EASTERN BROWN-HEADED COWBIRD 433 



The temperament of the host species is also of importance; the 

 hawklike character of the shrikes makes them absoultely free from 

 cowbird molestation; and the pugnacious kingbirds are seldom 

 imposed upon. Birds nesting in holes are mostly free from cowbird 

 interference; Friedmann (1929) says that woodpeckers, house wrens, 

 nuthatches, chickadees and bluebirds are "very seldom molested, in 

 fact the Bluebird is the only one of these birds for which I have found 

 more than a very few records." Some birds are intolerant of cowbirds' 

 eggs; He mentions the robin, catbird, and yellow-breasted chat as 

 "examples of absolutely intolerant species. Others such as the 

 Yellow Warbler are intolerant to a certain extent." I should except 

 the catbird, as I once found a catbird sitting calmly on a nest that 

 contained four eggs of the cowbird and only one of its own! Fried- 

 mann adds: "Birds react to Cowbirds' eggs in several ways. The 

 great majority of species seem not to mind the strange eggs in the 

 least and accept, incubate and hatch them. Of these birds, some 

 occasionally cover over the parasitic eggs by building a new floor over 

 them if they have no eggs of their own at the time. This is true of 

 such birds as the Red-eyed, Warbling, Blue-headed and Yellow- 

 throated Vireos, the Prothonotary, Yellow and Chestnut-sided War- 

 blers and the Redstart. This has also been recorded in the following 

 species — Meadowlark, White-crowned Sparrow, Cardinal and Indigo 

 Bunting, but only a single time in each case." 



He conducted experiments with a robin's nest and a catbird's nest to 

 learn what the reaction of these birds to strange eggs would be. The 

 robin threw out a song sparrow's egg, which was spotted much like a 

 cowbird's, but accepted a chipping sparrow's, which was mainly blue 

 like the robin's, though much smaller. The catbird ejected both the 

 song sparrow's and the chipping sparrow's eggs. Of the yellow- 

 breasted chat, he says: "The eggs of the Chat are very similar to those 

 of the Cowbird, but nevertheless the nest is almost invariably deserted 

 if a parasitic egg is laid in it. This is doubtless due to the extreme 

 shyness and nervousness of the Chat, rather than to any superior 

 ability to distinguish the strange eggs from those of its own. * * * 

 Nevertheless on at least two occasions Chats have hatched and reared 

 Cowbirds." Some other birds, perhaps more than we know about, 

 eject the alien eggs from the nests, B. H. Warren (1890) says: "I 

 have twice found broken eggs of Cowbirds on the ground near nests of 

 the Yellow-breasted Chat, and on three occasions have discovered the 

 shattered remains of these eggs directly beneath the pendant nests of 

 Baltimore Orioles." 



Among the birds that show their intolerance by burying the cowbird's 

 egg in the bottom of the nest, or by building a second story over it, 

 so that the alien egg fails to hatch, the yellow warbler is the star 



