This order includes all species of pigeons and doves, which represent about 350 species. These animals are cosmopolitan, and some species are adapted to urban life. The greatest diversity is found in tropical rainforests. They have small heads with short beaks. The flight muscles are highly developed, which gives them excellent flight abilities over long distances. Carrier pigeons are a perfect example of this as they were used to deliver messages for many centuries. They often have bare skin at the base of their beaks and around their eyes. Their crop produces a protein- and fat-rich nutrient called “crop milk”, which is produced by both males and females and uses to feed their chicks. Pigeons and doves are monogamous, i.e. they breed with the same partner from one year to the next. They feed on either seeds or fruit. They form the sister group to the Cuculiformes (cuckoos).