Fret marks on the train of an eyass hawk are often caused by which of the following?

Fret marks on the train of an eyass hawk, which refer to the feather damage or wear often seen on young birds, are primarily associated with a period of inadequate parental care. When the parents fail to provide sufficient food during critical growth phases, the young hawks may exhibit signs of stress and malnutrition. This can lead to physical manifestations such as fret marks as the bird's body is not receiving the nutritional support needed for proper feather development.

While the other choices refer to potential sources of stress or damage to the hawks, they do not specifically correlate to the primary cause of fret marks. The stress from being taken from the nest can lead to a host of issues, but it may not consistently result in fret marks as prominently as food scarcity during the early developmental stage. Similarly, external parasites can cause discomfort and health issues in birds, but their impact on feather health differs from that caused by inadequate food. By focusing on parental care and nutrition, one can see why a lack of adequate food directly influences the physical condition of the hawks, leading to the appearance of fret marks on their train.