The ducts

in the case of fishes and in biped and quadruped
ovipara are attached to the loin under the stomach and the gut, in
betwixt them and the great vein, from which ducts or blood-vessels
extend, one to each of the two testicles. And just as with fishes
the male sperm is found in the seminal ducts, and the ducts become
plainly visible at the rutting season and in some instances become
invisible after the season is passed, so also is it with the testicles
of birds; before the breeding season the organ is small in some
birds and quite invisible in others, but during the season the organ
in all cases is greatly enlarged. This phenomenon is remarkably
illustrated in the ring-dove and the partridge, so much so that some
people are actually of opinion that these birds are devoid of the
organ in the winter-time.