In some species, like the Rock Pigeon, the male and female will both sit on the nest and incubate the eggs, to keep them warm and protected while the chick inside the egg grows and develops. Usually the male pigeon sits on the nest during the day so the female can go look for food when its easier to find food. She spends more total time on the nest because she will sit there all night, as well as in the early morning and late evening.
The time for incubation varies widely from species to species. You can get this information for any of our focal species you’re interested in by going to its page in our bird guide, or by visiting AllAboutBirds.org