Neck-breaking, disembowelling, constricting and fishing - the violent world of raptors

The role of Velociraptor's infamous claw has received much attention from scientists ever since they clicked their way across a movie kitchen. In comparison, the formidable claws of living raptors (birds of prey) have received little attention. Eagles, hawks, falcons and owls are some of the most widespread and well-liked of all birds. They are superb hunters and even though it's always been suspected that they use their talons to kill, we know amazingly little about their techniques.

Denver Fowler (great name for an ornithologist) and colleagues from Montana State University have changed all of that, through the first comprehensive study of raptor feet. Their work reveals that these apparently familiar birds use a striking variety of killing strategies including some rather grisly ones. Some raptors use their talons to attack with high-speed killing blows, and others suffocate their prey to death in constricting fists. Some give their victims a merciful death by broken neck, but others eat their victims alive after slashing them open.

Fowler unveiled this macabre and violent world by measuring and photographing the talons and feet of over 34 birds from 24 raptor species. He also considered over 170 video sequences of raptor attacks, as well as many published accounts of predatory behaviour. By linking shape and size to actual behaviour, he managed to document the wide range o uses that curved claws can be put to.

Fowler found that raptors use their talons in a similar way when tackling small prey. Their feet are used to imprison their prey, with talons deployed as a cage rather than as weapons. Falcons use a notched ridge on their upper beak - the 'tomial tooth' - to sever the spine or crush the head, while owls sometimes break their prey's neck with a swift twist. Accipitrids (eagles, hawks, kites, harriers and the like) have weaker bites than falcons and no 'teeth' - they use their feet to constrict their prey, cutting off its air supply much like a python uses its coils.

Owls tend to ambush their prey on the ground and their chances of landing a killing blow are slimmer. So they have evolved feet that are better at restraining struggling prey. Their toes are shorter and stronger than those of other raptors, and one of them can swivel backwards so that the owl can grip with two pairs of opposing toes. That makes them powerful constrictors, capable of crushing small animals in a suffocating 'fist'. It also means that they specialise on smaller victims, and rarely tackle the larger prey that falcons and eagles do.

Larger prey simply can't be enclosed by feet, so falcons and accipitrids use different strategies when their meals get bigger. They'll stand on top of the animal, pinning it down with its full body weight. If the prey tires and stops moving, it's all over, but death only comes after a "prolonged and bloody scenario". The raptor plucks any fur or feathers, especially around the belly, and starts to feed, often using the large second claw to slash open the body and expose the innards. Grimly, the prey is sometimes still alive when this happens - it's only the ensuing blood loss or organ failure that finishes them.

Accipitrids are more likely to consume their victims alive, and to subdue any final struggles, they have two unusually massive talons on the first and second toes that provide extra grip. These piercing anchors give them the ability to cope with the most powerful of struggling prey, and it's no coincidence that the accipitrids include the mightiest of the raptors.

Falcons, on the other hand, often kill their prey with a neck-break to avoid a protracted struggle, so they can afford to have smaller talons. Their prey is also more likely to be seriously injured already. Falcons specialise in high-speed assaults, striking their prey with rapid dives and swoops that can potentially cripple them or even kill them outright. 

Aside from size, the type of prey doesn't have much of an impact on the shape and proportions of the raptor foot. The only exceptions are those species that are specialist fishermen, such as the osprey, the bald eagle and the fishing owl. Their talons are like fishhooks - exceptionally large, highly curved and equal in size on all the four toes.

Considering how popular and common the birds of prey are, it's amazing that a study like this has never been attempted before. Even now, Fowler sees it as just the beginning. There's no reason why the same sort of analysis shouldn't apply to meat-eating dinosaurs, the extinct relatives of today's raptors, and that will form the plot of his sequel study.

Reference: Fowler, D., Freedman, E., & Scannella, J. (2009). Predatory Functional Morphology in Raptors: Interdigital Variation in Talon Size Is Related to Prey Restraint and Immobilisation Technique PLoS ONE, 4 (11) DOI: 10.1371/journal.pone.0007999

More on birds of prey: 

• South African wildlife - Martial eagle
• Owls use poo and plumage to mark their territories