Birds fertilize their eggs through copulation or male and female sexual interaction. The male bird produces sperm which is deposited into the female's cloaca during mating, and this results in the internal fertilization of the eggs.
Birds normally mate according to breeding seasons and for most species this takes place in spring. Once a female is ready for reproduction, she allows the male partner to jump on her back in order to allow for a cloacal kiss that only lasts for a few seconds. It is normal to observe birds engaging in this process for several days as this increases chances of successful fertilization.