194 U.S. NATIONAL MUSEUM BULLETIN 233 



probably be those of the younger birds. Then too, by the time the 

 year-old birds were ready to begin laying, all the district would have 

 been already staked out into territories by the adults. From this it 

 follows that there would be a tendency for the yearling birds to con- 

 gregate in whatever places were left. Also most small birds would be 

 through breeding and their nests either destroyed by the elements or 

 hard to find as the Cowbirds usually find such nests by watching the 

 birds coming and going. ... if the year old Cowbirds found large 

 nests it would not be surprising if many of them would lay in each one 

 and not only once at that. It should be remembered that of 217 

 Ovenbirds' nests found early in the season (September through De- 

 cember), only 20 contained Cowbirds' eggs, so that these are not [so 

 greatly] favored by the Molothrus when they can avail themselves of 

 nests of smaller birds, as Brachyspiza, Muscivora, etc. I think that 

 the large clutches of 25 and 37 eggs are the product of year-old birds 

 without territories . . . ." It may be added that Miller estimated, 

 from the appearance of the eggs in a nest containing 25 of them, that 

 at least 12 hen cowbirds were responsible. It may also be mentioned 

 that a fair number of these late-breeding males still showed some of the 

 Juvenal remiges; thus they were clearly year-old birds. The plumage 

 of the females unfortunately is of a kind as to give no opportunity for 

 such revealing criteria of age. 



The pertinence of the above is, as already stated, merely suggestive. 

 The younger buds show an active interest in these domed nests and 

 less so in open ones. Since atavistic traits and tendencies seem to 

 crop up more often in younger than in older birds, these factors may 

 be operative here. It must be admitted that these late breeding, year- 

 old birds also may occasionally use open nests for their wasteful 

 multiple ovulation. Nests of the yellow-breasted marsh bird, Pseudo- 

 leistes virescens, have been reported with from 10 to 17 cowbird eggs 

 in them. One nest of a mockingbu-d, Mimus saturninus modulator, 

 with 14 cowbird eggs, the product of at least seven hens, was found 

 very late in the season (February 12). Ottow (in litt.) studied 14 

 cowbird eggs, found on January 10 in another nest of this mockingbird, 

 and attributed them to 14 different hen cowbirds. Nests of Leistes 

 militaris supercUiaris also have been reported with as many as 19 

 cowbird eggs in them. 



That the interest in domed nests is more atavistic than currently 

 useful is indicated by the frequency with which the various kinds of 

 hosts actually are chosen. The data in our present host catalog 

 involve between 825 and 900 parasitized nests; of these, 165 were nests 

 of Zonotrichia capensis, no less than 10 races of which species are known 

 to be parasitized. The second most frequent, although geographically 

 restricted, host is Diuca diuca, with 80 records; the third, Muscivora 



