Pigeons are monogamous and typically mate for life. Pigeons build a flimsy platform nest of straw and sticks, put on ledge, under cover, often located on the window ledges of buildings. In captivity, pigeons commonly live up to 15 years and sometimes longer. In urban populations, however, pigeons seldom live more than 3 or 4 years. Natural mortality factors, such as predation by mammals and other birds, diseases, and stress due to lack of food and water, reduce pigeon populations by approximately 30% annually. Pigeons are found to some extent in nearly all urban areas around the world. It is estimated that there are 400 million pigeons worldwide and that the population is growing rapidly together with increased urbanization. Sexes look nearly identical, although males are larger and have more iridescence on their neck. Juveniles are very similar in appearance to adults, but duller and with less iridescence. Pigeons are highly dependent on humans to provide them with food and sites for roosting, loafing, and nesting. They are commonly found around farm yards, grain elevators, feed mills, parks, city buildings, bridges, and other structures, although they can live anywhere where they have adequate access to food, water and shelter.