Professor Cooke (1904) says of the fall migration route: “The principal line of migration is from the Mississippi Valley across the Gulf of Mexico to Mexico and Central America. The eastern part of this route probably extends from the southern end of the Alleghenies across northwestern Florida to the coast of Yucatan and Honduras.” A. H. Howell (1932), however, gives several records for central and southern Florida, and says: “In autumn, Weston reports a large migration on October 26 and 27, 1925, when 31 birds were killed at the lighthouse near Pensacola on the two nights, and large numbers seen on the morning of October 26 in vacant lots in the city.”
Dickey and van Rossem (1938) say of the migration in El Salvador:
During the fall migration of 1925, Tennessee warblers arrived in the vicinity of Divisadero on October 13. No advance guard, that is, individuals arriving ahead of the main flights, was observed in this case. On the above-mentioned date they were suddenly found to be present in numbers, and from then on were common in every lowland or foothill locality visited. In point of relative abundance this was by far the most common warbler (resident or migratory) throughout the coastal plain and in the foothills, but it was greatly outnumbered by Dendroica virens above 3,000 feet.
The manner of occurrence was usually as small flocks of six or eight or even twenty or more birds. These combined with several other species to make up larger flocks which worked ceaselessly through the crown foliage of low, semi-open woodland. However, many were found even in the tall, dense swamp forests along the coast and also in the oak woods on Mt. Cacaguatique.
Winter.—Dr. A. F. Skutch has contributed the following account: “The Tennessee warbler winters in Central America in vast numbers. Coming later than many other members of the family, the first individuals appear in mid-September; but the species is not abundant or widely distributed until October. During the year I passed on the Sierra de Tecpán in west-central Guatemala a single Tennessee warbler appeared in the garden of the house, at 8,500 feet, on November 7 and despite frosty nights lingered into December. On November 19, 1935, I saw one on the Volcán Irazú in Costa Rica at 9,200 feet—the highest point at which I have a record of the species. At the other extreme I found a few of these adaptable birds among the low trees on the arid coast of El Salvador in February and among the royal palms at Puerto Limón, on the humid coast of Costa Rica, in March. But Tennessee warblers are most abundant as winter residents at intermediate altitudes, chiefly between 2,000 and 6,000 feet above sea-level. From 3,000 to 5,000 feet they often seem to be the most abundant of all birds during the period of their sojourn. They travel in straggling flocks and form the nucleus of many of the mixed companies of small, arboreal birds. At times ‘myriads’ is the only term that seems apt to describe their multitudes.
“I think ‘coffee warbler’ would be a name far more appropriate than Tennessee warbler for this plainly attired little bird; it was merely a matter of chance that Alexander Wilson happened to discover the species in Tennessee rather than at some other point on its long route from Canada to Central America; but the warblers themselves manifest a distinct partiality to the coffee plantations. The open groves formed by the shade trees, whose crowns rarely touch each other, yet are never far apart, seem to afford just the degree of woodland density that they prefer. It matters not whether these trees are Grevilleas from Australia with finely divided foliage, or Ingas with large, coarse, compound leaves, or remnants of the original forest—a mixture of many kinds of trees with many types of foliage: from Guatemala to Costa Rica the Tennessee warblers swarm in the coffee plantations during the months of the northern winter and are often the most numerous birds of any species among the shade trees. Possibly they may at certain times and places be as multitudinous in the high forest as in the plantations. Although I have never found them so, the negative evidence must not be allowed to weigh too heavily, for such small, inconspicuous birds, devoid of bold recognition marks, are not easy to recognize among the tops of trees over a hundred feet high.
“Tennessee warblers are fond of flowers, especially the clustered heads of small florets of the Compositae and Mimosaceae, and of the introduced Grevillea that sometimes shades the coffee plantations. They probe the crowded flower clusters, perhaps seeking small insects lurking there rather than nectar. The white, clustered stamens of the Inga—the most generally used shade tree of the coffee plantations—are especially attractive to them. Local movements within their winter range appear to be controlled by the seasonal abundance of flowers. So, in the valley of the Río Buena Vista in southern Costa Rica, at an altitude of about 3,000 feet, I found Tennessee warblers very abundant during December and January. Here they flocked not only in the forest and among the shade trees of the little coffee groves, but also in great numbers through the second-growth thickets that filled so much of the valley, where at this season there was a profusion of bushy composites with yellow or white flower-heads, and of acacia-like shrubs (Calliandra portoricensis) with long, clustered, white stamens. But during February, the third dry month, the thickets became parched and flowered far more sparingly. Now the Tennessee warblers rapidly declined in numbers, and before the end of the month disappeared from the valley. During the following year, which in its early quarter was far wetter, a number remained through March, and a few well into April.
“Tennessee warblers pluck the tiny, white protein corpuscles from the brown, velvety bases of the long petioles of the great-leafed Cecropia trees, taking advantage of these dainty and apparently nutritious tid-bits when the usual Azteca ants fail to colonize the hollow stems; for only on trees free of ants does this ant-food accumulate in abundance.
“While the Tennessee warbler departs during February from some districts where it is common in midwinter, it remains until April in regions where the dry season is not severe. After the middle of April it is only rarely seen in Central America; and there appears to be no record of its occurrence in May.”
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