NESTING HABITS OF OROPENDOLA- — CHAPMAN 385 



ings of tlie reproductive system which are believed to have been the 

 fundamental motivatin<j: factors in the orifjin of bird mi^^^ration. 



Data on a second or supplementary nesting season are not con- 

 clusive. 



The females outnumber the males about six to one. If this dis- 

 parity of the sexes is an actual characteristic of the species it may 

 be the cause of the colonial association that permits one male to mate 

 with several females. The males show no marked sexual jealousy. 

 Courtship begins with nest building. A male may woo several 

 females but he apparently has but one mate at a time; the length 

 of this association covering only the period when the ova are ready 

 for fertilization. A similar type of sexual relation appears to exist 

 in the humming bird and cotinga that nest in the oropcndola tree. 



The males take no part in the selection of the site, gathering of 

 building material, construction of the nest, incubation of the eggs 

 or care of the young. They are, however, in constant attendance on 

 the females either as wooers or accepted mates until the eggs aro 

 laid. As watchmen of the colony they play an important part in 

 the protection of the females, particularly in the early stages of nest 

 construction. 



Only two other colonies of Zarhynchus being known on the island 

 there is no question of colonial territorial rights; but there is often 

 pronounced competition among the females for possession of a nest 

 site. 



A new nest is built each year, about one month being required for 

 the completion of the bag and its contained nest. The females 

 then begin to sleep in the nest, leaving the males, who never enter 

 the nest, to return unaccompanied to their roost in the forest. 



Two eggs are laid. The period of incubation is approximately 

 17 days. The young leave the nest about one month after hatching. 



The terminal twigs of the nest tree are brittle and in a strong wind 

 sometimes break. Nests that fell from this cause contained both eggs 

 and young but were not found to be infested by parasites. The only 

 ascertained enemies of the oropendolas were other birds. They are 

 parasitized by a cowbird {Cassidix), and a flycatcher (Legatiis), no 

 larger than a phoebe {Sayornis /^Aoe&e), constantly harries them with 

 the evident purpose of securing possession of one of their nests for 

 its own uses. The building female is at times susceptible to attack 

 by hawks, from which it appears to be the duty of the male to guard 

 her. In response to his alarm call the entire colony dives hastily into 

 the lower forest growth. An owl {Fulsatrix) makes an opening in 

 the lower part of the nest bag and the oropendolas appear to be 

 at the mercy of this foe. The inaccessibility of the nest site, and 

 impregnability of tiie pendant, strongly woven nest bag are, there- 

 fore, more apparent than real. Furthermore, the conspicuousness 



