The most basic driving force behind seasonal migration is not that different for birds than it is for humans. It is seasonal change, prompted by the transition from a warm and hospitable environment to a frigid state—at least for places like Minnesota—when plant life and much of animal life will soon be immobilized.

There is one major difference, of course. For birds, the habit of migration is a matter of ensuring the survival of their species. For human snow birds it’s a matter of comfort, though you could plausibly count snow shoveling-induced heart attacks, and broken hips due to falls on icy sidewalks, a threat to survival. Even so, for us it’s at worst a matter of personal survival, not survival of our species; our winter risks usually arise long after we have had our chance to pass on our genes to the next generation.

Only certain species of birds migrate between warm climates and northern destinations like Minnesota and Canada. I’ve seen varying estimates of how many of the world’s roughly 10,000 bird species migrate annually between nesting and wintering areas. The estimates I find most credible are between 15 and 20 percent; thus, roughly one in five bird species migrates annually.

Some birds live out their entire life in the tropics, subtropics or at least well to the south of Minnesota, We’ll only see them if we do the traveling to the places where they’re found. Others live in northern latitudes, like ours, also the whole year through. That may be the bigger mystery, considering how hostile to life Minnesota can be in January and February. Those that do so have evolved special abilities to find food, and plumage that has sufficient insulating properties to retain body heat when it is most vital.

So why haven’t birds evolved as stay-at-homes, staying where they are rather than braving the hazards they may face when flying distances which—for some species—are measured in thousands of miles? The answer is opportunity. This opportunity comes in part in the form of a spring explosion of insect and plant life, here where the land has been basically frozen solid for months. To arriving birds this means not only abundant food for themselves, but—so critical—for the nestlings they will soon be raising.

The other dimension of opportunity that northbound bird migrants find here is less competition for favorable nesting sites than they might have encountered to the south where they wintered. This makes sense, since the winter bird population in the North is much reduced due to limited winter food availability, and significantly fewer bird species well-adapted to survive here under deep-freeze conditions.

There is a fairly dependable sequence to bird departures southbound from Minnesota each fall. By now, mourning doves have begun to leave the state, which is why the hunting season for them begins Sept. 1 each year. Hummingbirds will also be departing in large numbers soon. Some of the 30-something species of warblers that can be found in Minnesota have already left us, and others will before very long.

Blue-wing teal and wood ducks are among the earliest ducks to leave Minnesota; blue-wings especially. Many believe hunting pressure plays a role in pushing these migration-ready birds out of the state in the early days of the season. For this reason some oppose the early youth waterfowl season, believing that some teal will be induced to leave before the general waterfowl hunting opener; this year on Saturday, Sept. 21.

Conversely, some of the latest waterfowl to make their migratory move through Minnesota are tundra swans and goldeneyes. The latter are often called “whistlers” by hunters, due to the distinctive sound made by a goldeneye’s rapidly beating wings in flight. I have heard and seen whistlers late in the duck hunting season when the temperature was single digits above zero, and the water surface took on an almost oily appearance as it neared the freezing point. By then most other duck species have long since departed. On the lake where the family cabin sits, swans seem to hang around until the last open water begins to close in November.

The robin, our all-American bird, often begins migration in early October. One of the heaviest such migrations ever recorded dates back to 1988, when migration watchers reportedly estimated some 60,000 moving through the Duluth and Lake Superior North Shore area in a single day. There are exceptions, and some robins stay here through the winter if they can find a dependable food source, typically wild or cultivated fruits.

But, as the insects and earthworms that robins depend on heavily become unavailable, most will migrate far enough south to find a dependable supply. Some go as far as Georgia, Texas or Florida. Some stop at destinations much closer to us. Some do one thing one winter, and the other the next. Thanks to this unpredictability, some label robins “wanderers” rather than migrators in the strictest sense.

Mid-September is also a peak time when raptors are moving southward. Duluth, at the lower end of Lake Superior, is a concentration point, since raptors in general prefer not to fly long distances over water. Broad-winged hawks, the much smaller kestrel, bald eagles and even some owls will be seen moving through now. One of the best locations, famous among birders the world over, is Hawk Ridge overlooking the city. In October, northern goshawks—some say their name is a contraction of “grouse hawk”—will come through in larger numbers.

In October, the lanky, bomber-sized, almost prehistoric-looking sandhill cranes begin migrating. Some duck species, like the canvasback, are thought to be more influenced by the calendar than by the weather; October is typically their time, too.

The most mysterious part of bird migration is “How do they do it?” Some say it’s the angle of the sun that helps orient birds, but birds migrate when it’s cloudy, and also at night. Some say it’s the earth’s magnetic field, combined with bird-brain circuitry that can use it to home in on a distant migratory destination.

Other say that—in significant part—it is behavior learned from others of the same species, using landmarks imprinted when a journey is first made. One proponent of this theory was H. Albert Hochbaum, a researcher at Canada’s famed Delta Marsh, who wrote the compelling book Travels and Traditions of Waterfowl.

One objection or another can be found for just about any explanation of bird migration. Some believe it’s a combination of more than one mechanism: learned, inherited or yet-to-be-discovered. Maybe that’s part of the mystique, that humans—who can unravel mysteries as big as the cosmos, and as small as the atom—still don’t have bird migration quite figured out.