Breeding endangered species in captivity and releasing them after proper preparation has worked to keep many species alive–California Condor for example.
The release part is still not a guaranteed for the Hawaiian Crow (Alala), however. Click here to read about their current status, with most Alala living still in captivity. Their threats in the wild include endangered hawk, endangered owl (Pueo) and toxoplasmosis from feral cat feces. There are fewer than 200 of these birds alive.
One pioneer of captive breed and release as programs for endangered species was Gerald Durrell. The TV series, “Durrells in Corfu”, is fun viewing those of us who enjoy animals as much or more than other people.