Daylight and hormones

The cardinals and chickadees in our neighborhood have been in spring song for at least three weeks. Woodpecker males are chasing females from tree to tree. This breeding behavior is triggered by daylight. It's called photoperiodism. There is a point at which a particular length of day stimulates development of the gonads, and migration in species that make such a journey. The testosterone and estrogens produced stimulate development of the reproductive system, the molt into breeding plumage for some species, singing, and other physical changes such as bill color. Aggressive behavior is another product of the gonadal hormones. Courtship displays lead to pair formation, nest building, and copulation. Some bird species winter north of the equator. Their trigger day is a day with more light. Birds that winter in the southern hemisphere, where days now are growing shorter, understand that days are lengthening here, and migration is at hand. They begin moving north. And in the fall when days are shortening, all of them move south. It could be that one precise amount of daylight, a threshold presented in both spring and fall in both hemispheres, stimulates this behavior. Except, some of our breeding birds begin moving south as early as July, when day length is synonymous with day length in January. Scientists are still working on understanding these behaviors.