What Triggers Birds to Migrate?
By Marilyn Shy, Kalkaska Conservation District
The geese are honking and flying in V’s across the sky, robins are flocking up together, and Sandhill Cranes are gathering in groups in the fields. It’s something we observe every year at this time. In addition to the vibrant colors in the trees, we see these animals behaving in predictable ways across the landscape, as they have each fall for millions of years.
But what causes this behavior? What are the triggers that cause some birds---and some insects---to migrate?
Well, there are a number of factors involved, and some are quite obvious. For birds and butterflies that love insects and nectar, food resources are dwindling. The days are growing shorter and the temperatures are dropping. All of this cause changes inside of the bird’s body. Glands release special chemicals, called hormones, which influence the behavior of the birds causing them to fly restlessly and gather in flocks.
It also causes a feeding frenzy.
At my feeders, all of the birds seem to be packing it in, which includes the birds that will migrate as well as the ones that will stay around all winter. But I especially like to watch the migrants, the Rose-breasted Grosbeaks, the Ruby-throated Hummingbirds, and the occasional odd bird that shows up once for just a few minutes and then flies off. This year I had an unidentified warbler and a juvenile Baltimore Oriole land on one of the hummingbird feeders this fall, in an attempt to get a few sips of nectar before heading back out on their journeys to the south.
“Male Rose-breasted Grosbeak (left) and female (right), summer residents in our area, have already headed south for the winter.”
The birds, who seem to be spending every minute of the daylight hours stuffing themselves, are doing so in order to “fatten up.” This fat forms a layer underneath their skin which is converted into energy as the birds fly. Smaller species can gain 3-4 percent of their body weight per day during this time.
Once the migrants are loaded with food and ready to go, they wait for just the right weather conditions. A spell of calm weather with clear skies is usually the signal. Rainy, foggy, blustery weather can ground the birds, causing them to wait a few days until conditions improve.
Many migrating birds, but not all, fly at night, navigating in the dark. Why? Colder air temperatures reduce the dangers of overheating and allow the birds to fly further without having to stop and cool down. Equally important, dark skies tend to hold fewer predators. Unfortunately for the birds, migrating at night is riskier than it used to be. Lights on top of communication towers attract and disorient birds. It has been estimated that television, radio, and cell towers cause approximately 7 million bird collisions each year in North America.
Migratory birds can remember and return to the exact location where they were born. We know they navigate using the sun, stars, and earth’s magnetic field. But exactly how they do this remains a mystery. Scientists can now accurately track their travels by using tiny geolocator backpacks that transmit the birds’ locations.
The birds travel at the same speeds we usually do while driving. These range from 15-55 mph, depending on the species, wind speed, and air temperature. At these rates, migratory birds can fly 15-600 miles or more in a day!
The typical migratory bird may weigh a few ounces or less, and safely flies thousands of miles, braving storms and evading predators. Multiply this by several billion-----which is the number of birds that migrate annually-----and it’s hard to deny that this phenomenon is truly one of nature’s grandest miracles.