HOST RELATIONS OF PARASITIC COWBIRDS 73 



Many years ago I conducted some experiments to determine whether 

 the robin distinguished foreign eggs from its own by differences in 

 color or size or both, and it was fomid that color was the most decisive 

 factor. Chipping sparrow eggs, which are even smaller than cow- 

 bird eggs, were accepted by the robins, in whose nests sparrow eggs 

 were inserted along with their own, whereas larger eggs, more nearly 

 the same size as robin eggs but dissimilar in color, were rejected. 

 Nice (1941) made similar tests, using house sparrow eggs, and found 

 that the majority were rejected (although some were accepted). 

 Howell (1942, p. 560) independently tried similar experiments and 

 found that alien eggs were removed but that an addled robin egg was 

 accepted. Although cowbird eggs usually are not tolerated by 

 robins, the latter will accept and rear very young nestlings of the 

 parasite. On one occasion I put two very young cowbirds in a 

 robin's nest which at the time contained only eggs. The robins took 

 care of and reared the young cowbirds. An early case of a robin 

 tolerating only the parasitic eggs was mentioned by Walton (1879, 

 p. 78), who recorded an instance wherein the female robin was found 

 sitting on a nest containing a cowbird's egg in addition to three 

 of her own. 



The robin's relative immunity from parasitism is due not solely 

 to its ejection of the cowbird eggs; Leathers (1956, p. 68) observed 

 an incubating robin vigorously attacking and driving from its nest 

 an intruding female cowbird. 



Wood Thru8h 



Hylocichla mustelina (Gmelin) 



The wood thrush is a frequent victim of the brown-headed cowbird. 

 Although it is not the commonest host in any one area, in some regions 

 half or more of the nests of this bird are parasitized. The thrush is 

 not as abundant as some of the hosts which outrank it in these areas, 

 such as the song sparrow, the red-eyed vkeo, or the yellow warbler. 

 Over 75 records have come to my notice, ranging from Ontario, New 

 York, Massachusetts, Rhode Island, Connecticut, New Jersey, 

 Pennsylvania, Delaware, Maryland, Vu-ginia, and West Virginia to 

 Ohio, Indiana, Michigan, Mmnesota, Wisconsin, Iowa, Illuiois, 

 Missouri, Kansas, Tennessee, Arkansas, Oklahoma, and Texas. The 

 typical eastern race of the cowbird is involved in all the records 

 except one from the vicinity of Houston, Texas, where the breeding 

 form of the parasite is the subspecies obscurus. 



When I fu'st compiled the data on this host (1929, pp. 257-258), I 

 noted that Iowa seemed to be the region of greatest parasitism for 

 the species; two-fifths of aU the records came from that one state. 

 I assumed that, as more records were published, this ratio would 



