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We studied the detailed pattern of seasonality in reproduction and the associated functions in the females of migratory yellow breasted bunting (Emberiza aureola) and resident tree sparrow (Passer montanus) under natural and artificial photoperiods. They exhibited seasonal cycles of ovarian development, moult and bill colour under natural photoperiods which were found linked to the annual solar cycle. Photosensitive birds of both the species, when exposed to 9L/15D, 12L/12D and 14L/10D for 18 months, showed follicular growth followed by regression and development of photorefractoriness only under artificial long photoperiods (12L/12D and 14L/10D). Fattening and body weight increase were noticed only in bunting exposed to long natural and artificial photoperiods. Both species exhibited complete postnuptial moult of body and primary feathers under long natural and artificial photoperiods (except primaries moult under 12L/12D in bunting) that progressed with gonadal regression suggesting their photoperiodic control. In addition, bunting also showed an incomplete prenuptial moult of body feathers during gonadal stimulation. Thus, the moult and body weight responses of bunting differed significantly when compared with those of sparrows. Exposure of photosensitive birds to 24 h light-dark cycles with increasing photophase suggested that the threshold photoperiods of about 12 h in bunting and about 11 h in sparrow initiate follicular growth exceeding which the rate of growth increases with increasing photoperiods. However, the threshold photoperiod was found to be response specific in bunting.

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