A pellet or casting found under your bird's perch is most accurately described as:

A pellet, or casting, found under your bird's perch is best understood as the indigestible portion of the raptor's last meal. Raptors, such as hawks and owls, consume their prey whole or in large pieces, and their digestive systems process the digestible parts of the food. However, bones, fur, feathers, and other non-digestible materials cannot be broken down by their digestive enzymes.

The bird expels these materials in the form of a pellet after digestion. This is a natural and essential aspect of a raptor's life cycle and is indicative of a healthy bird. A well-formed pellet can provide insights into the bird's diet and health. Recognizing this behavior is important for falconers, as it helps monitor their bird’s wellbeing and feeding habits.

Considering other options, references to diseases or infections do not pertain to the nature of a pellet. They misrepresent the biological process involved in the raptor's digestion, focusing instead on health issues that are not relevant to pellet formation. In a falconry context, understanding the nature of castings is vital for managing a raptor's health and ensuring it is receiving appropriate nutrition.