134 Barrows on Birds of the Lower Uruguay. [July 
times. In general habits it is precisely similar to M. ater , and 
the eggs of the two birds are scarcely distinguishable. I am 
inclined to think that the Argentine bird differs less from our own 
in its parasitic habits than is generally supposed. Its great 
abundance and the comparative openness of the country will in 
great measure account for the larger number of eggs found as 
well as for the numbers sometimes observed in single nests. The 
largest number I ever found in any nest at one time is four, but 
not very rarely twice that number is found, as witnessed by many 
reliable observers. Of course this overdoes the matter so as to 
compel the rightful owner to desert the nest, but I suspect our 
own Cowbird would be no wiser under similar pressure. 
40. Molothrus rufo-axillaris Cass. — Much less common 
than the preceding, and usually met with only in the woods or 
close to them. Here little parties of ten or a dozen may be 
found at any season. Although so common, I was not able to 
satisfy myself by observation either that they did or did not build 
nests for themselves or impose on other birds. I have frequently 
seen them in small parties clambering about the bulky nests of dif- 
ferent species of Synallaxis , and occasionally even entering such 
nests as if they owned them ; but examination afterward never re- 
vealed eggs of any kind in the nests, so that I was at a loss to 
account for the proceeding. Twice I have found in the nests of 
Zonotrichia pileata a good sized white egg (in one case spotted, 
in the other immaculate) differing in size, shape, and color from 
the average egg of M. bonariensis , yet I am not ready to say 
that these were not abnormal specimens laid by that species. 
As I have never seen the eggs of M. rufo-axillaris I can of 
course draw no conclusions in that direction, but suspicion 
naturally falls on any member of a disreputable family who has 
not proved an alibi. 
41. Ageleeus thilius ( Mol .). — Abundant in flocks through 
the year, but the flocks seldom consist of more than fifty individ- 
uals, oftener of only a dozen. Breeds in the marshes during 
October and November, but rather sparingly ; probably the ma- 
jority breed farther south. The male in spring has a low but 
sweet and varied song. 
42. Xanthosomus flavus ( Gm .). — During the first year 
spent at Concepcion this species was not observed, but in Octo- 
ber, 1880, it made its appearance in flocks and remained to 
