HOST RELATIONS OF PARASITIC COWBIRDS 195 



tyrannus, with 57; the fourth is one of the domed-nest builders, 

 Furnarius rufus, with 38 instances; the fifth, Mimus triurus, with 28; 

 then Agelaius ruficapillus and Troglodytes musculus, with 15 records 

 each. The chingolo, Zonotrichia capensis, is, as far as present data 

 indicate, the primary host, with a very wide geographic range over 

 which it has been found to serve as a fosterer — Argentina, Chile, 

 Bohvia, and Brazil as far as Peru and Venezuela. In Chile, where 

 the shiny cowbird appears to be a fairly recent addition to the fauna, 

 the main host is the diuca finch, Diuca diuca. Goodall, Johnson, and 

 Philhppi (1946, p. 130) wrote that, of 100 nests found in Chile with 

 cowbird eggs in them, 72 were of the diuca finch. The domed-nest 

 builders other than Furnarius — -Anumbius, Synallaxis, Cranioleuca, 

 Phacellodomus , etc. — have only one or two kno\vn records of parasitism 

 apiece. In the case of the rufous ovenbird, Furnarius rufus, the one 

 species of domed-nest builder that is frequently parasitized, it ma^^ 

 be noted that, of 217 of its nests examined by me in Argentina, only 

 20 contained eggs or young of the shiny cowbird. In other words, 

 current use of domed nests as repositories for eggs of the shiny cow- 

 bird indicate that the nests are used less by the parasite than its 

 interest in such structures would suggest. 



In some areas, observers have credited other species as the most 

 frequently imposed upon hosts even though actual instances in suffi- 

 cient quantit}^ are not yet recorded. In British Guiana coastlands, 

 Young (1929) called the white-headed marsh tyrant, Arundinicola 

 leucocej)hala, the commonest victim of the shiny cowbird, whila others 

 have so characterized the wren, Troglodytes musculus. The last 

 named bird is stated by Haverschmidt (1955, p. 127) to be the princi- 

 pal host in Surinam. In response to an inquiry, Mr. Haverschmidt 

 has told me of at least 11 such records in 15 years in his own experience 

 in Surinam (and only a single record of another host species!). A 

 similar great use of this wren was reported also from Trinidad by 

 Herklots (1961, p. 222), who found as many as 3 eggs of the parasite 

 in individual nests of the small fosterer. This is certainly different 

 from the situation in Argentina. 



On the other hand, as Sick (1957, pp. 16-17) has noted, the 

 introduced and now wide-spread European house sparrow, Passer 

 domesticus, has not been "accepted" by the cowbirds as a host although 

 Sick has found, near Rio de Janeiro, mixed groups of the two species 

 sleeping together in small roosts. The fact that there is one instance 

 on record of the shiny cowbird's parasitizing the house sparrow does 

 not alter the general validity of Sick's comments. 



While no one has stated in so many words that the shiny cowbird 

 exhibits any tendency toward individual specificity in its choice of 

 hosts, the observations of Sick and Ottow (1958) in the vicinity of 



