Walking along a river trail recently, some birding friends and I stopped to stare in astonishment at an unusual sight, a hummingbird flitting near the base of a big oak. Almost at ground level, she jabbed her beak at the heads of dandelions gone to seed, and we realized she was gathering their fluff to line her nest.

Later that same morning, a female Baltimore oriole flew low over our heads and disappeared into a cottonwood tree's leaves. As she flew back and forth several times, we recognized what she was up to — weaving plant strips to make a neat, hanging nest at the tip of a twig. (Baltimore orioles make one of the most intricate nests in the bird world — there are videos on You Tube showing how much work and skill these take.)

Two very different bird species, but on a late spring morning they had the same thing on their minds: getting ready to raise a brood of youngsters. With the breeding season well underway in late May, female birds, especially, were spending hours each day building nurseries for their coming broods.

There's a great deal of hard-wired activity that goes into such preparations, including what kind of nest to build. Even though they've never seen their parents' construction process, each member of a bird species builds the same kind of nest.

Birds use grasses, sticks, mud, feathers, fur or moss to build the kind of structure particular to their species. Chickadee nests feature soft green moss and even softer fur, often from a rabbit. Tree swallows assemble a cup of dried grasses, then cover it with other birds' feathers, often those dropped by gulls. Cardinals bend pliant twigs into a well-hidden cup deep inside a shrub.

Many of us have come across a robin's nest after the family leaves, a heavy assemblage with a mud base holding twined grasses and twigs. No older robin shows first-time robins how to gather mud and tamp it down. And no older house wren demonstrates how to gather hundreds of twigs to stuff into a cavity. Birds go by plans stored in their brains, because they weren't around to watch their parents build a nest. In each subsequent year, birds' nest-building skills improve, but even so, they do a pretty good job the first time around.

There's no time to waste, because summer is short. Birds need to build a nest, breed and then spend several weeks incubating eggs. Most females produce one egg a day and don't begin to incubate them until the last egg is laid, usually for a period of 10 days to two weeks. Add about two weeks of feeding hatchlings in the nest, and still parent birds' job isn't finished: After young birds leave the nest, they need several weeks under their parents' wing, so to speak, to learn how to survive on their own.

One of the most amazing things about this mad-dash season is that it's only the first act for many birds. Cardinals and bluebirds, robins and ruby-throated hummingbirds, house wrens and phoebes turn right around and build a second nest and raise a second brood each summer.

Birds work hard all year long, and even harder in the spring and summer. This is not a group known for lounging, and they're embarking on their busiest season of the year. It's an astonishing feat to raise a new generation each year, but birds pull it off, year after year.

St. Paul resident Val Cunningham, who volunteers with the St. Paul Audubon Society and writes about nature for a variety of publications, can be reached at valwrites@comcast.net.