Migration Mysteries: Disappearing Neotropicals
By Dr. Jim Pease, Emeritus
Extension Wildlife Specialist (ISU-Retired)
Migration is the movement of animals from one place to another. We are all familiar with the migration of birds like the American robin that arrives in our backyards with the coming of spring. These birds have returned from the places where they spend the winter, to our area where they will nest and raise young birds.
People have been fascinated with this annual migration of birds for thousands of years. Aristotle was an ancient philosopher who wrote about the wintering habits of birds 3,000 years ago. He noticed that some birds traveled to warmer places to spend the winter. He also mistakenly believed that some birds like swallows hibernated to survive the harsh winter weather. This theory persisted for 2,000 years!
Today, we know that birds do not hibernate. But it does show how long people have been trying to understand the disappearance of many birds from northern climates in the fall. So what do we know now about migration? Where do the birds go? How? Why? Today, scientists know far more now about migration than they did even 25 years ago.
When you see flocks of birds flying overhead in the fall, they usually are flying south toward their wintering grounds. How far south they go depends on the type, or species of bird. Some birds travel farther than others. For example, in some species females and young birds fly farther south than males.
The largest group of birds that we see during migrations are called neotropical migrants. They got this name because these species of birds migrate in the fall all the way to Mexico, the Caribbean islands, and other Central American and South American countries in the tropics. This means these birds fly thousands of miles every fall and spring. About 300 of the 650 bird species that nest in North America are neotropical migrants. They include warblers, vireos, orioles, hummingbirds, swallows, swifts, shorebirds, and some birds of prey. The neotropical migrants make up 50-70 percent of the bird species of deciduous forests and prairies in the central and eastern United States.
Migration of birds through the United States follows some bird highways known as flyways. The four main flyways are the Pacific, Central, Mississippi, and Atlantic. These flyways run north and south. Many birds cross open ocean during their migration between North and South America. This means that birds need a lot of energy to migrate. This energy is stored in the form of body fat. Smaller birds can not store a lot of fat to use as energy during long flights. During migration, some birds lose as much as one fourth to one half of their entire body weight, so it is very important that they store up enough fat for energy. Just think how much weight you would lose if you lost half of your body weight! How smaller birds ever store enough to make these flights is still a wonder to scientists. It was once believed that little birds, like hummingbirds, migrated by riding on the backs of larger birds. However, this myth is not true. These little birds make it entirely on their own!
Scientists have been studying how birds find their way along these routes. To successfully migrate from wintering grounds to breeding grounds birds must be able to navigate (judge their position while traveling) and orient (determine compass direction). Birds do this by using a variety of different cues which allows them to find their way in different weather and habitat conditions. There are five main ways that birds navigate and orient themselves: 1) topographic features (things like mountains and rivers that can also influence wind direction), 2) stars, 3) sun, 4) earth's magnetic field, and 5) sense of smell.
Some birds need to stop to rest and feed during the day. This is when insects they eat are most active and available. These birds, then, migrate at night. They can find their way at night because they learn to follow the rotation of the stars. On cloudy nights, things like wind direction also help them to orient themselves. Other birds, like barn swallows, migrate during the day and feed on flying insects while they are in the air. That way, they are not limited to traveling at night because they can feed during flight.
When birds migrate is closely tied to why they migrate in the first place. Primarily, birds go south for the winter to find lots of insects and other food. However, these birds need more room and even more insects during the breeding season when they have a nest of young ones to feed. To solve this problem, the birds migrate north for the summer. As a result, when their bodies become ready to breed every spring birds know it is time to migrate north. In the fall, their body puts on fat and signals that it is time to begin their long journey south. Actual migration begins when the birds are triggered by some other stimulus, such as a change in temperature or weather.
So, as you can see, migration is a very complex behavior that has evolved over many years. But, we are losing many migrating birds, particularly the neotropical migrants. Does it have something to do with the dangers of migration or are we humans the culprits? Scientists have uncovered many clues, but so far the numbers of birds continue to drop at an alarming rate. Some birds in trouble are the scarlet tanager, Swainson's thrush, ovenbird, and black and white warbler. Read on for more clues in the case of the disappearing migrants. Weigh the evidence and decide for yourself "who dunnit?"
Sidebar: Why should we care?
Many neotropicals--like warblers, vireos, flycatchers, and swallows--are some of our best insect controllers, eating tons of insects annually.
Neotropical migrants--like thrushes, warblers, tanagers, and vireos--are among the most beautiful birds in the world, both in song and color.
Neotropical migrants may be a indicators of the health of our environment. If their populations continue to decline, our quality of life declines with them. It is in our own best interest, then, to try to reverse it.
Photo Credit: Red-winged Blackbirds - © Carl Kurtz, St. Anthony, Iowa