324 BULLETIN 19 7, UNITED STATES NATIONAL MUSEUM 



as for the great majority of small passerine birds of the region. Since 

 in normal years the rains begin at the end of March or early in April, 

 the vegetation is by now in its fullest verdure and insect life abundant. 



Like other members of her family, the yellow-green vireo chooses 

 for her nest site a V-shaped fork between slender, nearly horizontal 

 branchlets. The 17 nests whose approximate height I have recorded 

 ranged from 51/0 to 40 feet above ground. Of the two nests 40 feet 

 up, one was in a shade tree of a coffee plantation near Colomba, 

 Guatemala, and the other in a roadside tree near Alajuela, Costa Rica. 

 Our local birds prefer low positions for their nests ; 9 of my 14 records 

 for El General are of nests less than 10 feet above ground; and the 

 highest was 25 feet up. Wlien nesting in bushy pastures, a favorite 

 site is a crotch of the tuete {Vernonia patens), a white-flowered com- 

 posite bush of no great height. But even when they choose a tall tree, 

 the vireos frequently attach their nest to the drooping extremity of 

 one of the lowest branches. 



It was in such a position, in a fork at the end of a low bough of an 

 aguacatillo tree {Persea caerulea) standing in an open pasture, that 

 I watched a yellow-green vireo build her nest on two bright mornings 

 of March 1937. Since the bird was shy, I sat well concealed in a blind. 

 The nest had already been given its final shape, that of a cup attached 

 by its rim to the supporting branchlets ; but the fabric was still ex- 

 ceedingly thin and delicate. The female alone worked. Although she 

 was exactly like her mate in plumage, her silence and his untiring song 

 served well to distinguish the twain. She brought strips of partially 

 decayed grass blades, fibers of various sorts, and cobweb. Standing 

 upon the supporting twig at one side of the nest, she would deposit 

 the material in its bottom. Then, frequently, she would enter the deep 

 cup, and press herself down into it, with bill pointed upward and wings 

 raised above her back, while she shaped it with her entire body. As 

 she flew away again, she usually uttered her sharp little churred call. 

 She did not work very hard; and her visits on the nest were widely 

 spaced. On the first morning, from 7 : 30 to 8 : 30, she brought mate- 

 rial only 10 times; yet in this hour she labored more actively than 

 during any other of the five that I kept watch. In the next hour she 

 came only eight times ; and on the second morning she worked even 

 more slowly. 



Her mate remained much of the time among the foliage of the 

 aguacatillo tree in which the nest was being built, singing mree in his 

 usual unhurried fashion. Early in the morning, before the female 

 had begun to build, he twice went to the nest, perched a few moments 

 upon one of the supporting twigs at its side, and quivered his wings 

 while he coiitinued to drop his bright notes at measured intervals. 

 Often he followed the female when she flew off to seek more material, 

 and accompanied her too, on her return to the nest. At other times, 



