Convergent evolution is the process by which unrelated species evolve similar traits due to similar environmental pressures or ecological niches. In the case of bat and bird wings, both species have evolved the ability to fly, but they have done so independently and through different evolutionary pathways.
Bats are mammals and are more closely related to primates than they are to birds. Bird wings are composed of bones, muscles, and feathers, while bat wings are composed of bones, muscles, and skin. Despite their different origins, both bat and bird wings have evolved to serve the same purpose: flight.