The other day someone reported to the Maine birding listserve that they had seen a bald eagle carrying off a big stick. Despite the fact that it is clearly still the dead of winter, this bird was planning ahead, ensuring that it would have a suitable nest ready for the breeding season.

Winter is a good time for us humans to get more of a glimpse into what is usually a hidden part of the life history of many birds. When the leaves are gone from the trees and bushes, a close look will reveal a variety of bird nests from the previous summer. And the diversity can be quite amazing.

Most of us are familiar with the circular, often grassy nests of American robins, which usually include a bit of mud to hold them together. Barn swallows build nests up against beams or eaves on porches or inside sheds or barns, often forming a half cup. Though always using lots of mud, barn swallow nests are always lined with warm, dry grass.

Cliff swallow nests are not seen regularly anymore in most areas, but these once-very common nesters attached their nests to the eaves of barns and sometimes homes all along Midcoast Maine. Those fortunate to host them see a gourd-like nest built completely out of about 1,000 mud pellets collected by the nesting pair and painstakingly pressed together. The inside is then lined with dry grass.

Many of our familiar backyard birds prefer to hide their nests as deep as possible in the cover of thick, and sometimes thorny, shrubs. Gray catbirds, those familiar gray birds with whose cat-like mewing calls have rudely awakened many in the early morning hours of a summer day, make a somewhat unruly nest of twigs hidden inside the thickest bush they can find. Northern cardinals, too, like to build their twiggy nests in such places.

Baltimore orioles are famous for their very unusual, hanging nests that look like a white sock. It is amazing that the female (males don’t do much to help) oriole can find ample long, fibrous pieces of bark and grass and can weave them together in such a way that this hanging bag can support the weight of the female and her eggs, and later, the growing young. Surprising, also, is how often one of these nests can hang in an area we pass by frequently yet fail to notice, until it is exposed by the stark branches of winter.

Then there are the cavity nests. Woodpeckers excavate holes in rotted trees and use nothing but wood chips to line the nest cavity. Smaller birds like black-capped chickadees and white-breasted nuthatches will find small cavities and do some excavation, adding some softer materials like fur or feathers. Eastern bluebirds and tree swallows don’t have bills that allow them to dig holes even in soft, rotten trees so they have to find abandoned woodpecker cavities or other cavities, and often use nest boxes.

When we come across a nest in winter, we usually take a moment to try to identify the builder based on the evidence of what the nest was made from. It’s also enjoyable to imagine the family that lived in it: the eggs hatching to young; the parents’ steady supply of food for the gaping mouths; and the moment of takeoff from the safety of the nest, out into the world. And in the midst of winter, we wonder where they are now.