Why do white swans and dark coots have differently colored body lice?

The variation in the coloration of body lice found on different birds is an adaptation to their specific ecological niches on the host's body, serving as a form of concealing coloration.1

By matching the color of the host's plumage, these lice are better able to blend into their environment, which helps them avoid detection and removal by the host during grooming and preening. Consequently, birds with white plumage, such as swans, host white body lice, while dark-plumaged birds like coots host lice that are almost black. This background matching is a common evolutionary strategy used by many organisms to achieve concealment from potential threats or selective agents.2134