A bird can be identified by the different splashes of color on its feathers, or its distinct call, but did you know that you can also tell a bird by the way it builds its nest, even if it's empty?
Just as birds in all their variety evolved from the very first species, their nests have evolved in equal variety over millions of years. Every bird builds a nest unique to its species.
Most likely the first nest was a natural cavity, readily available shelter from weather and predators. For modern birds, the longer a species' young remain in the nest, the more intricate its design has to be.
At one extreme, ducklings emerge from the egg able to move about and forage on their own. Because their time in the nest after hatching is short, most duck nests are just a simple buffering layer of materials scraped together on the ground.
At the other extreme, songbirds may make over 1,000 trips carrying materials for their nest, including spider silk, lichen, plant fibers and mud, twigs, and grasses. A lengthy nest-building process develops a strong pair bond between songbird mates, essential to raising their young, which hatch naked and totally dependent.
As for the expression, "to feather one's nest," the farther north the nest and the earlier in the season, the more insulating feathers are added. Tree swallow nests can have over 100 feathers--white for the most part, as well as small and downy.
In an earlier era, when horses were the main means of transport, a common backyard bird, the chipping sparrow, was known as the "hairbird" for lining nests with long, delicately coiled horse hairs. There aren't as many horses today, but find a small nest lined with animal hair, human included, and you'll know you've found a chipping sparrow.