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The Cowbird takes no interest in its offspring, but leaves their rearing 
entirely to other birds, usually to species smaller than itself. During the 
nesting season both sexes go roving about the country in flocks, from which 
the females detach themselves for short periods to place their eggs in the 
nests of other birds. Usually but one egg is deposited in a place, but 
sometimes two or three. The Cowbird sometimes removes an egg of the 
bird she is imposing upon. Roberts tells us that some two hundred species 
and sub-species of birds have been listed as thus victimized, and Dr. Fried- 
mann reports that ninety-one species have been definitely recorded as 
rearing young Cowbirds. “For every Cowbird that comes to maturity a 
brood of some other species must perish” (E. H. Forbush). Usually the 
fosterparents are the smaller perchings birds. I once saw a Cowbird’s egg 
in a Cardinal’s nest. Few birds seem to recognize the unwanted eggs 
imposed upon them, but the Yellow Warbler reacts by building a second 
nest over the first and sometimes, if need be, a third nest over the second. 
Several other warblers refuse to incubate any Cowbird egg found in their 
nests and a Robin or a Catbird will throw one out immediately. The eggs 
of the Cowbird are white or bluish-white marked with brown and gray, 
and are larger than those with which they are associated. The young are 
said to leave the nest seven days after hatching. 
Authorities differ in their views in regard to the Cowbirds establishing 
a particular territory within which the female operates. Eggs judged 
plainly to belong to different Cowbirds have been found in the same nest. 
Dr. Roberts says that he once observed two females trying at the same time 
to enter a Yellow Warbler’s nest. Dr. Friedmann states that not only does 
the female have a definitely marked off breeding area, but that the male 
bird has a definite post during the breeding season. However, Cowbirds 
do not defend their respective territories, if any, but restrict their methods 
of defense to an intimidation display. 
The incubation period of the Cowbird is but ten days, while the similar 
period of most birds it parasitizes, such as sparrows, warblers, and vireos, 
averages eleven or twelve days. This gives the young Cowbird a day’s 
start over the rightful nestlings, which is a great advantage, as birds do 
not feed their young in rotation and thus make sure that each has a right- 
ful share. When the parent bird brings food, the baby with the most 
gaping mouth and the longest neck receives the coveted morsel. In con- 
sequence the young Cowbird gets the greater part of the food while the 
other little birds starve or are crowded from the nest. 
When the young Cowbird leaves the nest it develops a most insistent 
food call and often it appeals to birds other than the foster parents and 
successfully begs food from them. For a time the obnoxious young bird 
may roost with English Sparrows about houses. Then he resorts to 
marshes with blackbirds and starlings and many cowbirds. 
Nobody loves a Cowbird, yet the male bird is really handsome in his 
glossy greenish black, with a coffee-colored head. He is the only American 
blackbird with a brown head. The female and young are very ordinary 
looking in dull brownish-gray. The Cowbirds have a sturdy daring that 
