TESTICULAR ASYMMETRY AND SEX RATIO IX BIRDS. 1 99 



the testes equal in size. The other three Molothrus bonariensis, 

 Molothrus ater, and Tangavius xneus, have sexual dimorphism in 

 plumage, are more or less promiscuous with a tendency towards 

 polyandry, and the males in all three outnumber the females by 

 at least three to two, or by not less than fifty per cent. In these 

 three species the left testis is much larger than the right. It is 

 interesting to note that of the five the first two (with equal testes 

 in breeding adult males) are the most primitive species of Cow- 

 birds in all respects. Agelaioides badius is more or less noimal in 

 its reproductive habits but all the others are parasitic, i.e., have 

 no nests of their own and lay their eggs in nests of other birds and 

 leave them to be cared for by these foster-parents. The develop- 

 ment of this habit allowed for an increase of males in proportion 

 to females as true mating and pairing were probably of less 

 importance to a parasitic species than to one tied down by 

 nesting and parental obligations. The parasitic habit is simplest 

 in Molothrus rufoaxillaris and this species still pairs off in its 

 ancestral monogamous fashion as does the most primitive, non- 

 parasitic Agelaioides badius. 



In most birds studied however no correlation between testicular 

 asymmetry and sex ratio can be made out. Many more data are 

 needed and I hope that bird collectors will make a point of noting 

 the relative size of the testes in all breeding adult males. 



BIRDS IN WHICH THE Two TESTES ARE EQUAL IN BREEDING 



ADULT MALES. 



Family Ciconiidx. 

 Euxenura galatea (Molina). 



Family Tinamidx. 

 Nothura maculosa maculosa (Temminck). 



Family Rallidx. 

 Gallinula chloropus brachyptera (Brehm). 



Family Charadriidx. 

 Charadrius pecuarius pecuarius Temminck. 



