Most bird species in the United Kingdom host at least one species of louse that is uniquely adapted to them, with these parasites unable to survive on other bird hosts. For example, the house sparrow carries up to three distinct louse species, while pheasants can support a more diverse community of around a dozen different lice.

Avian lice are characterized by their flattened, wingless bodies and have evolved in close association with their bird hosts. These lice primarily consume feathers and dead skin, with their body structures and mouthparts specialized to suit the specific plumage of their hosts. In contrast, louse flies, which are larger insects, engage in blood feeding. Among these, some species are generalists, capable of living on multiple bird species, whereas others show a high degree of host specificity. The swift louse fly, for instance, is known to overwinter in its larval stage within the old nests of its particular host species.

In addition to lice and louse flies, birds also carry various mite species, including bird mites and feather mites. Unlike lice and flies, these mites belong to the arachnid class. These tiny parasites further contribute to the complex micro-ecosystem found on wild birds across the UK, reflecting a wide range of coevolutionary relationships between parasite and host.