migration mysteries - Kate Crowley
By Kate Crowley
Every year about this time my birding thoughts are focused on migration. I can’t help it. Everywhere I look; up, down and straight ahead I see birds in motion and I know that they are heading south. I admit to feeling a bit melancholy about this annual procession, because I know it’s going to be a long seven to eight months before we will see them again.
Just how birds make this biannual trip remains in some ways as much as a mystery as it was for our ancestors. We have developed different technologies that give us a great deal of information about the routes they follow, but just how they manage the journey continues to inspire researchers and amaze the rest of us.
We know that for millennia before humans walked the earth birds have been flying between North and South America. They are called neotropical migrants and we know that they come up from the south in the spring because the food sources in the northern regions are so overly abundant at that time (insects and flower nectar – known among scientists as a ‘seasonal protein pulse’). This is what most birds need in order to feed and raise their young. It is the decreasing daylight (photoperiod) combined with the decreasing food supply that causes the birds to leave each fall as well. It is not the cold that sends the birds south, that it reserved for the realm of human ‘snowbirds’.
In North America, the migrating birds follow four different ‘flyways’. You can think of these as bird highways. They are the Eastern seaboard, the Mississippi River, the Rocky Mountain Range and the West Coast. All are large natural/geologic features that funnel the birds toward their southern/winter ranges. Along flyways, wind currents can aid or inhibit flight. As it happens, the prevailing winds in the spring along the Mississippi River valley are from the south and in the fall they reverse and blow more often from the north and northwest.
We are in the Mississippi River flyway and nearly half of North America’s birds (more than 325 species), and about 40 percent of its waterfowl, spend at least part of their lives here. It is a vast region extending north to Canada’s tundra and boreal forest down to the mighty Mississippi through America’s heartland to the Gulf Coast. From there many birds will continue south, some as far as Patagonia at the tip of South America. The Arctic tern actually heads to parts of the Antarctic.
The Mississippi River historically was an ideal flyway, because it provided the critical resources needed by birds in transit – water first of all, and then a wide and varied habitat along its shores that provided food and shelter. Before the river was tamed for domestic use, vast floodplains were rich sources of food, and included vibrant grasslands, forests, and wetlands. We have done much to change the river’s freedom with locks, dams and levees and as a result have lost some of the richness in resources needed by the birds in migration.
The ‘how’ of migration is the hardest question to answer. So far scientists have concluded that some birds get compass information from the sun, the stars, and by sensing the earth’s magnetic field to orientate themselves in a particular direction, or as a map to help them determine their location. They also get information from the position of the setting sun and from landmarks seen during the day. There’s even evidence that sense of smell plays a role, at least for homing pigeons.
Whatever the ‘how’ and ‘why’ of migration, it is a perilous journey. The number of obstacles birds must avoid (cell towers, skyscrapers, the Viking football stadium, wind generators) grows in number each year and changes to the landscape – (new human developments and loss of wetlands and forests) add to the challenge. We who enjoy feeding the birds can help during migration. In the fall, many of the migrant songbirds switch from insects to fruit. As homeowners we can have a big impact by the way we landscape our yards, planting fruiting and flowering trees and shrubs that we are willing to share with the birds. And we can continue to fill our bird feeders with seeds and suet, giving us one last shot at seeing some of the summer visitors, even if they aren’t wearing their colorful breeding plumage any longer. Look up, look down and look into the trees on these beautiful early autumn days – you might be surprised by what you see.