Cowbird. I43 



it from the nest. Other species, notably the yellow warbler 

 and frequently the prothonotary warbler, determined to 

 avoid the task of rearing the intruder, sometimes build a 

 floor over the contents of their nest, thus incarcerating 

 their own eggs as well as those of the cowbird, and pro- 

 ceedto lay a new complement of eggs. When the parent 

 continues the task of incubating or of laying the remainder 

 of her regular complement, if the eggs are examined at 

 any stage of incubation, it will be seen that the Qgg of the 

 cowbird is rather more advanced than the eg^s of the 

 owner of the nest. This fact has been partially explained 

 by reasoning that the generally larger size of the parasitic 

 egg brings it into closer contact with the body of the sit- 

 ting bird, and hence it receives a larger portion of the 

 heat necessary to incubation. Observations appear to 

 warrant the conclusion that the eggs of the cowbird hatch 

 m eleven or twelve days. 



The eggs of the cowbird average .88 by .65 of an inch 

 the length varying from .95 to .67 of an inch, and the 

 width varying from .72 to .58 of an inch. The ground is 

 a dingy white or gray, and the markings vary through all 

 the shades of brown, sometimes evenly distributed over 

 the surface, and at other times predominating around the 

 larger end. In some instances the spots are quite fine 

 and m others the markings are moderately large, hence 

 there is much diversity in the appearance of different 

 specimens. Frequently the investigator is puzzled in 

 distinguishing the true eggs of the towhee, cardinal and 

 other species from the products of the parasite. 



It is a fact not generally noted by naturalists that in 

 the breeding season the male grackles and red-winged 

 blackbirds, and the cowbirds of both sexes nightly con- 

 gregate to roost in suitable places. I first became aware 

 of the fact on a visit to the swamp-lakes early in May 

 and described a roost situated near Havana, Illinois, in 

 the fet. Ijouis Globe-Democrat, June 7, 1896. In the Ob- 

 server for July, 1896, Dr. A. K. Fisher has an interesting 

 article on "Summer Boosts of Swallows and Eed- winged 

 Blackbirds." The roost which attracted my attention 

 was anaong willows growing on a sand-bar about one 

 hundred yards out in the mouth of Quiver Lake The 



