The Dromaeosaurs were a group of small, feathered dinosaurs closely related to the birds. They are commonly referred to as 'raptors' on account of an enlarged claw on each foot which was held clear of the ground when walking and is generally assumed to have been a weapon; this claw resembles that of a bird of prey, which are also referred to as 'raptors'. This claw was also present in the other group closely related to birds, the Troodontids, though in these it was not as prominent. The Troodontids were apparently adapted for pursuit of prey, placing additional demands upon their limbs that the Dromaeosaurs, thought to have been ambush predators, lacked. Thus the foot of a Troodontid is a compromise, that of a Dromaeosaur a more specialized tool.
A graphite drawing of Velociraptor, a typical Dromaeosaur, by artist Matt Martyniuk. (Note: this is probably a more realistic interpretation of this pheasant-sized dinosaur than the things in Jurassic Park).
The Dromaeosaurs, Troodontids and Birds together are referred to as the Paraves; all three groups show certain similarities, that were presumably present in the common ancestor, including advanced plumage, wings with flight feathers and prominent hooked claws on their feet. The presence of wings with flight feathers in Dromaeosaurs and Troodontids is interesting, as neither of these groups appears to have been able to fly, implying that these strictures predated that ability in birds. Unraveling what these structures were used for in Dromaeosaurs and Troodontids would therefore provide useful insight into the evolution of the first birds.
The use of the enlarged claw by Dromaeosaurs and Troodontids has been a subject of discussion for decades. It has been widely speculated that this was used as a slashing weapon, enabling the dinosaurs to dispatch prey with one or more lethal kicks. Another theory was that the claws were used to leap onto and even climb up much larger prey in pack attacks by the small dinosaurs.
A paper by a team lead by Denver Fowler of the Musuem of the Rockies and the Department of Earth Sciences at Montana State University, published in the journal PLoS ONE on 14 December 2011, examines the use of the raptorial claw in the small Dromeosaur Dienonychus, using modern Raptors as an ecological model.
Modern Raptors do not use their claws to slash at their prey (some do kick, but they hold their claws clear when they do so), nor do they use their claws to overcome prey larger than themselves; they do not generally attack animals larger than themselves at all. The preferred method of attack by birds of prey is to hit it very hard with either their feet (not their claws) or their beak, killing it straight away, then use the claws to carry it off. Where this is not possible they hold prey down with their claws, while they dispatch it with their beaks. This is the strategy that Fowler et al. believe was adopted by Dienonychus. When holding down struggling prey, modern Raptors use their wings to stabilize themselves, flapping vigorously to maintain their balance. Many also 'mantle' their prey, using their wings to cover it while they eat it, so that it is hidden from rivals or larger predators; some do this with still living prey, which is believed to deprive the prey of sight, making it harder to escape.
Both of these tactics would have worked perfectly well with wings that were not capable of flying, which provides a plausible explanation for the evolution of wings, flight feathers and flapping muscles in early Paravians, prior to the evolution of flight. Thus Fowler et al. believe that birds were pre-adapted to flight before they took to the air.
Illustration from Fowler et al. (2011) showing feeding in Dienonychus as they interpret it. (A) Grasping foot holds onto prey. (B) Large claws used to maintain grip on prey. (C) Predators bodyweight pins down victim. (D) Elongated tail acts as balance. (E) Rear of foot held low to help restrain victim. (F) Wings flapped to maintain stable posture. (G) Arms encircle ('mantle') prey to prevent prey seeking escape route. (H) Head reaches between feet, tearing of strips of flesh.