Doves and pigeons build relatively flimsy nests, often using sticks and other debris, which may be placed on branches of trees, on ledges, or on the ground, depending on species. They lay one or (usually) two white eggs at a time, and both parents care for the young, which leave the nest after 25–32 days. Unfledged baby doves and pigeons are called squabs and are generally able to fly by 5 weeks of age. These fledglings, with their immature squeaking voices, are called squeakers once they are weaned or weaning. Unlike most birds, both sexes of doves and pigeons produce "crop milk" to feed to their young, secreted by a sloughing of fluid-filled cells from the lining of the crop.