119 
The best way to become familiar with the anatomical conditions is to 
carefully cut up a rooster and a hen, because the sex is known in each 
case, and the organs are large enough to be easily observed. The sexual 
organs in all other birds are substantially the same except in size. 
The comparative size and development of the reproductive organs 
show the sexual maturity of the bird, as well as the season of reproduction, 
and should be noted on the label: “testes (or ovary) enlarged,” “some- 
what enlarged,” “slightly enlarged,” or “not enlarged.” In small species, 
this is still better indicated by an outline sketch of the true size and 
shape, or in large species by the measurements in millimetres. Any abnor- 
malities should be noted, as they often have an effect upon the plumage, 
or other secondary sexual characters. 
Finally, never fail to put a question mark after the sex mark on the 
label if there is the slightest doubt about it. 
Determining Age of Birds 
Some large birds do not attain their full development until their third 
or fourth year, and the age may be told from the condition of the 
plumage. Most birds, however, attain maturity the first spring after 
hatching, when they put on the first nuptial or adult spring plumage. As 
many adult birds moult into winter plumage, which is essentially the same 
Figure 44. Determining maturity of birds by the skull. 
as the first winter plumage of young birds, it is of interest to know the 
age of such birds when possible, marking on the label “ad.” (adult) and 
“juv.” (juvenile) or “im.” (immature) for any bird that is in its first year. 
In buying poultry it is possible to tell a young bird by the smoothness 
of the legs, or by feeling the posterior end of the sternum (breast bone), 
which ossifies late and has a cartilaginous rim in young fowls. The 
late C. Lehn Schipler, a Danish ornithologist, claimed to be able to tell the 
age of eider ducks to a month, up to the age of 4 or 5 years, by the 
development of the sternum, but very little is known about the detailed 
bony development of most species of birds, and the application of osteo- 
logical data is only practical to a limited extent in the field. 
