118 U. S. NATIONAL MUSEUM BULLETIN 211 



since they are usually separated by short intervals. At the beginning 

 of the main part, the throat is swelled, tail spread wider than usual, 

 and sometimes the wings are slightly opened; at the final note, the 

 breast is thrown forward, the neck stretched upward and the head 

 snapped back, so that the bill is pointing almost straight up." 



Field marks. — The adult male yellow-headed blackbird is too 

 conspicuously marked to be mistaken for anything else; the head, 

 neck, and upper breast are bright yellow, in marked contrast to the 

 black of the rest of the plumage, and the white patch in the wing 

 coverts shows plainly in flight and slightly when at rest; in fall the 

 yellow of the head is partially obscured by dusky tips. Females and 

 young males are dark brown, instead of black, with much dull yellow 

 or yellowish buff on the throat and chest, even in the juvenal plumage. 

 The females are always much smaller than the males, and have no 

 white in the wings. 



Yellowheads can sometimes be recognized at a considerable distance 

 in flight. Their flight is somewhat undulating, like that of redwings 

 and not like the straight-line flight of grackles; they differ from the 

 redwings in their flock formations, which are long, irregular, loose 

 flocks, like those of the grackles and not like the wide, company-front 

 flocks of the redwings; they can also be distinguished from the grackles 

 by their shorter tails. 



Enemies. — The eggs and young of the yellow-heads are preyed 

 upon by various forms of furred and feathered enemies. Small 

 mammals that can swim are likely to climb to the nests and rob them. 

 Crows, and perhaps grackles, sometimes steal the eggs or small young, 

 which are found in abundance in the colonies. The defensive response 

 in the colony to the appearance of a falcon, marsh hawk, or even a 

 harmless nighthawk or bittern, shows that almost any large bird is 

 regarded as a potential enemy, to be driven away by concerted action. 



The nests are not uncommonly invaded by cowbirds; Friedmann 

 (1929) cites several authentic cases in various parts of the bird's 

 range, and mentions one case in which six eggs of the cowbird and 

 four of the blackbird were found in a single nest. It would seem that 

 a young cowbird would have small chance of survival in the nest of a 

 species of this size; there seems to be no record of such survival. 



Dawson (1923) once found a large "blow snake" coiled just below a 

 nest full of young blackbirds. 



According to information given to Ammaiin (MS.) by Paul L. 

 Errington, he names the chief predators on 3 7 oung and adult j^ellow- 

 headed blackbirds, in the probable order of their importance, as 

 mink, great horned owl, marsh hawk, red fox, and muskrat. Said 

 Errington, "I would judge that the heaviest pressure by mink upon 

 the blackbirds occurs in late summer and early fall, probably to a 



