72 U.S. NATIONAL MUSEUM BULLETIN 233 



about 500 nests — but he made no mention of a cowbird egg in any 

 of them. This apparent immunity to parasitism may be due partly 

 to the early start of the thrasher's breeding season; eggs have been 

 found as early as February 24, more than seven weeks before the 

 first cowbird eggs are laid in the same region. However, since from 

 late April until mid-July both birds are breeding, some as yet unknown 

 factor may keep them apart. 



Curve-billed Thrasher 



Toxostoma curvirostre (Swainson) 



The curve-billed thrasher is still known as a victim of the dwarf 

 race of the brown-headed cowbird on the strength only of information 

 given me (1929, p. 254) by the late R. D. Camp, who had found the 

 bird parasitized near Brownsville, Texas, sometime prior to 1924. 

 The local race of the thrasher is oberholseri. 



Sage Thrasher 



Oreoscopies montanus (Townsend) 



The sage thrasher has been recorded a single time as a victim of 

 the western race of the brown-headed cowbird, M.a. artemisiae. 

 In the smiimer of 1937, Twomey (1942, p. 456) found a parasitized 

 nest in the Uinta Basin, Utah. 



Robin 



Turdus migratorius Linnaeus 



This familiar bird is an uncommon victim. It is possible, however, 

 that the brown-headed cowbird may lay in robins' nests more often 

 than the records indicate; the robin's habit of throwing out strange 

 eggs necessarily would do away with the evidence. All in aU, I 

 have learned of only 26 records, ranging from Quebec, Connecticut, 

 New York, Maryland and the District of Columbia, to Ohio, Michigan, 

 Wisconsin, Iowa, Missom^i, North Dakota, Utah, and Alberta. 

 Three races of the robin are involved : propinquus in the Uinta Basin, 

 Utah (Twomey, 1942, p. 456) and Wahpeton, North Dakota (Jensen, 

 1918, p. 347); achrusterus in Marj'land with four records (Stewart and 

 Robbins, 1958, p. 329); and typical migratorius in the other listed 

 areas. Two forms of the parasite are involved: artemisiae in North 

 Dakota, Utah, and Alberta; and ater in all the other records. When 

 one considers that hundreds of robins' nests are found each year, it is 

 obvious that a total of merely 26 instances of cowbird parasitism 

 over man}^ years can mean only that the robin generally is unmolested 

 by the cowbird. To mention a single example of the evidence: in 

 the files of the British Columbia Nest Records Scheme there are 

 data on 486 robins' nests, not one of which has been parasitized. 



