Diego Sustaita is fascinated by raptors. ‘Most people are keen on raptors because of their flight capabilities, but for me its their shear carnivory,’ says Sustaita. According to Sustaita, from the University of Connecticut, accipiter hawks tend to subdue and suffocate their prey with their talons, while falcons catch victims with their feet and sever the spinal cord with a neat bite to the neck. ‘There have been studies describing the morphological characteristics that are presumed to be associated with differences in the way they attack, capture, kill and ultimately feed on their prey,’ explains Sustaita, who adds, ‘This study was an attempt to identify the performance or functional implications of these morphological characteristics’. Working with his Masters thesis supervisor, Fritz Hertel from California State University, Northridge, USA, Sustaita began measuring the forces generated by the beaks and talons of accipiter hawks and falcons as they attacked force transducers (p. 2617).

‘Gaining access to the raptors was the most difficult part of this study,’ remembers Sustaita. Working with raptor rehabilitation organizations in California and Hawkwatch International in New Mexico, the duo had access to four species of falcon and two accipiter species. Designing a force transducer built from strain gauges and a crescent wrench, Hertel restrained the birds while Sustaita offered them the force transducer to bite on or grip with their talons. Sustaita admits that this was sometimes risky and says, ‘It helps if you trust the person holding the bird,’ and adds that sometimes his fingers got nipped but says, ‘It was worth it to get the data’.

Analysing the data, Sustaita found that the falcons bit with greater force than the hawks, while the hawks gripped tighter than the falcons. However, the grip forces produced by the birds were consistently lower than Sustaita and Hertel had predicted based on the bird's musculature and smaller falcons' grips were much weaker than they had predicted. ‘Smaller falcons may be capable of producing higher grip forces, but they tend not to as far as we can measure,’ says Sustaita.

The duo also compared the raptors' bite forces with those of other birds and mammals and was surprised to find that the raptors' bites were at the weaker end of the spectrum. ‘Based on data that have been published for seed eaters these forces are comparatively low, absolutely and relative to body size, which may be due to real functional differences,’ says Sustaita. ‘For example, some of the birds adapted to crush seeds have skulls built to deliver force and the forces that raptors may contend with in nature may be different because raptors are dealing with skin tissue, muscle and bone. There might not be such strong selection for crushing as for ripping and tearing, and these capabilities involve a different suite of modifications that involve not only muscular strength but also sharpness of the beak and talons,’ explains Sustaita. ‘There is a whole list of things that might explain the low bite force away and we would need to look at a greater diversity of birds to get at the heart of that,’ he adds.

In vivo bite and grip forces, morphology and prey-killing behavior of North American accipiters (Accipitridae) and falcons (Falconidae)
J. Exp. Biol.