Use of English PRO

The swifts are a family of highly aerial birds (0) THAT people think belong to the same family (1) ............ swallows. This is not true, because they are, (2) ............  fact, more closely related to hummingbirds. It cannot be denied that there are some strong similarities (3) ............ swifts and swallows. This is due to some coincidences in evolution, (4) ............ reflect similar lifestyles based on catching insects in flight. Although there are many candidates (5) ............ the title of the fastest birds in flight, the winner would definitely be the swift. Scientists have investigated this, and proved that the larger species are amongst (6) ............ fastest fliers in the animal kingdom, and the common swift can cover enormous distances in a single year. This ability is (7) ............ to the fact that their bodies are perfectly designed to fly fast, because their wings have large wingtip bones compared to other birds, and they have perfectly aerodynamic bodies. This allows (8) ............ to alter the shape and area of their wings while in flight, and to take advantage of any wind currents they might find.