120 
However, with practically all passerine birds (perching birds, the 
ordinary song birds and their relatives) it can be determined from the skull 
whether the specimen is a bird of the year or not. As the bird’s skull 
goes back into the skin again, it is obvious that the examination of the 
skull must be done by the collector in the field. 
In the nestling the bony roof of the brain case is very thin and trans- 
parent, but in the adult bird it is more opaque or nearly white, being formed 
of two layers of bone separated by air spaces. When an adult skull is 
held up to the light all parts of the roof of the skull show fine dark 
specks. As the young bird develops, the transparent area becomes smaller 
and the dotted structure crowds in on its edges until after 3 months 
there is only a relatively small clear area, where a single layer of bone 
persists (Figure 44). This is usually near the middle, back of the eye, 
but in some families of birds, as in the swallows, it may be farther back. 
After about 6 months this sign of immaturity is lost. A very few excep- 
tional species of Passeres retain a thin area in the skull throughout life, 
and woodpeckers and some other families have a different development. 
Information about this character of the skull is desired for all species 
of birds, and the collector should always note it on the label. The writer 
puts down “skull clear,” denoting immaturity, or “skull granulated,” 
denoting adults; others write “s. n. o.” (skull not ossified), or “s. o.” 
(skull ossified). The science of ornithology is becoming more exact and 
technical every year, and the student of bird skins wants his data where 
it can be readily found, and not buried in some absent collector’s notebook. 
It is not desirable to increase the size of the label, but if there is not room 
enough for the accessory information demanded, tie on a second label. 
Stomach Contents 
The economic value of a species largely depends upon its food habits, 
so the crop and stomach of a skinned bird should always be examined, 
and preferably preserved. A field investigation can seldom be complete, 
as the average ornithologist is seldom able to name all the other animals, 
insects, seeds, or fruits that have been eaten by the bird. If a bird of 
prey has been eating other birds or mammals, the material in the crop can 
often be determined at sight, and a rough estimate may perhaps 
be made of the insect or vegetable contents of the stomachs of other 
birds. It is advisable to make a routine practice of removing crop and 
stomach, tying both ends, labelling with the number of the bird specimen, 
and putting it into a bottle or friction-top can with alcohol or formalin. 
It is important that a crop or gullet containing food be kept with the 
associated stomach. A season’s collection of small bird stomachs can usually 
be contained in a quart fruit-jar or can. The contents of stomachs of 
small mammals are usually much more difficult to determine than those 
of birds’ stomachs, as most mammals have teeth and chew their food 
before swallowing. Mammal remains can be identified by the presence of 
teeth, feet, claws, and hair, and birds by feathers, feet, claws, and bills. 
Considerable information has been given by Dixon (1925) and Hamilton 
(1930), and much more attention should be given the subject by both 
ornithologists and mammalogists. (See also “Directions for preservation 
and care of material collected for food habits studies”; U.S. Biol. Surv., 
Wildlife Research and Management Leaflet BS-29, Dec. 1935.) 
