Endoscopy, which is the internal examination of body organs, joints and cavities using an endoscope, first began in the early 1900s when medical doctors developed telescopic systems that could illuminate the internal cavities of human patients. In avian medicine, rigid endoscopes were first used regularly in the 1970s to determine the sex of parrots, many of which are sexually monomorphic (i.e. there are no distinguishing external physical differences between the sexes). Endoscopic examination enabled the identification of gonads, located between the lung and kidney, leading to a rapid and accurate identification of sex which revolutionised the breeding of many species. It was quickly realised that the unique avian anatomy, with its air-filled body cavities, made birds an ideal candidate for endoscopy. Insufflation is not required, reducing the costs and technical difficulty involved. New techniques and instruments have since been developed to take advantage of this, and endoscopy is now a widely utilised diagnostic tool in avian medicine around the world.