What are fret marks in a bird's feathers?

Fret marks in a bird's feathers are specifically lines or markings that indicate physical stress or nutritional deficiencies, typically resulting from starvation or disease. These marks are visible alterations in the feather structure and coloration, which reflect the bird's health status over time. When a bird experiences poor nutrition or health issues, its feather growth may become irregular, leading to these distinguishing fret marks. This phenomenon is a way to visually assess the well-being of a bird, as healthy feathers should appear unblemished and robust. Understanding fret marks is important for falconers and bird keepers as they offer insight into the care and condition of the birds. Other options, while they reference feather characteristics, do not accurately describe the specific nature of fret marks.