The term 'Early Whirlybirds' refers to the first generation of helicopters, which were developed in the early 20th century. The most famous of these was the Giro, a five-letter word that is one of the solutions to this crossword. The Giro was developed by the German engineer Henrich Focke in 1936 and was the first successful helicopter to take off, hover, and land vertically. It was also the first helicopter to be mass-produced and used for military purposes.