Avian floodgates open

It starts with a trickle. One March day you notice a spot with open water on a city lake, then the next day several ring-necked ducks are swimming in it. A few days later an American robin "kuk-kuks" from the top of a bare maple tree, and somehow you know he's just arrived in town.

Bluebirds drop onto the top of nest boxes in late March, shivering through the first few weeks of spring, and red-winged blackbirds gargle "konk-a-ree" from snow-topped marsh grasses.

As spring advances, so does bird migration, with waterfowl in the vanguard, then progressing to solitary sandpipers and other shorebirds, a group few of us ever notice. Last of all are the songbirds, the species most of us associate with spring. Mid-April brings catbirds and rose-breasted grosbeaks, early May ushers in Baltimore orioles and their sweet whistled songs, then ruby-throated hummingbirds and many species of sparrows. By mid-May the woods are alive with the six-note song of the indigo bunting and the raspy notes of the scarlet tanager.

The river of migratory birds becomes a flood, including brightly colored warblers like the black-throated green, dropping down only briefly on their way to breeding areas in Canada's boreal forest.

New neighbors

I sometimes wonder what our resident birds, the ones we watch all winter, think about all this. Something like 35 species occupy our backyards, parks and open spaces in winter, but suddenly they're among 250 or more different kinds of birds hopping on the ground, perching in shrubs or exploring tree leaves for a meal.

Spring migration is a matter of exquisite timing, with arrivals targeted to food availability. Robins flood in just after ice leaves the soil and earthworms begin migrating up to the surface. Great blue herons drop down to lakeshores within days after ice-out. Yellow-bellied sapsuckers drill into trees just as sap begins flowing again. And warblers and other songbirds arrive as the first tiny leaves appear on deciduous trees. When leaves are small they're not yet full of the tree's natural toxins, so are ideal for billions of leaf-chewing insect larvae that hatch at this time. And songbirds snatch these caterpillars to nourish bodies depleted by the rigors of migration.

Right on time

Many species start and finish migration during a definite time range, so much so that ornithologists can fairly accurately predict their arrival dates. But because conditions along their migratory routes vary from year to year, birds' arrival times vary by several days from one year to the next. One example: Great blue herons appear around March 14 each year, give or take a day.

As Peter Berthold writes in "Bird Migration: A General Survey," migrants "cross all our oceans, deserts, mountains, and icefields … and there is not a single month in the year in which birds do not migrate somewhere over the Earth's surface."

Many of the birds we're used to seeing in our backyards exhibit a behavior called "nest site fidelity," meaning that if they survive the rigors of migration, they'll more than likely return to last year's nest site. Robins are champs in this regard: In one study, more than 70 percent of the birds banded on Vancouver Island, British Columbia, returned in subsequent years. (So, yes, in answer to a frequently asked question, that might well be last year's male robin singing from the top of the maple tree.)

Spring for birds and bird-watchers is an intense season, with a rush of activity compressed into a few short weeks. Keep your eyes on the skies, the trees and the ground to enjoy one of the most awe-inspiring spectacles the natural world has to offer.

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.