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
Birds of the Danube Delta (29); Mongolian (33); Chinese (2). The transpalearctic type contains 43 species. For Romania's latitude – delta included – the north-originating species (Arctic and Siberian) appear as winter-and passage birds, while the south-originating species (European, Mediterranean and Mongolian) are met with as sedentary and summer species. Further comments upon the manner birds migrate - namely the causes, which start on migration - and upon the way they orient themselves during migration, resulted in the hypothesis of the ""inner time"" and ""inner space"", which, along with their known senses, allow the migratory birds to perfect the migratory act in time and space.