Baby pigeons are fed milk by both parent birds during the first two weeks. This milk is produced in the crop of the parent birds and contains more protein and fat than cow or human milk. Prolactin is the hormone which stimulates the production of pigeon milk, the same hormone which stimulates the production of breast milk in mammals. Flamingos and Emperor Penguins also produce milk for their young. There are no bird species that bear their young alive.
All birds reproduce by laying eggs.
However, there are also two mammal species that lay eggs, the platypus, and the spiny ant-eater. Size differences vary among species. About the only exception to this rule is among Burrowing Owls, where the sexual size difference is negligible. If you can't put the baby bird back into the nest, of if it appears injured, then put the nestling in a comfortable, quiet place and call a wildlife rehabilitator. When an untrained person tries to feed liquid to a baby bird, the attempt is often fatal to the bird.