I presume you know what 'maternal' means--it means 'motherly'--and maternal care is what the mother bird does to take care of her eggs and chicks up to the time they become independent. The mother bird (and usually the father) of most species sits on the eggs (incubates them) to keep them warm so that their embryos can develop. When the eggs hatch after a short period (from several days to a few weeks), the parent birds forage for insects or other prey and feed the babies until they fledge (= get all their flight feathers) and are able to look for food themselves.
That is the basic maternal (motherly) and paternal (fatherly) care that most young birds receive.
What is more interesting, however, is the variations in this practice. Female megapodes, for instance, lay their eggs in mounds of earth or sand, where the eggs are incubated by the sun, and the young birds hatch as independent foragers after several weeks; the mother bird exercises no care over them at all, though the male birds guard and maintain the nest mound.
Among higher bird families, two basic strategies have emerged. The young of altricial species (like most of our songbirds) are naked and helpless at birth, and they remain in the nest and are cared for by the parents for a considerable length of time before they are able to leave te nest. On the other hand, the young of precocial species (like chickens and shorebirds) are born with a coat of warm down and are able to walk or swim about and forage for food within a few hours of birth.
For a really thoughtful essay, you might consider the advantages and disadvantages of the altricial and precocial reproductive strategies.
The main "maternal" care birds provide their young of course begins before the eggs hatch and lasts until the baby birds leave the nest. To simplify I'll use bullet points:
- the bird must protect the eggs and keep them warm
Once the eggs hatch the bird:
- protects the nest from predators
- brings the young birds food
- eventually "teaches" the young birds what to eat, how to find it, and how to avoid predators
- chew, swallow and regurgitate food for the young
- show the young how to fly
Finally, although this is called "maternal care" many of these behaviors are seen done by the male of the species.