Although many environmental cues influence reproductive activity, the seasonal breeder responds most strongly to long day length. THE MALE BIRD: Testicular interstitial cells secrete
testosterone, which influences reproductive behavior such as territorial aggression and song. Other changes observed in seasonal breeders include testicular
hypertrophy and enlargement of the ductus deferens and seminal glomus. THE FEMALE BIRD: Early changes associated with rising
estrogen levels in the hen include osteomyelosclerosis and
hypercalcemia. Ovulation is then induced by LH, which is followed by eggshell calcification, which is under the control of
progesterone. Sources of
calcium for shell production include intestinal absorption from the diet, renal control of
calcium levels, and mobilization of bone
calcium stores. During oviposition,
PGF2 alpha and
vasotocin stimulate powerful uterine contractions [32] in the presence of
calcium. Incubation is associated with falling LH levels and rising
prolactin levels. If the hen actually enters reproductive quiescence at this time, then molt will follow. Molt is associated with the total regression of the reproductive tract.