Birds are a group of endothermic vertebrates characterized by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong, lightweight skeleton. With approximately ten thousand species, birds rank as the class of tetrapods with the most living species, with more than half of those being passerines.
Collision with wind turbine blades can kill birds, though the impact of individual deaths on the greater population varies greatly based on species. Research continues to explore mitigation such as operation limitations, siting location, and lighting selection. Bird collisions with offshore wind turbines present an additional challenge of being unable to count bodies to determine the scale of mortality. Marine renewable energy devices likely pose little threat to diving birds colliding with tidal turbines, or birds colliding with floating wave energy devices. Birds are classified in the following categories: