This theory component begins with an overview of the avian taxonomy from classifications based on bird phylogeny to speciation mechanisms like dispersal. In a biogeographical perspective, birds are considered as ecological model organisms which can be divided into landbirds and seabirds. A focus on the Palaearctic region clarifies the subdivision of landbirds into continental and island birds. Further focusing on Macaronesia and especially Madeira, highlights the unicity of endemics. Seabirds of the Madeira Archipelago are documented in more detail in corresponding practical component.