MORTALITY AMONG WARBLERS 



33 



MORTALITY AMONG WARBLERS 



The death-rate among North American Warblers is doubtless 

 higher than that which prevails in any other family of American 

 birds. Their nest mortality is probably above the average while 

 a variety of unfavorable conditions encountered during their 

 exceptionally extended migrations, often cause them to perish by 

 tens of thousands. 



A discussion of the comparative safety of terrestrial and 

 arboreal nesting-sites will be found under the head of the 'Nesting 

 Habits of Warblers', here I may simply enumerate the enemies of 

 Warblers while in the nest. Chief among them are foxes, skunks, 

 weasels, martens, opossums, squirrels, cats, snakes, crows, jays, 

 and, except among the more northern species, probably most fatal 

 of all, the Cowbird. Cowbirds' eggs have now been recorded from 

 the nests of no less than twenty-four species of North American 

 Warblers. These species are included in the appended list which 

 is based in the main on Bendire (Life Histories of North Ameri- 

 can Birds) : Black and White Warbler, Prothonotary Warbler, 

 Worm-eating Warbler, Blue-winged Warbler, Golden-winged 

 Warbler, Nashville Warbler, Lucy's Warbler, Northern Parula 

 Warbler, Yellow Warbler, Black-throated Blue Warbler, Cerulean 

 Warbler, Blackburnian Warbler, Chestnut-sided Warbler, Black- 

 throated Green Warbler, Golden-cheeked Warbler, Prairie Warb- 

 ler, Oven-bird, Northern Water-Thrush, Louisiana Water-Thrush, 

 Kentucky Warbler, Northern Yellow-throat, Chat, Hooded Warb- 

 ler, and Redstart. 



The Cowbird's habit of selecting as a host a bird smaller than 

 itself is doubtless responsible for this long list of victims. The 

 Warblers may build cunningly concealed nests upon the ground, 

 they may place them in the densest thickets, or in trees at a height 

 of over eighty feet, it is apparently all one to this bird, which, 

 never having had a home of its own, has formed no attachment for 

 any particular site. It is not unusual to find three Cowbird's eggs 

 in a single nest, and, in one instance, four are recorded. 



Only the Yellow Warbler appears habitually to avoid incu- 

 bating the intruded egg by building a second, and, should occasion 

 require, a third story to its home, and the fact that with other 

 species the unfortunate Warblers devote the nesting season to the 

 care of their foster children makes this form of persecution far 

 more serious than the loss of merely eggs, which may be replaced 

 by a second or third laying. 



