In a tall, broken-topped spruce tree just 200 years from my house, a pair of bald eagles is adding to their nest. Nests aren't homes for birds, they're nurseries. Birds build nests for babies.
Eagles will reuse a nest year after year, adding material each spring. Over time, eagle nests came become huge platforms, meters across and weighing more than a thousand pounds. Eagles build the largest nests in the bird world. One of my neighbors told me he and his brother once climbed that spruce tree in the fall, after the nesting season, and laid down in the nest. It was more than big enough to hold two grown men.
Other birds, like woodpeckers, build a new nest each spring. Woodpeckers are cavity nesters, and well-equipped to make cavities, so that means they peck and chip a new cavity every year. That's really important for other cavity nesters, like owls, chickadees, wrens and flycatchers, that aren't built for drilling into trees. They need woodpeckers to do their nest building for them. They are also quite receptive to using nest boxes supplied by people.
Pine siskins are little finches, songbirds, but like eagles, they are early nesters, pairing up in late March in Southeast Alaska and building the familiar cup nests, little bowls of woven grass and twigs tucked into shrubbery and anchored to tree branches. I found a deer hide someone dumped at a roadside pullout this spring, and it was a siskin magnet. The birds repeatedly gathered beaksfull of hair and fur to line their nests, making a soft, warm bed for the eggs and chicks soon to come.