NORTHERN BLACK-THROATED GREEN WARBLER 303 



it does not form flocks, and except during the actual period of migra- 

 tion, 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 Tecpan 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 

 Tecpan 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. Dur- 

 ing the following days, it came every afternoon to forage in the low 

 cypress hedges that surrounded the dwelling. Possibly it was at- 

 tracted to these because of associations with its native land, for these 

 trimmed cypresses were the only coniferous trees in the vicinity — 

 indeed, in Costa Eica, 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, 1 heard a male black-throated green warbler singing 

 among the alder trees beside a rivulet on the Sierra de Tecpan. 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 1 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 



