Internal Annual Clocks and Returning Birds in the Spring

Dr. Douglas Robinson - Bird Migration

In the course of a single year, nearly the Earth's birds will migrate some distance at some time. During the spring in northern temperate regions, most of the birds that actually breed "here" will return from points South as the ice and snow melt in the warming sun of the spring. Some species, such as the American Robin, only flew to the southern Atlantic states; other species, including the Sandhill Crane, wintered along the coast of the Gulf of Mexico, while still others, including our brightest and best singers, the warblers, flew to the tropics and the southern hemisphere.

For species wintering along the equator, internal annual clocks promote increased restlessness and the signal to return to the northern breeding grounds. For other species, the urge to return is cued by increasing day length. Even longer day lengths after they arrive will stimulate production of hormones that trigger the growth of reproductive organs. But migrants may stop south of their final destination and then for the final push, use temperature, rainfall, atmospheric conditions, and food abundance to "fine tune" their arrival to their summer homes.

It is sensitivity to warming conditions, and perhaps earlier food abundance, that seem to be resulting in earlier and earlier spring arrivals of some species as global temperatures increase. The ability to time the arrival on breeding grounds has been strongly selected, as those who made mistakes or relied on the "wrong" cue paid with fewer offspring or even their own deaths. Look for those success stories of species to arrive in your yard in the near future!

After listening to Dr. Robinson's "academic minute" on the radio, it certainly reaffirmed what I was taught in biology during my freshmen year of college. The internal clocks birds have signal that it is time to return north, and others respond to the increase in day length. All of these sensitivities to outward stimuli and internal stimuli are fascinating for birds to be capable of doing- migrating back to locations many miles away at around the same time each season. As spring is approaching, I am indeed noticing the sounds of birds I have not heard all winter long. This must mean these particular birds have returned from the south already, and spring is on its way!

These instinctive clocks and responses to stimuli would be interesting to further research. Perhaps, as a class, my students and I could research the various migration patterns of birds in our area and wait for their arrival in the spring. These would be interesting projects to conduct in the classroom, and involve research by students while still incorporating the natural resources and nature surrounding our classroom. This project could then be possibly extended to research the migration patterns of other animals, such as sea turtles, who swim many miles each year to return to the beaches to lay their eggs.