265 
Nesting. Eggs laid in nests of other, usually smaller, birds. Entirely parasitic. 
Distribution. North America, south to Mexico. In Canada, across the continent; 
in the west, southern British Columbia and from the Mackenzie southward. 
SUBSPECIES. The subspecies recognized by the Check-list as inhabiting Canada 
is the type form, Molothrus ater ater. A western subspecies, the Nevada Cowbird Molo~ 
thrus ater artemisiae, has been proposed. It is postulated to have a slightly more slender bill, 
but this feature is not very marked or very constant. Western females show a faint striping 
below that is less apparent in eastern specimens. This is the form to be expected from 
Manitoba, westward. 
The Cowbird is our only habitually parasitic bird. It never builds a 
nest nor incubates nor cares for its young. In the absence of the rightful 
owners it takes the opportunity of depositing one of its 
own eggs in the unguarded nest of other birds. Usually 
the birds so imposed upon accept the foreign egg with- 
out protest, at other times there are strong objections 
and final resignation. In a few cases, the nest is deserted 
or a new nest is built over the offending egg, as is 
sometimes done by the Yellow Warbler. On incubation 
an interesting case of adaptation is shown. The Cow- 
bird’s egg usually hatches a few hours before those of Figure 255 
the original occupant of the nest and consequently Bl 0 t^aUize/ mt " 
the interloper is strong and well grown when the proper 
occupants of the nest break their shell. It can monopolize the food, 
thus increasing the difference in strength, and is able finally to hoist its 
competitors from the nest to perish on the ground while it receives the 
attention that should have been given to the rightful brood. Thus practic- 
ally every Cowbird is raised to maturity at the expense of a brood of another 
species and the Cowbird must be considered one of the greatest enemies of 
the species imposed upon. Once the foster-parents accept the intruding 
egg they do not make any distinction between it and their own. The 
Cowbird receives its name from its habit of following cattle, evidently 
attracted by the flies and insects which gather about those animals. 
In the early days, it probably followed the buffalo, which suggested 
its old, nearly forgotten name of Buffalo-bird. 
Economic Status. From a study of their food, Cowbirds would seem 
to be purely useful birds. They consume large amounts of weed seeds and 
harmful insects and only small quantities of grain or fruit, the grain largely 
waste and the fruit wild. Their effect upon other equally useful birds, 
however, puts a different complexion on their activities. Practically 
every Cowbird raised to the fledgling stage means the elimination of a nest 
full of other species. Perhaps the economic effects of the changelings 
equal those of the individuals they displace, but the substitution cannot 
be looked upon with equanimity. 
497. Yellow-headed Blackbird. Xanthocevhalus xanthocephalus. L, 10*0. Plate 
XLVII A. Slightly larger than the Bed-winged Blackbird, with a bright yellow hood over 
head, neck, and breast, and conspicuous white area at base of primaries on forepart of 
wings. Female: uniform dark brown with throat and upper breast dull white or dull 
yellow. Younger juvenile birds are like females, but with rusty-ochre hood. 
Distinctions. The adult male is unmistakable. In females and young birds, the white 
wing marks are missing, but the whitish or dull yellow throat and the well-defined rusty- 
ochre hood are distinctive. This latter description may suggest the autumn Rusty Black- 
bird, but the rusty colour of that species blends away on the body and never resembles a hood. 
