Although it was discovered in the 19th century that birds were closely related to dinosaurs, the current scientific consensus is that birds were, and always have been dinosaurs themselves. Fossil evidence demonstrates similarities between birds and other feathered dinosaurs, including hollow bones, nest building and similar brooding behaviours. Although the dinosaurian lineage of birds is largely undisputed, the evolution of powered flight in birds is still debated. Two theories of flight in birds are the “ground-up” theory, and the “trees-down” theory. Ground-up theorists suggest birds evolved powered flight from ground dwelling dinosaurs, and trees-down theorists suggest birds evolved from tree dwelling, gliding dinosaurs. Further research is required to conclusively verify the process in which birds evolved powered flight.