HOST RELATIONS OF PARASITIC COWBIRDS 77 



One Wyoming record (near Laramie) refers to the northwestern 

 form of the cowbird, artemisiae, while all the other records refer to 

 the nominate race of the parasite. In one case in Illinois, as many as 

 3 cowbird eggs were found in one nest with 6 eggs of the bluebird. 

 From Wisconsin and Missouri there are records of nests with 2 cow- 

 bird eggs each in addition to the rightful eggs. In all the remaining 

 cases there was but a single egg of the parasite. 



In her detailed study (1946) of the bluebird in Arkansas, Thomas 

 found one nest which was deserted because of cowbird parasitism. 

 This is the only instance of desertion by this host which has been 

 recorded definitely as such. 



Western Bluebird 



Sialia mexicana (Swainson) 



The western bluebird, subspecies bairdi, is in the present catalog 

 solely on the basis of its inclusion by Bendire in his list of victims of 

 the dwarf race of the cowbird. It is obviously a very rare fosterer; 

 Bendire gave no specific case, and none has come to attention else- 

 where. 



Mountain Bluebird 



Sialia currucoides (Bechstein) 



One record, a nest found by T. E. Randall, at Boyle, Alberta, 

 May 29, 1934, containing 4 eggs of the bluebird and 1 of the brown- 

 headed cowbird, subspecies artemisiae, first reported by me in an 

 earher paper (1938, p. 47), is still the only instance wherein this species 

 is known to have been used by the parasite. The absence of addi- 

 tional records is not due to a lack of frequency with which nests of 

 this bluebird have been found. In the files of the British Columbia 

 Nest Records Scheme there are data on 115 nests of this bird, not 

 one of which has been parasitized. In addition. Bent (1949, p. 288) 

 mentions 107 "egg dates," none of which apparently involve eggs of 

 the brown-headed cowbird. 



Blue-gray Gnatcatcher 



Polioptila caerulea (Linnaeus) 



The blue-gray gnatcatcher is not an uncommon host, and in some 

 areas it is even a fairly common victim. Two races, caerulea and 

 amoenissima, are known to be victimized, the former by two races 

 of the brown-headed cowbird, ater and ohscurus, the latter by artemisiae 

 and ohscurus. Due to a change in the accepted nomenclature of the 

 gnatcatchers, the name P.c. obscura, used in my 1929 book (p. 257) 

 for the western form, is now restricted to the race of Baja California; 

 the records formerly referred to this name now must be grouped 

 under P.c. amoenissima. All in all, some 39 records have come to 



