eight of the cowbird, on which the warbler was incubating. His photograph of the eggs suggests that they were probably laid by four different cowbirds.
Fall.—Several of the warblers show a tendency to stray from their breeding grounds soon after their young are able to care for themselves, perhaps even before the postnuptial molt is completed and long before the birds gather into the mixed autumn flocks. Among these early wandering birds the black-and-white warbler is a very conspicuous species, perhaps because it is one of our commoner birds or, more probably, because of its habit of feeding in plain sight on the trunks and low branches of dead or dying trees and shrubs instead of hiding, like other warblers, high up in the foliage. It may be that the warblers we see at some distance from their breeding grounds thus early in the season have already begun their migration toward the south: they often appear to be migrating.
Behind the house in Lexington, Mass., where I lived for years, there was a little hill, sparsely covered with locust trees, to the southward from my dooryard. This hill was a favorite resort for warblers in late summer. No warbler bred within a mile of the spot, except the summer yellowbird, to use the old name, yet soon after the first of July the black-and-white warblers began to assemble there. Not infrequently I have seen a single bird come to the hill, flying in from the north across Lexington Common, and join others there. The small company might remain for an hour or more, frequently singing (evidently adult males) as the birds fed in the locust trees.
Later in the season, as August advances, migration appears more evident. The birds now gather in larger numbers, sometimes as many as eight or ten; they pause in the locust trees for a shorter time before flying off; they are no longer in song; and the majority of the birds have white cheeks, most of them presumably young birds. Although they are almost silent as they climb about feeding, if you stand quietly in the midst of a company of four or five, now and then you may hear a faint note, and at once the note comes from all sides, each bird apparently reporting its whereabouts—a sound which calls to mind the south-bound migrants as they roam through the quiet autumn woods. Other warblers, unquestionably migrants, visit this hillside in August, notably the Tennessee, an early arrival who has already traveled a long way.
The fall migration of the black-and-white is long-drawn-out. The bird does not depend, like many of the warblers, on finding food among the foliage, so it may linger long after the trees are bare of leaves, sometimes, here in New England, well into October. I saw a bird in eastern Massachusetts on October 23, 1940, a very late date.
Winter.—Dr. Alexander F. Skutch (MS.) sent to A. C. Bent the following comprehensive account of the bird on its winter quarters: "None of our warblers is more catholic in its choice of a winter home than the black-and-white. Upon its departure from its nesting range, it spreads over a vast area from the Gulf States south to Ecuador and Venezuela, from the Pacific coast of Mexico and Central America eastward through the Antilles. And in the mountainous regions of its winter range it does not, like so many members of the family, restrict itself to a particular altitudinal zone, but on the contrary scatters from sea level high up into the mountains. As a result of this wide dispersion, latitudinal and altitudinal, it appears to be nowhere abundant in Central America during the winter months, yet it has been recorded from more widely scattered localities than most other winter visitants. On the southern coast of Jamaica, in December 1930, I found a greater concentration of individuals than I have ever seen in Central America during midwinter.
“Wintering throughout the length of Central America, from near sea level up to 9,000 feet and rarely higher, the black-and-white warbler is somewhat more abundant in that portion of its altitudinal range comprised between 2,000 or 3,000 and 7,000 or 8,000 feet above sea level. It is found in the heavy forest, in the more open types of woodland, among the shade trees of the coffee plantations, and even amid low second-growth with scattered trees. It creeps along the branches in exactly the same fashion in its winter as in its summer home. Solitary in its disposition, two of the kind are almost never seen together. The only time I have heard this warbler sing in Central America was also one of the very few occasions when I found two together. Early on the bright morning of September 1, 1933, when the warblers were arriving from the north, I heard the black-and-white’s weak little song repeated several times among the trees at the edge of an oak wood, at an altitude of 8,500 feet in the Guatemalan highlands. Looking into the tree tops, I saw two of these birds together. Apparently they were singing in rivalry, as red-faced warblers, Kaup’s redstarts, yellow warblers, and other members of the family solitary during the winter months will sing in the face of another of their kind, at seasons when they are usually silent. Often such songs lead to a pursuit or even a fight; but I have never seen black-and-white warblers actually engaged in a conflict in their winter home.
“Although intolerant of their own kind, the black-and-white warblers are not entirely hermits; for often a single one will attach itself to a mixed flock of small birds. In the Guatemalan highlands, during the winter months, such flocks are composed chiefly of Townsend’s warblers; and each flock, in addition to numbers of the truly gregarious birds, will contain single representatives of various species of more solitary disposition, among them often a lone black-and-white, so different in appearance and habits from any of its associates.