NESTS AND NEST-BUILDING IN BIRDS 351 



pare figs. 19 and 20.) A rim and bottom are gradually produced, 

 and the remaining gap finally bridged at the side opposite to 

 that represented by the first hanging mass. 



The first threads are wound around definite twigs and form 

 what I shall call the primary points of support. Much labor 

 is bestowed in building to the primary nest mass ; secondary 

 points of support are added later, when more than two twigs 

 are used, for the suspension proceeds pari passu with the growth 

 and conformation of the free mass. In the vireo these processes 

 of building downward and adding new fibres to the support 

 continue for a long time before much progress seems to be made, 

 and it ma}" be two days before a single molding movement 

 is observed. Moreo\'er, the vireo 's molds are somewhat difterent 

 from those practiced by the robin (see p. 353). 



This vireo 's nest occupied 4^ days in the building, and with 

 the exceptions to be noticed later was the work of the female 

 alone. It was composed of hundreds of minute increments, 

 like a single bast fibre or bit of cocoon silk, some indeed so small 

 that it was difficult to discern them at a distance of ten feet, 

 even with aid of a glass. The record of a single hour's 

 work at this early stage of operations is gi\'en in table VII. 

 During this period visits were made by one or both birds on 

 the average of once in every 4J minutes. A light gray fibrous 

 mass about one inch long and extending over each fork to about 

 the same distance, had now been loosely secured beginning a 

 little way from the angle of the twigs. It was composed of 

 spiders' silk, chiefly from their egg cocoons, and fine bark 

 strippings. 



The female vireo in this instance took the lead and did all 

 the constructive work, although the male frequently escorted 

 her to and from the nest, and twice was seen to ofter material 

 which the female quickly took from bill or mouth. The fibrous 

 matter brought was sometimes passed around the twigs, but 

 always carefully incorporated by quick movements of the 

 bill with the suspended mass, and from half a minute to a 

 minute was usually spent in this work. The bird moved quietly 

 and effectively, more time being spent in the readjustment of 

 old material than with incorporation of the new. A \'ery char- 

 acteristic movement at this stage was to pick up the free end 

 of a fibre, pass it over the twig and quickly secure it with the 



