To help you better understand bird behavior, here is some basic information on the breeding cycle of birds. This is generalized information for most songbirds, certain species may vary from this. Watch your breeding birds from a distance and do not disturb them. Birds are very resourceful and adaptive. If something goes wrong during breeding, they will know what to do to fix it and will continue trying to breed. Rarely is human intervention needed.
Breeding begins by a male singing, forming a territory and trying to attract a female. If he is lucky, a female bird will choose him and join him on the territory. He then usually diminishes or stops singing.
The pair will mate, then the female lays 1 egg per day until the clutch is complete. Most songbirds lay 3-6 eggs. She usually lays the egg in the morning and does not stay near the nest the rest of the time. So if you see 2, or 3, or 4 eggs in a nest and no birds around, it does not mean it is abandoned, chances are the female will come back the next day and lay another egg until she has a complete clutch.
Then and only then, incubation begins. She will sit on the nest full time, taking brief breaks to feed, until the eggs hatch. Incubation is done mostly by the female and usually lasts about 12 to 14 days. During this time the female is quiet and the male stays somewhat near and does not sing.
When the eggs hatch both parents become very active bringing food to the nest. They carry away from the nest fecal sacs, little white packages that are the droppings of the young. This keeps the nest clean. The young, called nestlings, stay in the nest for about 12-14 days (longer for birds thart nest in birdhouses). The young at first have very few feathers. Then they have "pin feathers", feathers enclosed in sheathes. By the time they are ready to leave, the feathers have broken out of the sheathes, the young are fully feathered, and they call loudly.
If during the breeding cycle, one of the parents dies, the other parent will continue to try and raise the young by themselves and often they are successful. If it is during incubation and the female dies, the male cannot incubate and raise the young. In any case, the remaining parent will try and get a new mate as soon as possible.
When the young "fledge", or leave the nest they are called "fledglings". At first they may not be able to fly that well and for the first few days stay in the vicinicy of the nest. They are still fed by the parents for another several weeks. The fledglings often stay scattered in trees and call constantly. So if you hear constant chirping and see adult birds carrying food to different bushes or trees, chances are they are feeding fledglings. During the fledgling phase the adults may start a new brood. The male may sing again, mate with the female, and she will start a new clutch of eggs. even while he is still feeding fledglings from the first brood. The fledglings will eventually learn to feed themselves and the parents stop feeding them, and so the whole thing starts over. Many birds can have 2 or even more broods a season, especially in warmer climates.