In nature, species, or subspecies (races) do not mix, if they have choice, because of their genetic differences, but also their lifestyle (social / solitary, different reproductive periods, etc. .) and their geographical location.
We will use the example of the hawks. There are indeed many species or races of hawk (Falco) in Europe.
The Kestrel, Eleonora’s falcon, the Barbary falcon, the Merlin, Gyrfalcon, the Eurasian Hobby, the Red-footed, the Lanner, the Peregrine, and the Saker.
All these hawks live in Europe, in the same geographical area, but in nature, they do not mix. The only hybridization between Falconidae have as origin the hand of human, which mixes the races for the domestication and use in artificial falconry. Other features, different from the natural qualities, are then sought et cultivated, and domestication is probably much easier when the hybrid falcon has instincts in conflict with its real capabilities, making his life in nature difficult.
One example is very common and ancient: Gyrfalcon x Saker hybrid, but there is also Gyrfalcon x Peregrine, or Merlin x Peregrine, products fertile. All these hybrids produce fertile chicks.