Chapter X: Section III: ASCERTAINING THE SEX OF BIRDS.--Although the sex of many

birds can be ascertained with tolerable certainty by the plumage, yet this is never an infallible guide, and to make perfectly sure of every case the internal organs should be examined. I always advise dissecting such plainly-marked birds as scarlet tanagers or red-winged blackbirds, and by practising this habit I was once fortunate enough to discover a female painted bunting in full male livery. The sex of birds can be readily ascertained in the following manner: Lay the bird's body on its left side, with the head from you; then with a knife or scissors, cut through the ribs and abdominal walls on the _right_ side; then raise the intestines, and the organs will appear.

In males, two bodies, the testicles, more or less spherical, will be seen lying just below the lungs on the upper portion of the kidneys (Fig. 6, 3, 3). These vary not only in color from white to black, but also in size, depending upon the season or age of the specimen. Thus, in an adult song sparrow, during the beginning of the breeding season, the testicles will be nearly or quite a half inch in diameter, whereas in autumn they will not exceed a number eight shot in size; and in nestlings of the same species they are not larger than a small pellet of dust-shot. At this early age, the sex of birds which have become somewhat soft is quite difficult to determine, and the same is true at any season if the specimens be badly shot up. There are other organs, however, in the male. For example, the sperm ducts are always present, appearing like two white lines; and in the breeding season the plexus of nerves and arteries about the vent becomes swollen, forming two prominent tubercles on either side (Fig. 6, 3, 3).

In the female the ovaries lie on the right side (Fig. 7, 2) in about the same position as is occupied by the testicles in the male. The ovaries vary in size from that of half the size of an egg to minute points, depending, as in the male, on the season of the year and age of the specimen. In very young birds the ovaries consist of a small white body which under a magnifying glass appears somewhat granular. In both male and female are two yellowish or whitish bodies, in the former sex lying above the testicles, but further forward, and consequently just in front of the kidneys; and in the female they occupy about the same position. In addition to the ovaries in the female, the oviduct is always present (Fig. 7, 3), large, swollen, and convoluted during the breeding season, but smaller and nearly straight at other times. In young specimens it appears as a small white line.

The denuded breast and abdomen seen in birds during the breeding season, cannot always be depended upon as a mark of sex, as this occasionally occurs in males as well as in females.