Animal genetic resources for food and agriculture crop and livestock species in the early neolithic time period changed our human evolution and lifestyles. This ability to control food production led to major demographic, technological, political and military changes. Consecutively, thousands of years of natural and human selection, genetic drift, inbreeding, and crossbreeding have contributed to the diversification of animal genetic resources and increased the variety of environments and production systems that livestock keeping takes place. Relatively few species have been domesticated; out of the world's 148 non-carnivorous species weighing more than 45 kg, only 15 have been successfully domesticated. The proportion of domesticated birds used for food
The Life of Birds raven from raiding a nest by collectively raising an audible alarm. However, in an English wood, all species co-operate to warn each other surreptitiously of approaching danger. By contrast, a sunbittern is shown expanding its plumage to discourage a group of marauding hawks. The members of the finch family exemplify how colour aids recognition. Birds have excellent colour vision, and the feathers of many species react to ultraviolet light. Flocking birds, such as sparrows, also have a 'ranking system' that determines seniority. In Patagonia, Attenborough demonstrates the effectiveness of sound: he summons a Magellanic woodpecker by knocking on a tree.