What is more, the acceptance of urban life among animals is becoming common. It appears among more and more species and in more numerous populations. Warsaw, which is the largest agglomeration in Poland, is a perfect example of this phenomenon. For the last several years, 213 species have been observed - 170 of them live here permanently or appear so often that they are perceived as permanent elements of nature. To compare, in Poland there are 360 species, and in Masovia 300. In Warsaw about 190-370 bird couples nest on 1 km2, which is twice as much as in a forest and many more than in a field.
The main limitations posed by the urban environment which animals need to face result from its separation from nature. In areas which became cities, the soil, fauna and flora which existed from the very beginning are suddenly destroyed and replaced with artificial structures - buildings, roads, lawns. The remaing green belts are separated with spaces full of roads and buildings. Flora which in cities is very barren is not a good source of food for animals and it does not provide them with enough hiding places. Cars, walls of skyscrypers, wire nets, chemicals, water contamination, air pollution, dogs, cats and omnipresent human beings make it difficult or even impossible to live in an urban environment. .
The mainstays of birds are parks, gardens, allotments and cemeteries. The House Sparrows, the Pigeons and the Jackdaws fly in here from housing estates but species from the woods nest here too. However, problems with finding natural nourishment and the lack of hiding places for nests make it difficult for birds to settle. Raking out plant litter, removing dead boughs, overdrying the soil and contaminations made nourishment in parks much poorer than outside the city. Green spaces in the cities which are poor in undergrowth and bushes are often inhabited by birds nesting in treetops and hollows.
