External parasites typically impact which of the following?

External parasites primarily affect the plumage of birds, as they often reside on the surface of the skin or feathers. These parasites can cause significant damage by feeding on the blood or tissue of the host, leading to issues such as feather loss, irritation, and even infection. This can compromise the bird's ability to maintain insulation and waterproofing of its feathers, ultimately affecting its overall health and survival.

In contrast, the other options stem from internal functions or different aspects that external parasites typically do not directly impact. For example, while internal parasites may affect the digestive system, it is external parasites that are more concerned with external surfaces like plumage. Similarly, the bottom of the feet and the nervous system are not the primary areas of concern when discussing the effects of external parasites. Therefore, the correct focus on plumage highlights the distinctive impact of external parasites on birds.