426 BULLETIN 203, UNITED STATES NATIONAL MUSEUM 



cessful fire prevention can actually extirpate Kirtland's warbler from 

 a given region — as happened recently in Alpena and Kalkaska 

 Counties. 



Leopold (1924, p. 57) was probably correct in considering the cow- 

 bird the most important enemy of Kirtland's warbler. As indicated, 

 a very large number of Kirtland nests are parasitized by the cowbird ; 

 frequently the competition is too great, and the warbler young do 

 not survive. However, there seems to be no reason to share Leopold's 

 fear that the cowbird "may soon" exterminate this warbler. It is 

 probable that the major changes that have been observed in the num- 

 ber of Kirtland's are the result of changes in the amount of suitable 

 habitat in Michigan or in the Bahamas. 



I have seen a female cowbird spend hours apparently watching a 

 female Kirtland building a nest. The nest was entirely unguarded 

 when the warbler was not actually working on it, and as soon as the 

 main structure was finished — even before the lining was added — a 

 cowbird (presumably the one that had been watching the nest-build- 

 ing) came early in the morning and laid in it. After watching many 

 hours at recently completed, or nearly completed, Kirtland nests, I 

 would judge that cowbirds laying in a Kirtland's nest during this 

 early part of the cycle, which is the period most favorable for the 

 cowbirds' chances of producing young, run very little risk of de- 

 tection and attack. But after the warbler has begun incubation, the 

 nest is rarely left unguarded, and the female warbler will attack 

 violently and drive away any cowbird she finds in the vicinity. 



The only predator I have actually observed to kill a Kirtland's 

 warbler is the sharp-shinned hawk (Accipiter striatus velox), al- 

 though the conditions were not natural in that instance, for the warbler 

 was in a trammel net. However, this hawk, as well as Cooper's hawk 

 {Accipiter cooperi) and, more frequently, the marsh hawk {Circus 

 cyaneus hvdsonius)^ occur regularly in the habitat of Kirtland's war- 

 bler, and they undoubtedly take an occasional warbler. Crows {Cor- 

 vus hrachyrhynchos) and blue jays {Cyanocitta cristata), common in 

 jack-pine areas, presumably rob the nests of this warbler as well as 

 of other birds. Red foxes {Vulpes fulva) , red squirrels {Sciurus hud- 

 sonicus) , spermophiles {Citellus tridecemlineatus) , and masked shrews 

 {Sorex cinereus) are potential mammal enemies that occur regularly 

 in the warbler habitat. I have some reason to attribute a few cases 

 of nest destruction to red squirrels or to spermophiles. Nesting fe- 

 male Kirtland's, under observation from a blind, have shown great 

 excitement at the approach of a shrew, and have attacked it and! 

 driven it away. 



