Fall migrations are in flightMigration is a phenomenon among many animals, but I’m focusing on birds because of the 338 Nearctic-Neotropical bird species that migrate into and out of the U.S. and Canada, 333 of those species move through Texas. Our state is in the middle of the Central Flyway, but also acts as a funnel for birds that use the Mississippi Flyway. Four other flyways exist–two along the Atlantic coast and two along the Pacific coast.
Nearctic-Neotropical includes birds that breed in the U.S. and Canada (temperate), and spend the winter in Mexico, and Central and South America (tropic). Actually, quite a few of the birds who migrate through Texas end up overwintering here. Migration occurs for a variety of reasons. Birds migrate to follow the food, to have access to more nesting sites, to take advantage of milder climates, as well as day length.
If the reasons above are true, then one would expect the birds could stay in the tropics and have all those things. However, what started as a way to avoid habitat invaded by the severe winters of the Ice Age has become a major factor in balancing ecosystems that experience seasonal fluctuations of food availability.
Migrating birds go to where food is most abundant in a particular season, particularly insect eaters. This is to take advantage of food surpluses that would occur if the birds were not present. You will not encounter many species of migratory birds in places where food availability is constant year-round.
Migration is both a learned and inherited trait. Short-distance migrants learn when and where to migrate from older birds. Long-distance migrants inherit the knowledge genetically. Over multiple journeys, these birds will incorporate learned experiences into their next migration.
Migration is such an ingrained instinct that birds born in captivity want to migrate. It’s called “Zugunruhe”, which means “migratory restlessness”. At the time their wild brethren begin migrating, these birds start fluttering their wings and face toward the direction they would fly. They stop when the actual migration ends.
Migration is triggered by changing barometric pressure and temperature. Cooler temperatures, along with cold fronts (high pressure) pushing south, tell the birds it’s time to move. The same thing happens in the spring when warm fronts (low pressure) begin moving north over the Gulf of Mexico. Also, depending on the season, days are either lengthening or shortening; and the sun is getting more or less intense.
About two to three weeks before the migration, the environmental changes cause the pituitary glands in the birds to begin the physical changes necessary for the journey. They begin to eat more, increasing their body weight, mostly fat, by ten percent a day.
The birds also have a large, rapidly beating heart to sustain the energy needed for long flight. They have two lungs plus air sacs. The lungs remain inflated at all times. The air sacs act as bellows to provide the lungs with a constant supply of fresh air. Migrations can take anywhere from two weeks to four months. Most birds, including solitary birds, will make the trek in flocks. They use landmarks to navigate, and can sense the earth’s magnetic field, using it like a compass. Birds that travel at night use the stars to get their bearing. The moon’s glare can actually disrupt them.
Most birds fly at night. Birds that soar and glide, like hawks, fly during the day to take advantage of the currents created by the sun’s heat. Birds that eat daytime flying insects also fly during the day. Birds also fly faster when migrating. The average airspeed ranges from fifteen to forty-five miles per hour.