What does "hard penned" mean in falconry?

"Hard penned" refers to a stage in a raptor's development when its eyass feathers, which are the young feathers of a fledgling bird, are fully grown and have begun to harden. During this stage, the feathers have reached maturity and are considered strong enough to support the bird in flight. This is a crucial phase in a bird's growth, as well-developed feathers are essential for effective flying and hunting.

The other options illustrate different concepts related to raptor development or behavior. For example, the notion of a raptor not being fully fledged signifies that it hasn’t developed the necessary flight feathers yet, while the beginning of the moulting process indicates a time when a bird will shed old feathers for new ones, not necessarily linked to the strength or state of the current feathers. Additionally, the age at which a bird starts to fly pertains to a different developmental stage. Thus, understanding that "hard penned" specifically indicates fully grown and matured feathers is key in falconry practices and terminology.