The eggs belonging to the robin on my porch hatched. The baby birds are very tiny, just lumps. They are not opening their mouth and cannot even seem to lift their heads. How do they get fed when they are so helpless?
The hatchling's neck muscles are stronger than you think, because hatching from the egg requires strong neck muscles. Newly hatched songbirds and others do indeed open their mouths after they have had sufficient rest following hatching, though you don't seem to have witnessed this. They have little neck control and their heads wobble initially, but feeding is so important that they are born with this ability.
Songbirds and most seabirds have altricial young, meaning that the newly hatched birds are blind, featherless, and helpless. Immediately after hatching, altricial birds can do little more than open their mouths to beg for food. They remain in the nest where the parents can feed and protect them while they continue to develop. For the first week of life, most altricial birds cannot control their own body temperature and must be constantly brooded (kept warm) by their parents.
This makes sense, as most altrical birds only spend about two to three weeks before flying off to start their own separate lives.
Newly hatched poultry chicks (precocial birds, which also include ducks and sea birds) can live for 48 hours on the last of the egg yolk that they take in just before the strenuous work of hatching out of the egg.
Incubation periods are longer for precocial birds than altricial birds, allowing for increased embryonic development in the egg, and therefore they have relatively advanced motor and sensory functions at hatching.
Feeding poultry is actually not necessary during that time. The chicks are strong enough to stand, drink, and peck for food soon after birth, but it's not necessary for life. What they cannot do is regulate their body temperatures well.