Since the two species co-inhabit the same habitat, habitat isolation can be disregarded as a factor keeping the two species separate. There are however some other broad categories of mechanisms for reproductive isolation. One is behaviour. Many birds in particular have very specific mating rituals, including mating dances, particular bird song, the development of specific mating plumage, the release of specific pheromones and the building of a particular nest structure. Another broad category is mechanical isolation. You will find that the genital organs of particular species are very specific, and preclude other close species from successfully mating. Another broad category is gametic isolation. For most species combinations, it is not possible for the gametes to fuse successfully to form a hybrid. If hybrid games do fuse, in many cases, the hybrids are sterile, such as mules for example.
