Depends on the birds and the age. Here is a resource.
“Insect-eating passerine birds belong to the order Passeriformes that make up approximately half of all the World’s bird. They all have feet adapted for perching and all produce chicks that are naked and immature at birth. Adults are easily identified by their long, slender beaks. Unfortunately, their young are indistinguishable from seed-eating birds. The insect and insect and seed-eating Passeriformes include all the crows, ravens, cowbirds, blackbirds, grackles, starlings, magpies, shrikes, mockingbirds, jays, warblers, vireos, thrashers, and orioles. Young of this group open their eyes when they are between 3 and 5 days old and leave the nest when they are between 14 and 21 days old. Adults of this group have ventricular stomachs (gizzards) that contain stone grit to help digest their diet. If you are not sure whether your birds are seedeaters or insect-eaters, use the insect-eater formula, which is higher in protein, just to be safe.
I like to prepare a diet for insect-eating birds that contains 25–38% % crude protein, 2–3% fat, 6–12% and 5–9% fiber. I begin with a base ration that approximates these levels. An excellent base ration is Mazuri’s Omnivore-Zoo Feed “A” 5635 (http://www.mazuri.purinamills.com). After grinding, Mazuri’s product can be fed as it comes. I pass the pelleted chow through an old-fashioned crank meat grinder to powder them and store the powdered product in the freezer until it is used to prevent weevil and moth infestation. I reconstitute it with drinking water to the consistency of pabulum or watery grits.
If, the above diet is unavailable, you can produce a good insect/omnivore diet yourself: In a bowel, mix 2cups of ground Hills Science Puppy chow, one cup of instant oat meal and two cups of grated whole, hard boiled egg. To this mixture add 2 crushed 750mg calcium Carbonate antacid tablets (Tums or generic equivalent), 500 iu of vitamin E, 500mg of vitamin C and any multivitamin/mineral tablet that contains among its other constituents, approximately 2500 iu of vitamin A and 200 iu of vitamin D3. Since many vitamins do not specify vitamin D isomer, I use as a vitamin source one-half teaspoon full of Plex-Sol C (Vet-A-Mix) multivitamin, or one-half Mazuri 5M24 tablet per batch of food. I feed young birds every hour during the day with forceps or tweezers. Taping the tweezers on the side of the container or brushing it over the bird’s beak will cause it to open its mouth (gape). Feed until the bird no longer trembles it wings when it opens its mouth. Do not feed a healthy chick to the point where it only half-heartedly opens its mouth. When you are done, clean the bird’s beak and face with a moist facial tissue. Do not give the nestlings water until they are moving freely about the bottom of their cage.
As the birds mature and begin walking and hopping about, sprinkle very small amounts of oyster shell grit on the floor of the cage as well as raisins, diced apple grapes and berries (for omnivores), mealworms and crickets. Also place jar caps of water in its cage at this time. Be sure the bird is in a secure cage when they begin to test their wings.
Fledglings are reluctant to stop begging for food in captivity and in the wild. When they are fully feathered (fledged) they should be able to eat on their own. To be sure the bird is eating adequately and loosing only moderate weight, a postage scale is helpful.
. A good judge of your feeding technique is the absence or near absence of stress bars on the tail and wing feathers (primaries) of the fledged nestling. Multiple stress bars or band-like creases in the feather fletch are due to periodic decreases in blood nutrients. This is usually a sign of too infrequent feedings. ”
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