What is a common effect of external parasites on captive raptors?

The common effect of external parasites on captive raptors is feather damage. This occurs because parasites, such as mites or lice, can infest the feathers and skin of the bird, leading to various issues. The parasites may cause irritation that triggers the bird to preen excessively, leading to broken or damaged feathers. Additionally, the parasites can directly harm the feather structure as they feed on skin or feather tissue, ultimately affecting the bird's ability to fly, thermoregulate, and maintain its overall health. Thus, feather damage is a significant concern for the welfare of raptors in captivity, as it can impact their quality of life and ability to engage in natural behaviors.

The other options do not accurately reflect the typical effects of external parasites on captive raptors. Greater social behavior is not a direct consequence of parasite infestation, nor is weight gain typically associated with parasites, which generally lead to stress and weight loss instead. Color enhancement is also not a common effect; rather, the health and plumage condition of raptors may deteriorate with the presence of parasites.