You can tell the creature by how it nests

In winter, chickadees and tufted titmice and house wrens and cardinals appear on the open stage of our backyard birdfeeders to squabble and shove for seeds and suet. But within a few months, all that energy will turn toward a more private occupation: the secretive life of nests.

Suspended high overhead or tucked into a thicket, nests are all around us, many appearing at first glance to be nothing more than a windblown clump of grass. Now that the leaves have fallen, revealing the birds' hiding places, it's a good time for nest-spotting, said naturalist John Doyle of the South Park Nature Center.

Each species has its own building style, preferred location and trademark lining -- titmice, for example, like to tuck in a piece of shiny snakeskin -- that help identify who's built what, he said. And for the birds, much of that nest-building hoopla has to do with finding true love.

"Everything they do is for attraction," said Verna McGinley of Mt. Lebanon, a former education director at Pittsburgh's National Aviary who was among the dozen participants at Saturday's hike and nest lesson at South Park. "Decorate the nest right and you get a good woman."

The lesson, led by Mr. Doyle, focused on the identifying factors of nests: size, location, structure, shape and materials.

Nests can range in size from 1 inch and a few ounces (hummingbirds) to 10 feet and more than 4,000 pounds (bald eagles), he said as he led participants across a muddy field and down to the trees growing alongside Catfish Run.

"If eagles like the tree they're in, they'll keep adding to that, year after year after year, until the top of the tree breaks off," he said.

If a clump of leaves high in a tree is far away from the trunk on thin branches, it is probably a squirrel's nest, called a drey, he said. If close to the tree, or in the solid crook of a limb, it is likely a hawk's nest because the bird's greater weight requires a sturdier foundation, according to Mr. Doyle.

About half of all nests are built less than 6 feet from the ground, while about a third are built less than 2 feet from the ground, Mr. Doyle said. And many are built by ground nesters -- one reason why South Park has stopped mowing many areas to let them return to meadow -- with a relatively small percentage of nests built in the high branches of trees, where most people look for them.

While some birds build new nests each year because of winter damage or parasites, other species reuse the same nesting spots and get rid of parasites creatively: layering cedar bark and leaves in open nests, or smearing pine sap and insects around the entrance of cavity nests, according to Mr. Doyle.

Many birds are very specific about what nest sites to use -- so-called cavity builders such as chickadees and bluebirds that use holes in rotted trees and fenceposts, for instance, like an eastern or southern exposure to keep out rain and let in sunshine to help warm their eggs. Bluebirds get even more particular. They want boxes about 20 feet inside a field, not on its edge, he said.

In the trees along Catfish Run, Mr. Doyle and his group found several hawks' nests and then, in a snag, a rotted tree with several holes. The higher hole, where the wood was softest, probably was a chickadee nest while the lower cavity probably was the nest of a pileated woodpecker or screech owl, he said.

Then, hiking up a hill into the woods, the group found a round ball of sticks and grass caught in a bramble along the woods' edge, likely the nest of a cardinal or a catbird. When the thicket was leafed out in the spring and summer, it would have been almost impossible for predators to see, Mr. Doyle said.

If nest-spotters can get close, a nest's lining is often the best identifier of its occupant.

Catbirds like rootlets, while cardinals and bluebirds prefer fine grass for their linings. Goldfinches use thistle down, while chickadees enjoy a moss-filled nest. And if you see mud inside a nest, a robin almost certainly has been hard at work, scooting its body around in the mud to plaster the "walls," Mr. Doyle said.

Vireos use spider webs to hold their grassy materials together and lots of birds decorate: eagles with flashlights and bicycle rims, cat birds with napkins, cellophane and shoestrings. And sometimes, people provide even more personal building materials -- like when titmice decide they need some fur to go with their snakeskin.

"Some guy will be sleeping on his porch and it will start picking the hair out of his beard and out of his hair," Mr. Doyle said. "It's interesting behavior, but I guess it's got to get material somewhere."

Amy McConnell Schaarsmith: or 412-263-1719.

First Published 2012-02-09 14:11:19