Scientists have unraveled how the talons of birds of prey have evolved to help them kill victims.
The killing techniques range from dismemberment to squeezing prey to death. The findings could help researchers understand how the claws of extinct dinosaurs and birds helped them hunt.
As common as raptors are, the specifics of how they capture prey often remain poorly understood. Even with modern technology, actually closely witnessing how birds of prey deal with victims remains largely a matter of luck. As such, surprisingly little was known about how talons are employed during feeding.
"One famous ornithologist from the 1920s said that he had only personally observed close up about 30 raptor kills his entire career," said researcher Denver Fowler, a paleobiologist at the Museum of the Rockies in Bozeman, Montana.
In the most thorough study up to now of raptor talons, scientists took detailed measurements of length and curvature of the claws from 24 bird of prey species and compared them with nearly 200 videos of attacks.
"In recent years we have been lucky enough that members of the public have been posting videos of raptor kills on YouTube — just things that they happened upon while out for a walk, or looked out into their garden to see amidst a blizzard of pigeon feathers a sparrowhawk furiously plucking away at its kill," Fowler said. "This is fantastic data."
Consuming victims while still alive
When it came to accipitrids, which include eagles and hawks, the researchers found they had giant claws on the first and second toes of each foot, evolved primarily to restrain large struggling prey. The raptors then often consumes victims while they are still alive, so long as they do not protest too vigorously — the prey eventually succumbs to massive blood loss or organ failure, incurred during dismemberment.
In contrast, falcons have only modest talons on each toe, and rely more strongly on the high velocity of their strikes to disable their prey, having evolved a "tooth" on their beaks to then aid in severing the spinal cords or crushing the heads of their prey.
The osprey has enlarged, highly curved talons on each toe. These are adaptations for catching fish, also seen to a lesser extent in fishing raptors such as the bald eagle.
Owls have enlarged talons on each short, robust toe, and each claw is less curved than seen in other birds of prey, part of a suite of adaptations to maximize grip strength. This enables them to squeeze victims to death — they specialize on small prey they usually swallow whole.
A box of claws
When the researchers began their study, they thought investigating a box of raptor feet stored at the museum would prove to be a small project they would complete over spring break, but it took months longer and became much more involved than they expected.
"My favorite, the goshawk, had a humungous claw on its second toe," Fowler recalled. "This was interesting to us, as dinosaurs like Velociraptor and Deinonychus also have huge claws on their second toe, so we wondered, 'What do these raptors do with this claw?' Well, after searching through the scientific literature, it turns out that no one had tried to answer that question before. In fact, no one had even noted that accipitrids — hawks and eagles — even had such a strangely oversized claw, so this is what prompted us to go and find out."
"Claws are very much understudied as a whole, which is strange because they represent the business end of the animal — they have direct interaction with pretty much everything the animal is doing," he added.
The videos the researchers found online proved extraordinarily helpful.
"We can watch them over and over again, making sure all that data is recorded, without missing anything," he explained. "Furthermore, often these videos are very long: up to seven to 10 minutes, so we see the full spread of behaviors. The videos are often unintentionally funny — either the music some people set to the video, or what is going on in the background, as the person calmly films the animal carnage outdoors while their kids are flinging toys across the room, or they are trying to have a conversation about sales with their boss on the phone."
"On a more serious note, these behavior videos represent a good way the general public can contribute to primary scientific research," he added. "There are some limitations of course, but generally it's a great resource that researchers are only just starting to use."
The researchers are now using the lessons they learned with birds of prey with research on the claws of extinct dinosaurs, including carnivorous theropods such as Velociraptor and Deinonychus and giant herbivorous sauropods such as Diplodocus.
"It's often helpful to look at modern species and make comparisons to how dinosaurs may have behaved," said researcher Elizabeth Freedman at the Museum of the Rockies.
Fowler, Freedman and their colleague John Scannella detailed their findings online November 25 in the journal PLoS ONE.