External parasites such as mites and lice are known to cause which of the following?

External parasites like mites and lice significantly impact the health and well-being of birds. They cause irritation to the skin and feathers, which can lead to feather damage and the potential for chronic feather loss. As the parasites feed on the bird's blood, they can lead to anemia, a condition resulting from a reduced number of red blood cells. This combination of issues illustrates how detrimental these parasites can be, causing not just discomfort but also serious health complications if left untreated.

The alternative options do not align with the typical effects of external parasites. For example, increased metabolism is not a direct consequence of infestations; rather, it involves more systemic physiological changes related to energy use. Chronic feather loss is a potential outcome but is a specific effect that falls under the broader umbrella of irritation and damage caused by parasites. Improved flying ability would not be associated with infestation, as the presence of parasites generally leads to decreased health and performance.