The science of studying birds or ornithology most likely dates back to human’s fascination (21) _______ flight. In the United States, some of the earliest recorded observations can be linked to the American naturalist and painter John James Audubon, in the early 1800s, tied threads to birds’ legs as a means to identify individuals. A century later, scientists affiliated with the Smithsonian Institution used metal bands as a means of identifying individual herons. If and when the herons were recaptured, the bands provided data on the individual birds. This method is known as mark and recapture. It is still used to (22) _______ the abundance of a population as well as track the movement of individuals.

Modern technology has provided an ever-increasing number of ways to gather data on bird populations and individuals. Electronic tracking devices were once limited to the larger breeds of birds like Trumpeter Swans and Wandering Albatrosses (23) _______ the size of transmitters. The rule of thumb was that birds not be outfitted with transmitters (24) _______ exceeded 3-5% of the bird’s body weight. Researchers needed some form of transportation to carry the cumbersome equipment. The emergence of transmitters that utilised the ARGOS satellite network in 1984 revolutionised ornithological studies. More recently, the development of lightweight sensors like geolocators that record light (25) _______ and the even smaller nanotags that emit radio signals have led to the automation of radio telemetry.

The science of studying birds or ornithology most likely dates back to human’s fascination (21) _______ flight.