It is always a surprise to see birds starting to flock and begin to move south during the heat of the summer, but despite the hot temperatures during July, August and September, many of our southbound migrants are already starting to wander around and begin to drift south.

To the scientists who study the seasonal movements of birds over the year, bird migration is a fascinating, yet complex system. To the everyday person, the long and arduous distances that many species of birds travel twice a year seem unbelievable and difficult to fathom. Overall, the process of migration is best described as a seasonal movement of birds between two geographical areas. For example: birds that winter in the tropics of South and Central America move north to breed in the United States and Canada. Here they take advantage of a seasonally abundant and relatively untapped food source, which enables them to raise young while avoiding competition with the many resident birds in the tropics. But the harsh climate of a northern winter forces many birds south each year. It is this factor, plus the disappearance of the food supply that prevents many species remaining far to the north.

The distances that some birds travel may be great, as in the Arctic Tern, which travels from its breeding grounds within the Arctic Circle to its wintering grounds at the edge of the Antarctic pack ice. In contrast, migration may be as short as a few hundred meters down a mountain to escape frozen water and colder temperatures, as in the North American Clarks Nutcracker which breeds across the higher mountains of the western states and Canadian provinces.

Simple studies of these movements show us some very interesting facts. For instance the Ruby-throated Hummingbird makes a non-stop journey of up to 800 miles to cross the Gulf of Mexico. It then spends the winter from the Yucatan Peninsula in Mexico and Belize south to Panama.

But just how do these birds find their way across miles of unfamiliar land and water back and forth to their precise breeding and wintering grounds? Navigation is the most mysterious and seemingly complex aspect of bird migration, but we have discovered considerably more about this process in the last 25 years.&bsp; Birds are now known to find their way by using a variety of cues, including topographical features, the sun, moon and stars, wind, and the earths magnetic field. Rather than use a single method, they may use a combination of several to reach their final destination.

Here in western North Carolina, fall migration is almost upon us, and every day and each approaching weather system will bring in more birds.

Simon Thompson has lived in WNC for the past 16 years. He owns and operates his own birding tour company, Ventures Birding Tours. www.birdventures.com.

If you have birding questions, please drop Simon an e-mail at the above site.