Migration began in early September and is just passing the peak period with several millions of birds in flight every night. Migration is defined as a seasonal and predictable event that is repeated annually. Other types of movements for bird populations include dispersal, occurring when a bird leaves the place where it was raised to find a mate, and irruption, occurring when a group of birds leave a site because winter is so harsh they are forced to move to find food.
Migration requires that many species move hundreds of miles or more across and between continents. There are high risks involved, so why do birds leave a territory they have established a home in to go elsewhere for a portion of the year? Insect eating birds must move south in winter as food sources die off in significant numbers in northern regions when temperatures drop. Neotropical species require constant warmer temperatures. Many birds that spend the winter in equatorial belts move north in spring to take advantage of abundant summer food supplies and fewer nest predators including parasites. These birds must migrate south in autumn to remain in seasonally warm weather.
Many species fly at specific times during a twenty-four hour period. Thermals, upward moving columns of air, are created by the sun. Soaring birds including sandhill crane, golden eagle, and turkey vulture flying during daytime hours, depend on thermals to get airborne and remain aloft. Some daytime species are dependent on food sources that are only out in warmer, daytime air. For birds that do not ride the air currents, including blue jay and white-crowned sparrow across the northern United States, using powered flight generates much body heat, which is easily compensated for by cooler nighttime temperatures. Nighttime air is also less turbulent, making the flight smoother and requiring less energy.
Migration distance varies widely among species and even within one species. In winter in northern Illinois, we often see dark-eyed juncos, Junco hyemalis, on our backyard feeders. This species breeds in summer in northern Canadian regions. Studies show that young males migrate only a short distance to southern Canada, while adult females migrate the greatest distance to the southern U.S. Adult males and young females migrate several hundred miles to winter in north-central regions. Observations of young males show they are first to stake out new territories in the spring for breeding. Adult males are aggressive enough to establish a territory upon their return. Adult females can arrive later to find a mate with an already established territory. And young females are simply learning the routine until they grow older.
Winds at flying altitudes are the main factor in determining whether a day’s travel will be long or short and use lots of energy or only a little. Strong headwinds reduce flight speed by up to half and require more energy. Strong tailwinds speed the flyer along with less effort. Birds that fly in flocks have more advantages. As air flows around a bird in flight, it is deflected upwards from the wingtips. Birds flying in a “V” formation, such as Canada geese, use the air coming off their neighbors wings to make their flight easier and use up to 40% less energy. As you watch a formation, notice the lead bird gets no energy savings but changes position every few minutes with another bird, so each takes a turn at the leader’s position.
Research in recent years has contributed to many sites having online tools for monitoring and learning about all aspects of migration. For fun facts and up-to-date information, take a look at:
- Migration Dashboard (birdcast.info) shows migration information including dates, birds, counts, and routes by U.S. state or county.
- Bird Migration Explorer (explorer.audubon.org) from Audubon shows maps of current migration activity.
