432 U. S. NATIONAL MUSEUM BULLETIN 211 



fourteen species in the A.O.U. Check- List are known to be victimized by the Cow- 

 bird, all of them very infrequently. Counting subspecies, six, of the thirty-six in 

 the North American fauna, are included in the roll-call of the Cowbird's victims. 



Paridae are of little importance in the economy of the Cowbird, and the latter 

 plays an inconsequential role in the lives of the Titmice. Five species of the eleven 

 in the A.O.U. Check-List are recorded as victims of the Cowbird; all of them un- 

 commonly. 



The kinglets and gantcatchers are, with one exception, infrequently molested 

 by the Cowbird. They are interesting in that they are among the smallest birds 

 definitely known to be affected by the parasite. Three of the six species in the 

 North American fauna are included in the list of hosts: one of the three being repre- 

 sented by two geographic races. 



The Thrushes are of considerable importance in the natural history of the Cow- 

 bird, and are among the largest birds commonly and regularly parasitized. Not 

 only do we often find Cowbirds' eggs in the nests of some of these birds, but fre- 

 quently they may be seen caring for the young Cowbirds. Even here, where the 

 rightful young are of approximately the same size as the young parasites, it is 

 rather unusual to find any but the Cowbird surviving in a victimized nest. * * * 



Seven of the fifteen North American species in this family are more or less im- 

 posed upon by the Cowbird, two of them being represented in the present list 

 by two races each. 



Bendire (1895) listed 91 species and subspecies victimzied by the 

 eastern cowbird, including a few victims of the Nevada cowbird, which 

 had not at that time been given separate status. Friedmann (1929) 

 listed 114 species and subspecies as victims of the eastern cowbird 

 alone, adding three species, the ruby-throated hummingbird, Nelson's 

 sparrow, and the brown creeper as hypothetical. In subsequent papers 

 (1934, 1938, 1943, and 1949), he increased the known hosts of the 

 eastern cowbird to 149 forms. 



Cowbirds' eggs are sometimes found in nests of birds that are wholly 

 unfitted to become foster parents for the young, in which cases the 

 eggs never hatch or the young never survive. If the eggs of the owner 

 of the nest are much larger than those of the parasite, the cowbirds' 

 egg will not receive enough warmth from the body of the incubating 

 bird to hatch. If the young of the selected foster parents are fed on 

 food unsuitable for the young cowbird, the latter cannot be expected 

 to thrive on it; one can hardly conceive of a mourning dove, which is 

 listed as a victim, feeding a young cowbird on "pigeon milk," or of a 

 kingfisher feeding it on fish. A swallow might hatch a cowbirds' egg 

 and feed the young one in the nest, but not afterward, as young 

 swallows are taught to feed on the wing. It is quite important for the 

 cowbird to select an open nest of some altricial bird that feeds its 

 young in the nest until they are nearly able to fly; the young of pre- 

 cocial birds leave the nest soon after they are hatched and the young 

 cowbird would be deserted; the egg of a cowbird has been found in a 

 killdeer's nest, but, if the egg ever hatched, the young must have been 

 left in the nest to starve or die of exposure. 



