on the ground if the nest was not complete. The normal clutch size appears to have been a single egg, but there is some uncertainty about this, as two have also been reported from the same nests. Occasionally, a second female laid its egg in another female's nest, resulting in two eggs being present. The egg was white and oval shaped and averaged in size. If the egg was lost, it was possible for the pigeon to lay a replacement egg within a week. A whole colony was known to re-nest after a snowstorm forced them to abandon their original colony. The egg was incubated by both parents for 12 to 14 days, with the male incubating it from midmorning to midafternoon and the female incubating it for the rest of the time. Upon hatching, the nestling (or squab) was blind and sparsely covered with yellow, hairlike down. The nestling developed quickly and within 14 days weighed as much as its parents. During this brooding period both parents took care of the nestling, with the male attending in the middle of the day and the female at other times. The nestlings were fed crop milk (a substance similar to curd, produced in the crops of the parent birds) exclusively for the first days after hatching. Adult food was gradually introduced after three to six days. After 13 to 15 days, the parents fed the nestling for a last time and then abandoned it, leaving the nesting area en masse. The nestling begged in the nest for a day or two, before climbing from the nest and fluttering to the ground, whereafter it moved around, avoided obstacles, and begged for food from nearby adults. It was another three or four days before it fledged. The entire nesting cycle lasted about 30 days. It is unknown whether colonies re-nested after a successful nesting. The passenger pigeon sexually matured during its first year and bred the following spring. Alfred Russel Wallace, in his historic 1858 paper On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection, used the passenger pigeon as an example of an immensely successful species despite laying fewer eggs than most other birds: Predators and parasites Nesting colonies attracted large numbers of predators, including American minks (Neogale vison), long-tailed weasels (Neogale frenata), American martens (Martes americana), and raccoons (Procyon