My baby sparrow is weak and not eating. What should I feed it?

The easies solution is either Recovery from Royal canin or a/d from hills, both are wet foods designed for cats and dogs but are suitable for baby birds as well, you can draw them into a syringe and feed this bird (many times a day!)

It is possible he/she may have aspirated some food into the lungs. This can lead to respiratory infection or even pneumonia. The bird should be examined by a wildlife veterinarian or a vet experienced in treating birds.

It looks like it may indeed be of the Warbler species. These birds typically eat insects, so small crickets or mealworms would be recommended. Ideally, you should contact a local wildlife rehabilitator for assistance. A rehabber will have the knowledge and experience to give the bird the best chance at eventual release.

The thump into the window appears to have caused physical injury and possibly neurological damage. This would have to be confirmed by a veterinarian. The bird may recover, but it could take some time and may require extensive supportive care. Ideally, it should be examined by a wildlife veterinarian or relinquished to an experienced wildlife rehabilitator. If treatment is not possible, humane euthanasia may have to be considered. Keep it warm and as stress free as possible. Prop the bird up with a towel and wash your hands thoroughly before and after handling. Use a dropper or small syringe to provide water. Offer a fortified seed mix for finches. You can also try a handfeeding formula for extra nutrition.

Contact the local veterinary clinic. Ask if the have contact information for a local wildlife rehabilitator. If you can get the bird to a rehabber, it will have an excellent chance at eventual release. House sparrows eat seeds and grains. In the meantime, you can offer a seed mix including millet and sunflower seed. Some moistened dry dog food can also be offered.

Their diet in the wild is mostly insects and berries, so purchasing some live crickets from a local pet store, or some berries from your store and offering them may help get them used to what they would forage for in the wild as they get weaned off the kitten food (and can be fun to teach baby birds how to catch food with live insects.) If there isn't a wildlife rehab nearby (India does have several), you can also see if there is a vet or group in your area that does wildlife rescue that may be better equipped for caring for the birds and releasing them back into the wild once they are old enough.

You can definitely try to feed it, you will need to buy wet canned cat food such as Recovery or A/D (from your local vet) as well as a syringe, you will need to feed it every couple hours during the day (no need to feed during the night).