YELLOW-GREEN VIREO 325 



AYhen he lingered in the tree during her absence, he hurried down and 

 alighted close beside her and soon as she returned to the nest. He 

 also undertook to guard it from intrusion, and drove a Lawrence's 

 elaenia {E. chiriquensis) from its neighborhood, then later chased 

 away a silver-throated tanager {Calospiza icteroce'phala) that had 

 ventured too near. But afterward, while the vireos were absent, a 

 Mexican honeycreeper {Goercha mexicana), which was doubtless 

 building a dormitory nest somewhere not far otF, stole a blade of 

 grass from their nest and flew off unmolested with his small booty. 



The completed nest closely resembles that of the red-eyed vireo in 

 form and materials. It is a thick-walled, compact structure, the 

 outer layer composed of dry grass blades, strips of papery bark and 

 of plant epidermis, or similar ribbonlike material, while the inner 

 lining is of fine fibers. It is bound together and attached to the arms 

 of the fork between which it hangs by cobweb used in liberal quantity, 

 and frequently ornamented on the exterior with skeins of white spider 

 web and empty silken egg cases. 



Eggs. — Four days may elapse between the completion of the nest 

 and the appearance of the first egg. The others follow on consecutive 

 days. Three is the usual set ; but about one-fifth of the nests I have 

 found contained only two. In one case the nest was visited daily dur- 

 ing the period of egg-laying, and only two eggs appeared, indicating 

 that at times two constitute the set. The eggs are white, finely speckled 

 with some shade of brown (ranging from light brown to umber and 

 chocolate) , the spots usually aggregated in a wreath or cap upon the 

 large end, with perhaps a thin scattering over the remaining surface. 

 The average measurements of eight eggs temporarily removed from 

 their nests in El General are 20.5 x 14.6 mm. ; those showing the four 

 extremes measure 22.2 by 15.1, 20.2 by 15.5, 19.4 by 15.1, 19.8 by 13.9 

 millimeters. 



Incubation. — Incubation is performed by the female alone. Com- 

 monly she sits facing the crotch in which her nest is hung — I cannot 

 recall having seen a bird face outward from the tree as she incubated. 

 As a rule, she sits steadfastly, allowing a close approach before she 

 quits her eggs. Then she may rise to a higher branch and peer down 

 at the intruder, frequently scolding with her nasal chaaa^ while she 

 spreads her tail and raises her crown-feathers in a questioning atti- 

 tude. I have watched two nests for a number of hours continuously 

 during the course of incubation, and since the behavior of the vireos 

 differed materially, it may be well to discuss them separately. 



One of these nests was situated about 25 feet up in a sotacaballo 

 tree {Pithecololium) growing along the Kio San Antonio, at the edge 

 of a pasture. On the morning of May 12, 1940, 1 watched for 3 hours, 

 and found the sequence of the female vireo's sessions and recesses to 

 be as follows (the recesses in italics) : 39, 19, 40, 11, 9, 13, 16, and 21 



