Bird migration is like an aerial rush hour

More birds are in the world right now than at any other time of the year.

Bird populations are their highest at the end of each summer, as millions of young birds, fledged just weeks ago, join their parents in the big, wide world. Soon, depending on their species, they'll face a big challenge — either joining the migrant stream heading south to winter homes or staying here to face the elements.

Their parents have just come off an arduous summer of energy-consuming nest building, egg tending and chick raising. And then they must molt new feathers so they're in the best aerodynamic shape for long flights — up to 2,500 miles in some cases. There's not much time to rest in the bird world, because as soon as they've replenished their energy stores, it's time to leave.

Autumn's abundance of berries and seeds are a boon to the nearly 5 billion North American songbirds looking to put on weight before their long flights. (At the same time, some 4 billion birds are beginning to move south from Canada into the United States to spend the winter.) This hints at the major element driving so many millions of birds to wing out of the north — the coming lack of food.

Birds whose diet is primarily made up of insects, like swallows, flycatchers, bluebirds and warblers, would starve to death if they remained in the north. These birds are the first to leave, in August and September, because their food supply runs low as fall approaches. Birds with a wider menu, capable of digesting seeds and fruits — this includes most sparrows — have the luxury of being able to wait many more weeks before taking off. This allows them to put on more weight for their arduous journey and to avoid dangers like hurricanes.

Think of diminutive hummingbirds and their migration schedule. The males, with their bright red throats, began migrating in late August into September, a time when females were still caring for young birds. After female hummingbirds replace their body fat and feathers depleted over the arduous summer, they join the migration stream in September into October. Young hummingbirds are the last to leave, on their own, relying on skills they learned from their mom and on innate knowledge wired into their brains.

Since songbirds don't travel as families, this year's newly minted young birds are going to have to rely on all their senses and an inner map to get them from Minnesota to places like Costa Rica. Day migrants, which include nearly all the hawks as well as geese and swans, can navigate more easily than those that travel at night. Most songbirds fall into this latter group and they use their ability to read the positions of the stars and the Earth's magnetic field to guide their flights.

We know so much more about migration now, compared with even 20 years ago, thanks to breakthroughs like satellite transmitters attached to birds, and tiny geolocators that reveal where they've been, as well as radar tracking that shows density and direction of big flocks.

Even though migration is one of the grandest spectacles in the natural world, for most of us it passes by with little notice. Sure, we look up when we hear Canada geese honking overhead in their long V's and might notice some unusual ducks on a local lake. But the majority of songbirds migrate at night, escaping the notice of all except early risers.

The movement of billions of birds in fall and spring goes on whether we know it or not. This might be the season to keep your eyes on the skies or to look more closely at small birds flitting around in the shrubbery. You might be looking at an intrepid traveler with thousands of miles still to go.

St. Paul resident Val Cunningham, who volunteers with the St. Paul Audubon Society and writes about nature for local, regional and national newspapers and magazines, can be reached at valwrites@comcast.net.

To get a sense for songbird migration, head outdoors after 10 p.m. or before 6 a.m. in autumn. You won't see migrants but you might hear them — sometimes they call repeatedly, especially on overcast nights. They're doing two things with those soft peeps: staying in touch with other migrants and avoiding collisions in the dark.