Wednesday, November 25, 2009

How Raptor Talons Fit Their Prey

Most birders learn through field guides that different raptor groups have recognizable body shapes adapted to the way they hunt. Accipiters, for example, have short rounded wings and long tails to facilitate short pursuits through close quarters. This is, in fact, a key to identifying many raptors in the field. Close study of raptors reveals even more subtle anatomical differences. A newly published article in PLoS ONE relates differences in the shapes of raptor talons to how they kill their prey.

The first thing to understand is that raptors do not usually kill their prey by wounding them with their talons or beaks. Instead, most raptors kill their prey by constriction – squeezing their prey so tightly that death comes by asphyxiation. In a minority of cases, this squeezing motion may cause fatal injury if a talon pierces a vital organ. In other cases, a raptor may start dismembering and eating its prey before the prey is fully dead. In addition, falcons may attempt to break the spinal cords of their prey using a special tooth-like projection on their beaks. Here is an example of such a "tooth" on a Peregrine Falcon. The Merlin at right has another such "tooth." To my knowledge, no buteo or accipiter has such a feature.

Given the prominent role that a raptor's feet play in seizing prey, it makes sense that the shape of their feet might vary with how a raptor uses them. This is in fact the result reached by a team of graduate students after they photographed and measured the feet of hundreds of bird specimens, both raptors and non-raptors. (Most specimens were held by Montana State University; others were from the American Museum of Natural History.) The most significant differences in claw shape were among raptor families and distinguished raptors from non-raptors.

Figure 1. Feet of representative raptors. (A) Accipitridae: goshawk; (B), Accipitridae: red-tailed hawk; (C) Falconidae: peregrine falcon; (D) Strigiformes: great grey owl; (E) Pandionidae: osprey. Source: PLoS ONE.

Here are the significant characteristics of each raptor family:
  • Accipitridae (including accipiters, buteos, and eagles) have first and second talons (D-I and D-II) that are exceptionally large in proportion to their other talons.
  • Falconidae have talons that are more equal in size than Accipitridae but D-I and D-II continue to be proportionately larger.
  • Strigiformes have large talons that are more equal in size mounted on short strong toes.
  • Pandionidae have long and exceptionally curved talons. The largest talon, D-IV, can rotate so that an osprey can grip its prey with two talons on one side and two on the other.
Having a D-II talon that is similar in size or larger than a D-III talon separates all raptor families from non-raptors, which have a significantly larger D-III.

Table 1. Mean and standard deviation of claw sizes (outer arc lengths) of D-I, II, and III relative to D-IV, and D-II relative to D-III. Source: PLoS ONE.

The authors argue that the best explanation for the variation in talon proportions is hunting technique. Falcons can afford to have proportionately shorter talons since they strike their prey at high speed while airborne; this strike is often sufficient to kill or seriously injure their prey. If not, they have the option of using their false tooth to break the neck of their prey. Owls have short toes and long talons that give them maximum leverage to constrict small prey. Accipitrids, by contrast, have neither the powerful feet of owls nor the high-speed hunting techniques of falcons. Instead, they can either constrict small prey or use their longer first and second talons to their advantage against larger prey. These long talons allow them to grip large prey even as the prey struggles to escape. Once the prey is subdued, the hawk can begin plucking and eating, regardless of whether the prey is dead. Foot shape gives Accipitrid hawks more options for what they can capture and eat.
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