Photoperiodic Control of the Annual Cycle of Birds at Different Latitudes

Photoperiod is the major cue used by birds to time the stages of the annual cycle (breeding, moult and migration), but exactly how remains unclear. Birds at different latitudes will experience very different changes in photoperiod. If starlings, Sturnus vulgaris, are held under simulated ‘annual’ cycles in photoperiod that last between 3 and 24 months, or to cycles of different amplitude (equivalent to latitudes of 9° and 52°) the timing of gonadal maturation, regression and moult entrain to the imposed photoperiod. An advanced artificial summer solstice advances gonadal regression. These data suggest that neither a circannual clock nor the absolute length of photoperiods alone, play the major role in timing stages of the annual cycle. Birds may instead continually monitor change in photoperiod to estimate calendar time.