What does the term 'gleam' refer to in falconry?

In the context of falconry, the term 'gleam' refers specifically to a coating found in castings. This is the remnant of the undigested parts of prey that raptors expel, such as bones, fur, and feathers. The 'gleam' is significant in falconry as it can indicate the health of the bird, as well as its diet and feeding habits. Observing the condition and content of castings can help falconers assess whether their birds are receiving appropriate nutrition and if they are healthy overall. Understanding the significance of castings is pivotal for effective bird care and management in the practice of falconry.

While the other options present different concepts related to birds and falconry, they do not correctly define the term 'gleam' as it pertains specifically to the context of castings.