198 HERBERT FRIEDMANN. 



male birds in breeding condition collected during three expe- 

 ditions to South America, Africa, and the Texan- Mexican border. 

 Only adult males in full breeding condition were of value in this 

 study for in all the birds examined not in breeding condition the 

 testes were invariably equal in size and very small. 



In no species was the right testis larger than the left. The 

 two were equal in size in one hundred and four species and the 

 left testis was larger than the right (usually by a very considerable 

 amount) in sixty species. Five species showed both conditions. 

 This is hardly in keeping with Newton's statement quoted above. 

 It should be borne in mind that many of the species are included 

 in the following lists on the basis of the condition found in a single 

 adult male. With further data and the elimination of individual 

 variation the status of some of the species may require modifi- 

 cation in this respect. 



From my own field notes on all these birds and from all 

 available published accounts of other observers it seems that no 

 general correlation exists between the relative size of the testes of 

 the breeding male and the sex ratio of the species. That is, field 

 observations do not indicate a larger proportion of males in those 

 species exhibiting testicular size assymetry than in those in which 

 the two testes are equal. However it must be admitted that sex 

 ratio is a very difficult thing to determine in the field, particularly 

 as male birds are usually more conspicuous than females and are 

 therefore seen and collected more frequently. In some small 

 groups of species there does seem to be some such correlation as 

 Riddle found in his pigeons. Perhaps the most striking case (and 

 one on which I have sufficient material to eliminate individual 

 variation) is that presented by the Cowbirds. 



The Cowbirds are a group comprising three genera and six 

 species. One 2 of these six species is extremely rare, being 

 represented in the museums of the world by only one adult and 

 three immature specimens and I have no data on it. The 

 condition in the other five is interesting and suggestive. Two of 

 them, Agelaioides badius and Molothrus rufo-axillaris, have no 

 sexual dimorphism in plumage, are monogamous, and the ratio of 

 the sexes is even, one male to one female. These two species have 



2 Tangavius armenti. 



