What does "fledging" refer to in birds?

Fledging refers specifically to the developmental stage of young birds as they grow feathers and begin to learn how to fly. This period is crucial for their survival, as it involves not only physical development, such as the maturation of feathers that allow for flight, but also behavioral learning, where fledglings practice flying and gain essential survival skills. Successfully fledging signifies that the young birds are becoming independent and are prepared to leave the nest. This process is fundamental in avian life cycles and is closely linked to the overall success of a bird species in terms of reproduction and growth.

The other choices describe related concepts but do not accurately define fledging. For instance, nesting typically refers to the entire process of building a nest and caring for eggs or hatchlings, not just the maturation of fledglings. Consequently, while fledging occurs during the nesting period, it is a distinct developmental phase. Additionally, the notion of a form of nesting behavior does not capture the essence of fledging and age when birds start singing is not relevant to the fledging stage at all, as singing often occurs after fledging as part of adult development.