In nature, what is the selection pressure? (what causes adaptations to occur in bird populations? )

A cause that drives an organism to develop characters that help it survive a change in its surrounding is called selection pressure. Selection pressure is the cause of Evolution as it acts as a driving force in the development of new species from the original species.

Explanation:

Birds have several adaptations ,that allow them to have such a diverse habitat.These adaptations are modification of forelimbs into wings, different types of beak shapes, different types of claws and feet etc.All these adaptations are result of some kind of selection pressure. For example, the wings developed because it must have became difficult for birds to compete with terrestrial animals and thus they had to choose a aerial habitat ,fly long distance in search of food and shelter. Here the population of terrestrial organisms acted as Selection Pressure.Again the different types of beaks emerged depending on the feeding habits of the birds. This was followeed because a particular type of food was not sufficient for entire bird population and the second reason was that the birds migrated to different places and thus they were compelled to feed on the food resources availablein that particular place. Here the selection pressure is the availibility of food.