Figure 1.

The rate of testicular growth as a function of photoperiod for three species. Each point represents the initial rate of growth following transfer from a short photoperiod to each particular longer photoperiod. In Japanese quail, there is a rapid transition from non-stimulatory photoperiods to maximally stimulating photoperiods. In Gambel's white-crowned sparrow, which breeds at high latitudes, longer photoperiods are needed to induce testicular growth. Redrawn from data in Farner & Wilson (1957), Follett & Maung (1978) and Dawson (1989).