"Although a number of wood warblers which winter in the Central American highlands are gregarious, those that center in the lowlands are typically solitary. In this, the yellow warbler is no exception. Each wintering bird appears to have its own territory, from which it attempts to drive others of its kind. Trespassers are scolded with insistent _chips_; or more rarely, soon after his arrival, a male will sing while defending his claim. Near San Miguel de Desamparados, Costa Rica (4,600 feet), on October 1, 1935, I made the following note: 'This morning, which for a change was bright and calm, I heard a yellow warbler singing in the low fig trees near the house. Upon going out to look, I found that there were two yellow warblers in the trees. One was trying to drive the other away; but the pursued always circled around and returned. I watched them for a long time; but this indecisive action continued without any change in the situation. In the intervals of the pursuit, the warblers (or at least one of them) would sing, but in a low and imperfect fashion, far inferior to the yellow warbler's summer song.' Again, on October 31: 'After the Wilson warbler, the most abundant winter visitor is the yellow warbler. The bird who on October 1 drove its competitor out of the fig trees beside the house still retains these trees and the surrounding _Inga_ trees as its domain.' "The yellow warbler sings far less while in Central America than many other wintering species. Exceptionally, one will be found singing profusely. In early October, 1934, I came upon such a bird among the coffee groves of a great plantation on the lower Pacific slope of Guatemala. His behavior was so far out of the ordinary that I am tempted to copy in full the notes I made upon it at the time: October 5--On the afternoon of my arrival at 'Dolores,' I went out for a walk through the coffee groves. From among the 'chalum' (_Inga_) trees which shaded the coffee bushes, I heard a bird's song which seemed to belong to a warbler; but I did not recognize it as the utterance of any species I knew. After searching for a time among the tree-tops, I spotted the singer, and was surprised to find him a yellow warbler. He was apparently a young bird, for he lacked the chestnut splashes along the sides which distinguish the mature males. He repeated over and over again his little song of four or five notes, which was so unlike the familiar song of the yellow warbler in the eastern United States that I did not at first recognize it; but once I had identified the singer, I realized that I was listening to a shortened and modified form of the typical song.