Reproduction in most species of animals occurs during that season of the year when conditions for raising young are optimal. This study explores physiological control of seasonal reproduction in the American tree sparrow by focusing on the mechanisms that regulate secretion of an essential hormone of reproduction ?luteinizing hormone) (LH)!. Previous work has established that LH secretion increases when male tree sparrows are transferred from a short to a long daily photoperiod (for example, from 8 hr. of light per day to 20 hrs. of light per day). Two mechanisms control LH secretion: one is analogous to a throttle and drives LH secretion; the other is analogous to a brake and retards LH secretion. A long daily photoperiod is stimulatory because it augments the drive on LH secretion, rather than in the brake, and largely determines when the reproductive season begins and ends in birds exposed chronically to long daily photoperiods. The mechanisms that decrease the drive on hormone secretion or modulate the brake to terminate or control the transition from the non-breeding to the breeding state, respectively, are not understood. This investigation specifically explores (a) whether the drive on LH secretion in male tree sparrows exposed to short daily photoperiods is daylength-dependent; (b) whether the eyes are components of the mechanism that decreases the drive on LH secretion at the end of the breeding season and modulates the brake on LH secretion; and (c) the relationship between restoring the drive on LH secretion and intensifying the brake. The results will clarify general physiological principles underlying initiation and termination of seasonal reproduction and suggest improved management strategies for domestic and /or wild (including game and pest) species that breed seasonally.
