Other intimate pigeon behavior includes billing, often a prelude to mating, when the female sticks her bill down the male's throat and takes an offering of regurgitated food, and allo-preening, when pairs lightly preen each other's heads. Once paired, pigeons may mate for life. Pigeons often nest high on protected ledges, under bridges, or in abandoned buildings. When a nest site is selected, the male pigeon gathers twigs one by one, placing them next to the female, which sits on the nest and works them into a flimsy platform of twigs and grass or straw. Pigeons usually lay two white eggs that take about 18 days to hatch.
Parents share incubation duties, with the male usually taking the morning to afternoon shift.
The nest remains covered by one of the parents about 99 percent of the time. This protection from heat loss is especially important in cooler months. By the end of the first week, crop milk production dips and the parents feed their young increasing amounts of seeds, until by the second week, the crop milk is gone. Compared with many songbirds, in which about 30 percent of young survive, this is a good survival rate. Although pigeons are among the most thoroughly studied animals, great rifts remain in our knowledge of their biology, even in the Washington area.