Keep just a few of both in captivity for breeding purposes, just in case. Then be 'laissez-faire' about the birds in the wild. If too many crows get killed off

Keep just a few of both in captivity for breeding purposes, just in case. Then be 'laissez-faire' about the birds in the wild. If too many crows get killed off, the excess hawks will starve, thus allowing the crow numbers to rise again. With the availability of sufficient prey, the hawk numbers will rise again and complete the cycle. It's standard 'predator-prey' cycling.