After taking a nestling raptor, it's crucial to inspect the ears for which of the following?

When inspecting the ears of a nestling raptor after taking it in, the primary concern is checking for maggots. This is an essential part of ensuring the health of the bird, as maggots can infest the ears and lead to severe health issues, including infection. A nestling, being young and potentially weaker, is more susceptible to infestations that can significantly impact its overall health and survival.

Identifying the presence of maggots allows handlers to take immediate action, such as seeking veterinary assistance, which is crucial for the successful rehabilitation of the bird.

Other aspects like feather growth, hearing ability, and castings might be considered during assessments of the raptor’s overall condition, but they do not directly relate to an urgent health threat like maggot infestation. Thus, the inspection for maggots is critical and should be a priority when caring for a nestling raptor.