What do fret marks on a bird's feathers indicate?

Fret marks on a bird's feathers signify both arrested feather development and stress, which is why this choice is the correct one.

When a bird experiences stress, whether due to environmental factors, health issues, or inadequate nutrition, it can affect the growth and condition of its feathers. Arrested feather development occurs when the feathers do not grow properly, leading to deformities or irregularities in their structure. Fret marks are often associated with these issues, as they can indicate that the feathers have not had a healthy growth pattern, reflecting both the stress the bird is under as well as developmental problems with the feathers themselves.

Understanding the presence of fret marks is crucial for those involved in falconry and avian care, as it provides important insights into the bird's well-being and the necessity for potential interventions or changes in care practices. Recognizing these signs can ultimately contribute to more effective management and rehabilitation of birds in falconry and other settings.