446 U. S. NATIONAL MUSEUM BULLETIN 211 



Field marks. — The cowbird is the smallest of our blackbirds and 

 can generally be recognized in the mixed flocks by size alone, even if 

 the brown head and glossy black body of the male and the plain dark- 

 gray coat of the female cannot be distinguished. 



Enemies. — Some of the host species are hostile to the cowbird, 

 but few succeed in driving it away. Some remove the eggs of the 

 parasite and some bury them; many eggs are removed by human 

 observers, and the total egg loss must be considerable. An interesting 

 demonstration of hostility is mentioned by Dr. George M. Sutton 

 (1928). He noted that in Pymatuning Swamp the swamp-nesting 

 small species were nearly immune from cowbird parasitism, because 

 the red-winged blackbirds ganged up against the cowbirds and drove 

 them out of the swamp. He "saw a flock of Red-wings once pursue a 

 female Cowbird until she was utterly exhausted and plunged into the 

 water to escape. Her pursuers chased her to the edge of the Swamp 

 then headed her off and forced her back to the opposite bank." 



While roosting in the swamps, cowbirds are preyed upon by mink 

 and weasels and perhaps owls; during the day, they are subject to 

 attack by hawks and falcons. 



Harold S. Peters (1936) lists one louse, two flies, two mites, and one 

 tick as external parasites on the eastern cowbird. 



Fall. — As soon as the egg-laying season is over, the cowbirds begin 

 to gather into large flocks and wander about over the country, feeding 

 in the fields and pastures. The young birds join these flocks as soon 

 as they are able to fly. While molting, in August, the young males are 

 quite conspicuous, the glossy black feathers of the new plumage being 

 scattered among the old brown feathers of the juvenal dress and giving 

 them a curious, mottled appearance. 



The enormous flocks are often quite spectacular as the great, black 

 clouds of birds swoop down into the fields to feed or pour into their 

 roosts at night, sometimes in association with other blackbirds or 

 starlings. Elon H. Eaton (1914) says: "The flocks of cowbirds found 

 during September in the grain fields and pastures are so large that on 

 one occasion after discharging my gun into a flock which was passing 

 I picked up 64 birds from the two discharges of the gun, which will 

 indicate the density of the flock. My estimate of the flock referred to 

 was that there were between 7,000 and 10,000 birds. The usual flock 

 in the fall, however, consists of from 50 to 200 birds." 



The fall migration gets under way in September, but is mainly 

 conducted during October, some individuals lingering well into 

 November. 



Winter. — A few cowbirds sometimes remain to spend the winter as 

 far north as Massachusetts and southern Ontario. During mild 

 winters considerable flocks sometimes spend the winter on Cape Cod, 



