Pigeons mate for life and rear their broods together, although if one dies the other will take a new mate. Once the simple nest is built, the female lays an egg and then another a day or so later. The incubation period for common pigeons is 17 to 19 days. The female sits on the egg from late afternoon through the night until about 10AM. The male then takes over and does the day shift. Once the eggs hatch, both parents feed the young squabs. The first food is pigeon milk or crop milk, a cheesy substance that appears in the crops of the parents at hatching time and is fed for a week or so. Then the adults start regurgitating partially digested grains for the young. By the time the squabs are ready to fly, about 4 weeks, the father is doing most of the feeding. The squabs are fed for another week to 10 days after they are free-flying.