What causes fret marks on the feathers of a new bird?

Fret marks on the feathers of a new bird signify a developmental issue where the feathers have not grown properly, often due to either arrested feather development or stress experienced during the feather growth phase. Arrested feather development occurs when the growth of feathers is halted, which can result in irregularities and imperfections on the feather surface known as fret marks. This can be caused by various factors, including nutritional deficiencies, environmental stresses, or other health issues.

Additionally, stress can also contribute to the appearance of fret marks. Stress factors might include changes in the bird’s environment, handling, or inadequate living conditions, which can disrupt normal feather growth. When a bird is under stress, it may not be able to form its feathers as it normally would, resulting in these observable fret marks.

Thus, the correct answer is a combination of both arrested feather development and stress leading to fret marks, which makes the answer that includes both elements the most accurate choice.