34 SEX AND AGE DETERMINATION 



end of December the penis is not a reliable criterion of age in all in- 

 dividuals of a species. (The exceptions to this rule are the Mallard, 

 Black Duck, Pintail and Wood Duck in which the adult penis is as- 

 sumed by a few young males by the middle of November. The opening 

 of the bursa would then have to be relied upon as the age character.) 



Group 2. Species in which the juvenile penis persists in some or all 

 young males during their second autumn, and in which, therefore, the 

 presence of the juvenile penis, while indicating immaturity, does not 

 necessarily prove the specimen to be a young bird-of-the-year. This 

 group includes all the Geese and Swans, and probably all Ducks of the 

 following species: American and Barrow's Golden-eye, Buffle-head, Old- 

 squaw, Harlequin Duck, all Scoters and Eiders. 



The method of examination of the cloaca to disclose the penis char- 

 acter is given on the following pages. 



THE FEMALE OVIDUCT AS AN AGE CHARACTER 



The age of the female is told by the presence or absence of the 

 opening of the oviduct. 



The functional left oviduct of the female shows two stages of de- 

 velopment: (a) In adult females the opening of the oviduct shows as a 

 conspicuous slit on the left cloacal wall about midway between the lip 

 of the cloaca and the opening of the large intestine (fig. 29). (b) In 

 juvenile females the opening of the oviduct is blocked by membrane and 

 the left wall of the cloaca is unbroken. In those species which attain 

 maturity in their first year the oviduct membrane disappears before 

 they are one year old. Perforation of the membrane probably occurs as 

 a normal function of development towards breeding age. 



In considering the oviduct as an age character it is necessary to 

 class the birds in two groups: 



Group 7. Includes species in which the oviduct membrane of the ju- 

 venile female is known to disappear before the bird is one year old. 

 These are Mallard, Black Duck, Baldpate, Green-winged and Blue- 

 winged Teals, Shoveller, Pintail, Wood Duck, Canvas-back, Redhead and 

 Lesser Scaup, and probably the Mottled and Florida Ducks, European 

 Widgeon, European and Cinnamon Teals, Greater Scaup, and Ruddy 

 Duck. In these species the oviduct membrane has never been known to 

 disappear before the end of December. During the summer, autumn and 

 early winter, all these females therefore may be classed as juveniles or 

 adults by the presence or absence of the occluding membrane. 

 Group 2. Includes all of the Geese and Swans, and probably all Ducks 

 which do not attain maturity in their first year, namely, the American 

 and Barrow's Golden-eye Buffle-head, Old-squaw, Harlequin, Ducks, all 

 Scoters and Eiders. In these species the oviduct membrane is present 

 in some or all young females during their second autumn. Hence, while 

 the oviduct membrane is an evidence of immaturity it does not neces- 

 sarily prove the specimen to be a bird-of-the-year. 



