Bird breeding season is frenetic few weeks

One day in early spring a male cardinal carefully selects a seed from a feeding tray, then drops to the ground and presents it to a female cardinal. She accepts his offering, thereby engaging in a courtship ritual that's fairly common in the bird world.

This kind of symbolic activity helps the cardinals prepare for the breeding season. Two things are happening as that seed is passed from beak to beak: The male is showing he can be a good provider for his bird family, and the two birds are cementing their bond. Soon their bodies will begin releasing the hormones that propel them to mate, build a nest and raise offspring together.

Courtship rituals are an important first step, and it's one that occurs in many other bird species, from blue jays to catbirds to cedar waxwings, as adults turn toward the serious business of raising this year's "crop" of young birds. The schedule is tight: In just over a month's time (the average is about 35 days) adult songbirds race through all the tasks required to raise a family. A nest gets built, the female lays a clutch of eggs and incubates them until they hatch, and then parent birds keep their youngsters warm and well fed for several weeks until they fledge. (Some species, notably robins, cardinals and house finches, raise more than one brood each summer.)

Birds don't start this cascade of activities until food is abundant, and what constitutes food varies by species. Robins are early nesters because they feed on earthworms that begin to reach the soil surface by the end of March. Tree swallows nest fairly early because the flying insects they feed on begin hatching in late April. Orioles and tanagers enter the breeding season in mid- to late May, timed to the appearance of juicy caterpillars on tree leaves.

After their food source appears, birds turn to constructing their nests. In most species, the female handles most or all of this task, building a nest to fit her contours, since she'll be the one spending up to two weeks sitting in it. Female birds tirelessly gather grass, bark strips, pine needles, plant down and spider webbing to make a resilient structure, crouching and turning as the nest develops to pack things down and make sure she'll have enough room.

One of the most amazing things about birds is their adeptness at structural engineering. Each has a nest plan hard-wired into the brain that dictates the general size, shape and location for the nest-type built by their species. (After all, they weren't around to watch their parents build the nest they hatched in.) All orioles build pendant-shaped nests far out on the end of a branch of a tall tree. Catbirds fashion a cup nest hidden deep within a vine tangle and bluebirds build a grassy nest inside a tree cavity or nest box. Birds don't build "McMansions" or add an extra story or two; instead their final product is just large enough to hold their clutch of young birds as they grow.

Because nestlings are vulnerable to predators, adults hide their nests well, in thick tangles of twigs and leaves, within rock piles or in well-camouflaged dips in prairie grasses. In fact, they're so invisible that we tend to see bird nests only after trees and shrubs are bare — how often have you said, "I had no idea that was there"? Bird nests are sturdy enough to last for several seasons, although they're almost always used only once.

In most species, about 80 percent, both parents handle feeding duties after their eggs hatch. Most young birds must eat every half-hour to thrive, and with four to six nestlings in the average nest, that makes for a long day of foraging for adults. One research study showed a bluebird pair making 300 trips a day back to their nest box with food for the young.

If something happens to one parent, such as to predation, injury or illness, during this critical two-week period, the survival of the nestlings can be threatened because a single parent won't be able to bring food as frequently or guard the nest as often.

Which brings us back to that male cardinal offering a seed to the female. He may have a beautiful song, a great territory and the brightest coat of feathers in the neighborhood. But the female bird isn't completely swayed until she is assured that he'll be a worthy partner in rearing the next generation.

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

Producing eggs is a big drain on the bodies of female birds: A clutch of four to six eggs has about as much calcium as a small songbird's entire skeleton. You can help replenish those stores by putting out small pieces of chicken eggshells just before or during nesting season. Rinse the shells, then bake in a 250-degree oven for 20 minutes to sterilize them. Crush them into small bits, then offer these "calcium pills" alongside birdseed or alone in a flat pan.