Since the birds can interbreed in captivity, it is not a postzygotic barrier. Since the birds in nature live in different habitats that would be habitat isolation, a kind of prezygotic barrier.
Work Step by Step
See pp. 502-504 and scan through the different kinds of barriers. The hints in the question are that breeding is possible (not postzygotic) and that the birds conduct their lives in different places/at different altitudes (habitat).