Which of the following best defines an "eyas" in falconry?

An "eyas" in falconry specifically refers to a young raptor that is captured or acquired while still in the nest or as a nestling. This definition highlights the significance of the bird's age and developmental stage, as an eyas is typically untrained and unflown, making it different from other terms in falconry that relate to adult or experienced birds. This stage is crucial for falconers because it allows for the bird to be raised and trained by humans from a young age, potentially leading to a strong bond between the falconer and the bird.

The other definitions provided in the options refer to various stages and experiences of birds of prey but do not accurately capture the specific meaning associated with an eyas. For instance, a fully grown raptor refers to a bird that has matured and could be an adult or a juvenile, while a raptor that has never been handled by humans indicates a wild bird, and a bird that has completed its first flight could still be classified differently based on its age and training history. Thus, the unique characterization of an eyas as a nestling makes it the best choice in this context.