Use of a circadian clock in photoperiodic time measurement is demonstrated in the tropical photoperiodic weaver bird with the help of resonance, ahemeral, and asymmetrical skeleton photoperiods. Different asymmetrical skeleton photoperiods and seasonal scotophase scans indicate (1) that light entrains endogenous circadian rhythms (ECR) of photosensitivity and the position of the photoinducible phase shifts according to the length of the basic photoperiod, (2) a seasonal variation in response to asymmetrical skeleton photoperiods, and (3) dissociation in the two gonadotrophins LH and FSH and a possibility of two distinct ECRs of photosensitivity for LH and FSH. Annual phasing of the ECRs of photosensitivity of the two gonadotrophins and/or interaction of hormones might be involved in the seasonal reproduction and photosensitivity of this bird.