This formula has worked well for me over many years. It has worked well for young owls as well as hawks and eagles. Large rehabilitation centers have other options. They cannot mend all the small prey animals and songbirds brought to them and they  purchase frozen rats and mice in bulk from commercial sources. I have had several non-releasable screech owls go from chicks to mature egg-laying adults on this diet:

½ pound raw chicken necks

½ pound raw chicken gizzards

Take sewing scissors and cut the gizzards into strips about as long as the bird’s beak and a bit less wide than the base of the beak.

Take the chicken necks outside on a clean area of your sidewalk. Pound them with a hammer until the bones are thoroughly crushed.

Add one 5½ Oz can of Purina Friskies Turkey & Giblets Shreds in Gravy.

Add 2 tablespoons of tap water.

Mix well.

Put each days ration into a small plastic zip lock and freeze.

Put on a pair of leather welders gloves

Wrap the bird in a soft bath towel –like a burrito- to restrain its legs and wings with its head exposed. Not so tight as to restrict its breathing - not loose enough for it to escape. Never clip the sharp ends off of its claws (tallons).

Feed the diet at room temperature or slightly warmed once a day.

I feed owls at sunset, raptors in the morning. I hand feed until the bird begins to rebel. That is about the time when tufts of feather down will no longer be present on the tips of its head feathers. The bird's primary and secondary feathers will have fully erupted - although some sheath material will still be present at the feather bases and on the floor of the cage. At this weaning time, birds won’t sit still – somewhat like an overactive rebellious todler. They will flap and attempt to fly. Proper timing is hard to describe, but at that time, leave a bit of the mix in a small dish at the bottom of the cage. For hawks, leave it there in the early morning, for owls, at sunset. If it is gone in the morning for owls or by evening for hawks, it is time to stop feeding by hand. Leave food out at room temperature for no more than half a day. The bird should now be at its maximum body weight and will loose weight during weaning. That is normal.  Spend as little time around the bird or handling the bird as is possible. You want it to become fearful of you.

If you have the temperament to purchase live mice or rats now is the time to do so. Be sure these rodents cannot get loose in your house. Many feeder rodents are themselves diseased. Others believe that Hunting is genetically programmed into these animals and that they will learn to hunt on their own.

Whole chicken or chicken leg quarters (with the back included) also work well as these owls I raised in 2015 attest to. I buy the 10 lb packages and remove most of the meat in small strips with kitchen scissors. I discard the big lumps of fat. I put the the remaining bones and clinging meat in a Proctor pressure cooker with enough water to cover them and 1/3 cup of white vinegar. Then I cook that bone portion for 3 hours in a Presto pressure cooker or until the bones disintegrate or are crumbly. Then I mix that with the raw meat strips I removed earlier. That supplies the calcium these birds need to develop normally.