The activities of songbirds, as illustrated here for the white-crowned sparrow, are the expression of several interrelating factors. Photosensitive cells in the bird's brain respond to lengthening daylight by stimulating the release of hormones. Hormonal changes cause physiological changes (above, in green) such as the development of reproductive organs. In turn, the physiological cycle affects the behavioral cycle (in purple). The events in these parallel cycles correlate predictably. The cycles begin to repeat at right, the beginning of a new reproductive year.
