108 U.S. NATIONAL MUSEUM BULLETIN 233 



parasitized nests. The Ontario records, two in number, were found 

 near Wasaga Beach, South Georgian Bay, by Dr. Paul Harrington. 

 All the records refer to the nominate race of both warbler and cowbird. 



Kirtland'B Warbler 



Dendroica kirtlandii (Baird) 



Bjrtland's warbler is a species, perhaps the only one, the survival 

 of which is seriously threatened by the parasitism of the cowbird. 

 As is well known, the breeding range of Kirtland's warbler is extremely 

 small, being limited to stands of small jack-pine in a few counties of 

 north-central Michigan ; the cowbird probably extended its range into 

 this area after 1875 and it has increased rapidly there ever since. 



The earliest record I have found involves a parasitized nest dis- 

 covered in Crawford County on May 31, 1908, by Strong (1919, 

 p. 181). Barrows (1921) in Iosco County found a parasitized nest 

 containing 1 egg of the warbler and 3 of the cowbird while a 4th 

 cowbird egg lay on the ground just outside the nest. Subsequently, 

 the warbler laid another egg, and 1 of the cowbird eggs disappeared. 

 Shortly thereafter. Barrows wrote me that his "correspondence with 

 several collectors indicates that bird is frequently victimized ; in fact, 

 I believe the cowbird is one of the most serious enemies of this species." 

 A. K. Fisher collected another heavily infested nest, with 1 egg of the 

 warbler and 4 of the cowbird, about 15 miles east of Grayling, on 

 June 13, 1923; the set is now in the U.S. National Museum. Leopold 

 (1924, p. 53), like Barrows, concluded that the brown-headed cowbird 

 was a major reason for the numerically low status of Kirtland's 

 warbler. 



The recent publication of Mayfield's meticulous and exhaustive 

 study (1960) of this warbler not only makes incomplete all earlier 

 statements about its relations with the cowbird, but also provides us 

 with the most critical evaluation of host-parasite relations we have 

 for any of the cowbirds' hosts. The interested reader should consult 

 Mayfield's book (especially pp. 144-181) for fuller details than can 

 be given here. 



Kirtland's warbler has this peculiarity as a cowbird host: it is 

 victimized very frequently, but the area in which it breeds is extremely 

 limited, with the result that a mere enumeration of the known in- 

 stances of parasitism means very little when compared with those of 

 a wide-ranging but less intensively imposed upon species. Further- 

 more, since the parasite is a recent invader in the breeding range of 

 this warbler, the latter has not been able to develop any effective 

 defenses against its new enemy. 



Mayfield reported that, of 137 complete sets of Kirtland's warbler 

 eggs, 75 sets, or 55 percent, contained 1 or more cowbird eggs. Up 



