Normally the first birds to nest are great horned owls and bald eagles. Great horns are usually incubating eggs by late January, and bald eagles follow shortly thereafter. These large predators time their nests so the eggs hatch just as young cottontails and migrating ducks and ice-free water become available to feed the nestlings.
Mourning doves cope with feeding their nestlings by regurgitating "pigeon's milk," which is sometimes called crop milk. Adult doves eat primarily seeds. As incubation comes to an end, both sexes manufacture a nutritious milky liquid in their crops. Fatty cells slough off the inside of the crop and are fed exclusively to nestlings for the first five or six days after hatching. Thereafter the "milk" is mixed with an increasing amount of seeds as nestlings transition to an adult diet.
At higher elevations in coniferous woods, pine siskins nest in late winter when there is a bumper crop of conifer seeds. Siskins build well-insulated nests, and females incubate almost constantly. The male brings her food so she rarely leaves the nest to allow the eggs to cool.
But most songbirds wait for longer, warmer days to nest. This coincides with the emergence of protein-rich insects, spiders, and other invertebrates, which most birds feed their nestlings.
If you have any active bird nests right now, I'd like to hear about them. Egg and hatching dates would be especially useful. My earliest backyard nesters are usually Carolina wrens in early March.
Send questions and comments to Dr. Scott Shalaway, 2222 Fish Ridge Road, Cameron, WV 26033 or via my website, http://scottshalaway.googlepages.com.