Baby birds ... why (exactly) are they poor fliers?

Watching a batch of fledglings trying to fly right now (they all abandoned the nest last night/this AM), and was wondering: Why are they having such trouble flying? They can flutter short distances near the ground, but that’s it. Is it:

a) Insufficient practice/coordination
b) Insufficient muscle strength
c) Insufficient energy reserves/endurance
d) Insufficient wingspan-to-weight ratio

? Or a combo?

yes, yes, possibly, also lack of long flight feathers on wings. until those grow in the bird is effectively “clipped.”

For most small songbirds, I would guess a, b, and d in combination would be the most important factors. In this case it seems that for some reason they have left the nest somewhat prematurely - they shouldn’t be fluttering around so close to the ground.

Possibly the most important factors are the fact that the feathers are not yet fully grown, so that the flight surface is not as great as it should be. But they also need to practice flapping their wings and their coordination before they can fly efficiently. Many birds will stay in the tree in which they were hatched, fluttering from branch to branch, gradually becoming more proficient at flight over several days. As they become better, gradually their parents will lead them into other trees once they become capable of it. (But arboreal species in general should not end up on the ground - that means something went wrong).

There are some birds, such as certain petrels, that nest in burrows within forest, in which the young have to be able to fly on their very first attempt or they will crash into the trees and die. In these the fledglings wait until they are large and mature enough until trying. But these mostly nest on predator-free islands, so they can take their time about maturing.

These are barn swallows. They built a nest on our porch above the one door. They were starting to look a little crowded (6 nestlings!) yesterday, and today the nest was empty. There was one dead babe on the porch floor, and throughout the day I came across several others (alive) in various places throughout the yard. The parents were still trying to track them down and feed them.

Swallows in particular shouldn’t be on the ground. The parents usually lead them from the nest to another perch where they feed them for a while. Sometimes the young will go back and roost in the nest for several days after fledging. Swallow fledglings may be fed by the parents for a week to 10 days before becoming independent. However, it sounds that these guys flew the coop a few days before they were ready.