A male osprey bringing in nesting material. Osprey have among the biggest nests in the bird world. —Kevin Doyle photoA male osprey bringing in nesting material. Osprey have among the biggest nests in the bird world. —Kevin Doyle photoIt’s the season of nest building.

Rearing children is the most dangerous time for a parent bird, not to mention its offspring. Many creatures, from birds and snakes to squirrels and pet cats, would love nothing better than eggs, chicks or even the parents for dinner. Thus a nest has to be well hidden or well positioned to protect the contents and caretakers from predators — which is why we don’t spot most of the nests that are being built in our neighborhoods.

Birds hide their nests in two ways: They locate them in out-of-the-way places and they often camouflage them so that they are hard to notice, even when out in the open. Most nest-sitting birds are themselves colored or otherwise marked to blend in with their surroundings when they are nesting. That’s probably why many female birds are much less colorful than the males of the species.

The variety of nest sites is endless. Many birds build nests on tree branches or in holes they find or excavate in tree trunks or limbs. Others nest on cliffs or the sides of buildings where, while they might be visible, they are hard to reach on foot or even on wing. A surprising number of birds nest on the ground and some, even in the ground.

In his book, Winter World, Bernd Heinrich says that the wide variety of kinds of nesting sites makes it difficult for predators to focus on one kind of nest. If all birds used the same kind of nest, such as a hanging cup, he said, that design would become a sign to predators that food can be found here, just as McDonald’s arches indicate where to find hamburgers.

A few birds don’t bother with either hiding or camouflaging nests. They are so large that hiding would be difficult. But the birds are also big and, often, fierce enough so that they can easily defend their nest against most predators. The stick nest of an osprey, for instance, is hard to miss sitting atop a dead tree, utility pole, light stanchion, or even a platform designed as a nesting site.

Construction time for nests varies with species, and even within species, varies with bird couples. Many factors affect construction time. Some nests are more complex than others.

Beginners at nest-building usually take longer and are more apt to make nests that fail — i.e., fall apart — during construction. As builders have more broods their construction skills increase. Studies have found that the quality and sophistication of the work of many birds increases as the birds get older and build more nests. They also get faster at it. For instance, an American Goldfinch may take 13 days to build its first nest of the season, and only six for the second nest. This may be due not only to the experience gained in building the first nest, but also to the greater availability in August, when the second nest is built, of materials like thistle down.

Environmental factors affect construction times; a late arrival of spring, or stormy weather, can delay construction or force the birds to spend time on repairs. Or make materials harder to find. A Carolina Wren may spend less than a week in nest building if the weather is good, and two weeks if bad.

For most small songbirds, nest building requires three or six days. Robins vary from six to 20 days, while Song Sparrows take from three to 13 days. A Golden Eagle may take two months to build a nest.

Some species keep working on their nests all through incubation. This is especially true of large birds like eagles, osprey and gulls.

Urban hawks

Jason Kessler of Ridgefield writes: “I read with interest your Bird Note on the hawks in Tompkins (not Thomson, by the way) Square Park.

“They are indeed Red-tails, and are big crowd pleasers among birders and non-birders alike, with many people stopping to watch and photograph them whenever they appear. It’s great to see New Yorkers gathered around a bird’s appearance as they might a cab accident or police action.

“There’s a problem, though. Like parks all over the world, New York City parks have a problem with rats, which are dealt with in the usual way: poison. Often the hawks will eat the poisoned rats, who are either slowed down or stopped entirely by ingesting the poison. The hawks will, of course, be poisoned in turn. Many of the recent urban hawk deaths have been attributed to this cause.”

Jason recommends anyone interested in New York City hawks check out the website, urbanhawks.blogs.com. There are some amazing videos there.

Oops Dept.

My old eyes misidentified that woodpecker pictured last week: It as a Pileated, not Red-bellied, as a couple of eagle-eyed readers pointed out.

Coming up

Spring Migration Bird Walks: Bring binoculars and/or a camera, no charge or RSVP required, Saturdays: April 14, 21, 28, May 5, 12, 19, 26, meet in Audubon Greenwich parking area at 613 Riversville Road, 203-869-5272 x230, greenwich.audubon.org

Dance of the Woodcock at Ward Pound Ridge Reservation with Bedford Audubon Naturalist Tait Johansson, Thursday, April 12, 7:15 to 8:30 p.m., meet at the parking area just before the toll booth inside the Reservation, located off Route 121 in Cross River. Register at 914-519-7801 or [email protected]; www.bedfordaudubon.org

Birding by Ear, indoor discussion of how and why birds vocalize, plus recordings, then outdoor walk to practice, Saturday, May 5, 9:30 to noon, $15 adults, kids free, Audubon Greenwich, 613 Riversville Road, 203-869-5272 x230, greenwich.audubon.org

Copyright 2012 by Jack Sanders. Send sightings or comments to: jackfsanders [at sign] gmail.com, or to Bird Notes, Box 1019, Ridgefield, CT 06877. If you need help identifying a bird, try your local nature center. If you find an injured bird, call wildlife rehabilitator Darlene Wimbrow of Redding, 203-438-0618, Wildlife in Crisis of Weston, 203-544-9913, or Wild Wings of Greenwich, 203-637-9822. The columnist’s website is www. sandersbooks. com.