Winter.—The following notes are contributed by Dr. Alexander F. Skutch: “The black-throated green warbler is an abundant winter resident in the Central American mountains, where it is well distributed on both the Caribbean and Pacific slopes. In Guatemala, it winters from 1,000 to about 8,500 feet above sea-level, but is not abundant at either of these extremes of altitude. Farther south, in Costa Rica, it prefers slightly higher elevations. Here I have not recorded it between 2,000 and 2,900 feet, although the greater part of my bird-watching in the country has been done in this altitudinal belt. From 2,900 feet, where it is rare as a winter resident, it ranges up to nearly 10,000 feet. At this elevation, I found it abundant on the Volcán Irazú in late November. Less sociable than the Townsend warbler, it does not form flocks, and except during the actual period of migration, is more often seen alone than in the company of others of its kind.
“As a rule the black-throated green warbler arrives late, and has rarely been recorded before mid-October. But on August 9, 1933, I found a lone male in full nuptial plumage with a mixed flock of small resident birds in an open oak wood on the Sierra de Tecpán in the Guatemalan highlands. He sang his dreamy, unsubstantial song as he foraged along with his newly found companions. I saw only one other of his kind—or possibly it was the same individual again—before early October, when the species began to arrive on the Sierra de Tecpán in numbers.
“Another early arrival appeared on September 28, 1938, in the yard of the cottage I occupied at Vara Blanca, at an altitude of 5,500 feet on the northern slope of the Cordillera Central of Costa Rica. During the following days, it came every afternoon to forage in the low cypress hedges that surrounded the dwelling. Possibly it was attracted to these because of associations with its native land, for these trimmed cypresses were the only coniferous trees in the vicinity—indeed, in Costa Rica, the warblers find no native conifers save two species of Podocarpus, a genus whose center of distribution is in the Southern Hemisphere rather than in the North. At times the newly arrived warbler descended to the bare ground in the flower garden, where it appeared to find something edible. On October 2 it was for the first time accompanied by a second of its kind. Throughout the winter months a black-throated green warbler continued to visit these cypress hedges.
“This is another migrant warbler that plucks the dainty white protein corpuscles from the velvety cushions at the bases of the long petioles of the Cecropia tree. In excessively humid highland regions, as at Vara Blanca, the wide, hollow internodes of these trees are much of the time flooded with water, and therefore uninhabitable by the Azteca ants which at lower elevations usually colonize them. In the absence of the ants, whose food these tiny morsels are, the birds find an abundance of them on the Cecropia trees. A number of small native birds, including finches, tanagers, warblers, honeycreepers and ovenbirds (Furnariidae), share them with the migratory warblers.
“By mid-March the males are in resplendent nuptial plumage. On April 27, 1933, I heard a male black-throated green warbler singing among the alder trees beside a rivulet on the Sierra de Tecpán. On April 4 and 5, 1938, a male sang repeatedly at the edge of the forest at Vara Blanca; and from this date until the disappearance of the species from the region on April 14 I often heard their song.
“There is a certain amount of evidence that with the increasing aridity of the dry season the black-throated green warblers withdraw early in the year from districts on the Pacific slope where they were present during the wetter closing months of the preceding year. Thus, on the Sierra de Tecpán I met none between December 7 and April 20, when the northward movement was in progress, and the birds seen were doubtless transients rather than winter residents. And in the higher parts of the Basin of El General in southern Costa Rica I have recorded the species only in October, November, and December, after which the nearly rainless season begins. But in the wetter climate of Vara Blanca, they were seen throughout February and March until their northward departure in April.
“The black-throated green warbler withdraws from Costa Rica about the middle of April, and by the end of the first week of May has vanished from Guatemala.
“Early dates of fall arrival in Central America are: Guatemala—passim (Griscom), October 15; Sierra de Tecpán, August 9, 1933; Finca Mocá, October 29, 1934. Honduras—Tela, October 26, 1930. Costa Rica—Vara Blanca, September 28, 1937; San José (Underwood), October 16; Basin of El General, October 22, 1936.
“Late dates of spring departure from Central America are: Costa Rica—Juan Viñas (Carriker), April 17; Vara Blanca, April 14, 1938. Guatemala—passim (Griscom), May 4; Sierra de Tecpán, May 6, 1933.”
The following account of its winter haunts in El Salvador by Dickey and van Rossem (1938) is also interesting: