The Aerodynamics of Diving Peregrines

Falcons are well known for their speed and agility. They prey primarily on other birds and generally catch their prey on the wing. (Small falcons like kestrels often eat large insects or small rodents in addition to birds.) All falcons are sleek with pointed wings and capable of fast flight, but Peregrine Falcons in particular are noted for their high-speed dives. I have been fortunate to witness such dives on several occasions, and they remain impressive every time. A recent paper in PLoS ONE examines the aerodynamics of diving Peregrine Falcons through a series of experiments.

A typical sequence of images taken during a dive at the selected points. Source: PLoS ONE

Peregrines were trained by falconers to dive in front of the south wall of a dam to allow high-speed cameras to capture their flight shape and trajectory precisely. The resulting photographs were then used to build a model for wind tunnel testing.

Flow visualization on the surface of the falcon model (A) and the falcon at the same flight position (B). Source: PLoS ONE

At the top of the dive, the falcon initially accelerated and then flew at constant speed with its wings tucked into a diamond shape. During this dive, the falcon reached a maximum speed of about 50 mph. As it approached its intended prey, it spread its wings to generate lift and rapidly decelerated, finally pulling out of its dive and landing. The wind tunnel tests showed several regions where the airflow was uneven around the model. At one such region, near where the trailing edge of the wing is tucked close to the body, photographs showed feathers popped up to smooth the airflow.

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Benjamin Ponitz, Anke Schmitz, Dominik Fischer, Horst Bleckmann, & Christoph Brücker (2014). Diving-Flight Aerodynamics of a Peregrine Falcon (Falco peregrinus) PLoS ONE DOI: 10.1371/journal.pone.0086506

Review: The Crossley ID Guide: Raptors

In 2011, Richard Crossley introduced a new format for photographic field guides with his much-heralded Crossley ID Guide: Eastern Birds. Crossley tried to overcome the limitations of traditional photographic field guides by presenting each species in an array of plumages all crowded into a single plate that depicts each bird's typical habitat. The intent is to mimic how birds appear in the wild. While no printed guide can recreate actual field conditions, Crossley's guides are about as close as one can get.

Since then, Crossley has worked to expand his ID guide series into other geographic regions. A guide to the birds of Britain and Ireland is already published, and one for western birds is planned. Last year I received another installment in the series, The Crossley ID Guide: Raptors. The new guide is coauthored with Jerry Liguori and Brian Sullivan. Like the guide to eastern birds, this raptor guide features birds as they appear in the field: perched, flying, interacting with other birds, in various plumages, etc. Once again, the plates show a raptor's typical habitat with numerous bird photos added into the background with the help of Photoshop.

Like its predecessor, The Crossley ID Guide: Raptors features beautiful photography. I initially enjoyed just flipping through and looking at the plates. Since hawk migration is an emphasis in this guide, the backdrops are often coastal or mountaintop hawk watches, some of which are spectacular. I especially liked the plate showing how vultures might look from the top of the Cape May Lighthouse.

The focus on a single family allows even more detail than was possible in the guide to eastern birds. Most species are spread over multiple pages, with color morphs or regional plumages receiving separate attention. As in the first guide, the photos of individual birds are inserted into backdrops depicted a typical habitat. The raptor guide takes this a step further by showing separate plates for hawk watches and habitats where the hawk typically resides. This is useful because there are different identification challenges for raptors that are just passing through (especially at mountaintop hawk watches) than ones that are perched or hunting in a field. Some plates also mimic difficult lighting conditions, like overcast skies or the golden hour, and plates show both close and distant raptors.

New features introduced in the raptor guide are comparison plates and quizzes. These plates show groupings of similar species in typical habitat with individual birds numbered and with answers at the back of the book. These features ought to be useful for someone who is new to hawk study or for learning unfamiliar species. Another difference between this guide and the guide to eastern birds is the inclusion of detailed species accounts with range maps at the end of the guide.

While it is smaller than its predecessor, The Crossley ID Guide: Raptors is a little too large to carry as a field guide on most occasions. However, since hawk watching is a matter of waiting in a location for hawks to pass by, it could be used as a reference at a hawk watch (provided that getting to the hawk watch does not require a hike, as some mountaintop hawk watches do). The Crossley ID guides are intended for home study, mainly for preparation to know what to look for. On numerous occasions since I received Crossley ID Guide: Eastern Birds I have made use of it to help figure out difficult identifications.

Photoshopping aside, The Crossley ID Guide: Raptors occupies a similar niche as Jerry Ligouri's other guides, Hawks from Every Angle and Hawks at a Distance. All three guides attempt to show readers how raptors look under less-than-ideal field conditions: at a weird angle, backlit, at a distance, disappearing behind some trees, and so on. All three guides emphasize shape and flight style as a way of getting around the difficulty. I think the new guide actually improves on Hawks from Every Angle and Hawks at a Distance by showing those angles in their habitat contexts.

From reading reviews of Crossley's first guide and from conversations with other birders, I get the sense that some birders find Crossley's style far too cluttered. The new raptor guide is probably not for them since The Crossley ID Guide: Raptors shares much in common with its predecessor. However, if you like raptors and enjoyed the first Crossley guide, as I do, you will probably like The Crossley ID Guide: Raptors.

Crested Caracara in New Jersey

Last Saturday while I was off birding elsewhere, Vincent Nichnadowicz discovered a Crested Caracara in a field at Grover Farm, a historic preservation site in West Windsor, New Jersey. The bird stuck around, and since then numerous birders have gone to see and document it. Yesterday morning I finally had a chance to see it. Getting there took more than twice as long as it should have, as traffic on US 130 was backed up for miles because of a truck accident that shut down the New Jersey Turnpike. By the time we got to the site, the caracara had already left its overnight roost on a utility pole and was foraging on the ground in a depression on the far side of the field. Its head was just barely visible above the alfalfa.

Eventually it did move around a bit more as it foraged. Several times it stopped walking to pick at something on the ground with its bill. Most of its body became visible when it walked up on the small rise in front of the depression, which allowed for more complete views of the bird and (relatively) better photographs. I think the one above is the best of the ones I took. In it, you can see the caracara's distinctive facial features — the orange base of its hooked bill, the black cap and crest, and the white face and neck. The tail banding is also visible. In the hour and a half that I watched it, I never saw it fly. This was the 300th bird species I have seen in New Jersey. Several other birders were present yesterday morning, including two who had driven down from New York City.

Crested Caracara is not a regular visitor to New Jersey. This species is normally found much further south, in southern Florida, the Caribbean, Texas, and south through Mexico, Central America, and northern South America. It prefers arid or semiarid open habitats, such as brushland or agricultural areas. There are two other caracara species from which it was recently split: Southern Crested Caracara and Guadelupe Caracara (the latter now extinct). Caracaras are placed within the family Falconidae. Unlike other falcons, they run after prey on the ground and scavenge instead of pursuing birds in the air. Caracaras will feed on carrion or garbage if those are available; otherwise they prey on a variety of small animals like rodents, amphibians, reptiles, and invertebrates. When they hunt on foot, they use their feet to turn over objects that might conceal prey.

This is not the first time a Crested Caracara has been reported in New Jersey, but it is still a potential first state record. Bill Boyle's Birds of New Jersey (which I reviewed here) mentions two previous records, neither of which was accepted by the New Jersey Bird Records Committee. The first was in Colonia in 1976 and was so tame that birders assumed it had escaped from captivity. The second was seen at Sandy Hook on May 5, 2007. In both cases, the NJBRC rejected the records because they were unsure if the birds were actually wild. Historically, Crested Caracara was considered a sedentary species, unlikely to travel much beyond its normal range. However, as Nate Swick and various commenters discuss on the ABA Blog, there have been enough sightings of the species across the continent in recent years to suggest that caracaras may be more prone to vagrancy than previously thought. Some local birders are speculating that this individual was blown north by the remnants of Hurricane Isaac. That may be true, or it may have flown here under different circumstances. Many birders from the area are seeing and documenting this bird's appearance and behavior so hopefully we will have a clearer idea of this individual's status than the previous two sightings.

A Local Peregrine Falcon Nest

A few days ago, I got a tip from another birder about the presence of a local Peregrine Falcon nest. So yesterday morning I went to the site and checked it out. I had been waiting less than five minutes when one of the parents flew into sight and perched nearby (shown above). The chicks started calling and the adult called back in response. Around the same time, I got a brief look at what I assume was the other parent flying a short distance away.

Changing my position a few yards, I was able to get a better view of the nest. Unfortunately the angle was such that I was not able to see into the nest, but two of the chicks were perched at the edge. (I heard there are three, but I only saw two myself.) One of them was especially vocal and was flap-hopping around the ledge. Its wings looked developed enough that it probably will not be in the nest much longer. The adult made one delivery to the nest in the few minutes I was there, accompanied by even louder screaming.

Unfortunately my photos of the nestlings are not so good. I probably would have gotten better results on a sunny afternoon. I am not giving the exact location for this nest since it is still a state endangered species.

Jamaica Bay Osprey Being Tracked

One of the male Ospreys that nest at Jamaica Bay Wildlife Refuge in Queens, New York, is now wearing a satellite transmitter that will report its movements around the refuge and during migration.

On Tuesday morning, the banding team first startled a pair of ospreys off their nest, leaving behind four speckled eggs. The team then put a large mesh net over the eggs. The female bird flew back and became entangled in the net, and the team removed her and held her out of her mate’s view. When the male returned to the nest, he was captured and taken to the visitors’ center. The female, meanwhile, was released and returned to the nest within a minute.

There, Don Riepe of the American Littoral Society, who serves as the Jamaica Bay guardian, held the osprey still while Bob Kennedy, a prominent ornithologist enlisted by the conservancy, carefully sewed the harness — a kind of raptor backpack — that held the solar-powered transmitter, about the size of a pack of gum, on the bird’s back.

“He’s been a very good sport,” Mr. Kennedy said after the job was done, carrying the male bird back to the bay, where he was released.

Relatively few ospreys have been banded with satellite transmitters across the country, partly because the tags are expensive. The equipment alone can cost $4,000, Mr. Kennedy said, and there is a $50 monthly fee for the data downloads. Coleman P. Burke, a board director of the harbor conservancy, provided a grant to cover the expense of the Jamaica Bay operation. He had followed two other ospreys that Mr. Kennedy had helped band using GPS on Nantucket in the past few years. 

Birders will be able to follow this Osprey's movements via a website, jamaicabayosprey.org. Unfortunately the site will not be online until June

Pale Male and his Partners

Pale Male, the renowned Red-tailed Hawk in Manhattan, has had trouble finding a new breeding partner since the disappearance of his last mate, Lola. The trouble is not a dearth of potential mates. Rather, his mates have been disappearing too quickly to nest and lay eggs.

So what has life after Lola been like? Pale Male was not down in the dumps for long, unless he swooped by a landfill for dinner, but this is about his hunger for the opposite sex. “As ever, because Pale Male is a real stud, a new female showed up almost instantly,” Ms. Winn said.

That was the one the Pale Male-watchers called Ginger.

But just when things seemed as steamy as that sex club on “Law & Order: S.V.U.” the other night, Ginger became an ex, or so the birders speculate — suddenly she was nowhere to be seen, and there was someone new: Pale Beauty. (Some birders called her Paula.)

Then she, too, was gone, replaced by “the one there now,” as Ms. Winn described the female that has been keeping company with Pale Male. Some birders call her Lima, but others wonder if she isn’t really Ginger. Pale Male’s girlfriends are not banded, Ms. Winn said, so there is no way to double-check their identities.

Prior to Lola's disappearance, the pair had not nested successfully for several years. Whether due to nest structure, infertility, weather, or other factors, the pair would produce eggs, but the eggs would never hatch. It will be interesting to see if Pale Male fares any better with a new partner.

Hawk Rescued from the Library

Some of you may have heard that a Cooper's Hawk has been trapped for several days within the Library of Congress Jefferson Building. (For those who are not familiar with the library's layout, the Jefferson Building is the main library building with the domed reading room.) Apparently the hawk flew in while chasing after birds and took up a roost in the main reading room. While it managed to get in, it was unable to find its way out. So yesterday, staff from the Raptor Conservancy of Virginia captured the hawk and removed it from the library.

Here’s how they got their bird: 

The team put a pair of starlings – Frick and Frack, according to their owner – in a trap on a ledge inside the dome and waited, hidden beneath a tarp.

The starlings saw the hawk poised nearby and froze. But the noise of a truck passing by the Jefferson Building startled the pair and caused them to move.

The motion drew the attention of the hawk: She immediately flew onto the trap, where its talons entangled in the nylon nooses attached to the top of the wire cage.

The team grabbed the hawk, weighed and banded the bird, then placed it in a covered cardboard carrying box....

The capture occurred about 8:30 a.m., and the process took about 25 minutes from setup to completion, according to Craig Koppie, an eagle and raptor biologist with the U.S. Fish and Wildlife Service.

According to the post, the trapped raptor was a female, but it weighed only 424 grams. That is very light for a female Cooper's Hawk. Most females weigh between 500 and 600 grams, sometimes even more than that. This is clearly a very hungry bird. In addition to losing significant weight, the bird was also somewhat dehydrated, but it was otherwise healthy and sustained no injuries during its time in the library. After some time for rehabilitation, the hawk will be returned to the wild.

Oldest Living Red-tailed Hawk

A Red-tailed Hawk found in New York last month turns out to be the oldest wild Red-tailed Hawk.

She doesn’t have a catchy name (or any name), a fashionable perch or Twitter twitter (“Pale Male’s Central Park is infested with humans but he’s tolerant”), but the red-tailed hawk first seen just before Thanksgiving sitting on a dead rabbit on the white line in the middle of Route 17M near Monroe, N.Y., has quite a story to tell.

When finally contained, after a somewhat erratic journey, she was clearly old and infirm. But it was not until people were able to study the aluminum band that had been placed on her left leg when she was 6 or 7 months old that they realized just how old she was: about 27 years and 9 months. Most red-tails that survive their first year — more than 60 percent do not — live about half that long.

Among red-tails whose ages could be documented, she was the oldest ever found alive in the wild in North America.

She was captured as a result of injury, but she is on the mend:

She has almost certainly traveled far since then, but her current acclaim began on Nov. 15 when a motorist, worried that the bird would be hit by a car, stopped to pick her up after seeing her feeding on a rabbit carcass in the road. When the bird didn’t fight him and wouldn’t let the rabbit go, he figured there was something wrong with her and put her in the back of his van, where she perched on a mop handle.

There were a few stops and missteps. She escaped when a worker at Sterling Forest State Park in New York tried to transport her in a banker’s box, but she was picked up the next day on the same highway and taken to the Bear Mountain Zoo, and then to Suzie Gilbert, a wild-bird rehabilitator in Garrison, N.Y.

The hawk is currently undergoing treatment for a hairline wing fracture at The Raptor Trust in northern New Jersey. If she recovers and is able to fly, she may be released in the spring.

(via 10,000 Birds)

A Lingering Osprey

Yesterday afternoon I went back to Donaldson Park in the hopes of finding the Northern Parula that I had seen there on Tuesday and Wednesday of this week. I looked carefully and even sat for a while in the area where I had seen it, but it never appeared. One negative result does not necessarily mean that it is gone, but it might be. Instead of the warbler, I saw another lingerer. As I was finishing up my walk along the river, an Osprey flew upstream and landed on one of the poles along the river bank. The sighting was a real surprise; previously my latest sighting of an Osprey in New Jersey was November 26 at Sandy Hook, and my latest in the county was November 3. It was especially unexpected since the river is developing some ice patches. This one was cooperative enough to sit close by for a few photos before it flew back downstream.

A Peregrine's (Nocturnal?) Diet

Naturalists in the U.K. have been collecting feathers and other bird parts from a Peregrine Falcon nest box to monitor what sorts of prey items they catch. One of the biologists described some surprising finds to the BBC:

By studying the diet of the Bath peregrines I have discovered that they not only feed on the ubiquitous feral pigeon and common garden birds - they also consume more unusual birds that you wouldn't see in the city.

There was evidence of our smallest, dainty duck - a teal - and the lapwing, more associated with ploughed fields.

I then began to find prey remains of birds that are very difficult to spot in the Bath region or don't normally occur here.

Secretive water birds such as water rails, spotted crakes, little grebes and shy woodland wading birds such as woodcock were all appearing in the diet.

The one thing they all had in common was that they migrate at night, travelling hundreds of miles west from mainland Europe.

I was able to confirm the peregrines were behaving like owls, catching birds as they flew over Bath at night.

The street lamps in Bath light up the migrating birds as they fly overhead.

I have watched a Peregrine hunting things in floodlights, so I am not all that surprised that others would do it. At the time, though, I assumed that it was catching bats or nighthawks. Nocturnal migratory birds had not occurred to me, but it makes a lot of sense. One thing I wonder if whether this behavior is something that Peregrines do naturally (e.g., hunting by moonlight or the sounds of flight calls) or whether it is something they learned over the past century.

Or maybe Peregrines are just better at finding rare birds than birders. That pair must have quite a life list!

Vulture Overhead

Yesterday I did not have much time for birding, but in the short time I was outside it was clear that raptors were moving. First I saw a few accipiters circling high overhead; they turned and drifted south. Then a Black Vulture soared even higher as it made its way steadily south. With clear skies and such a high ceiling, I probably could have picked up more with more powerful optics.

Two Hawks Hatch at the Franklin Institute

Yesterday two chicks hatched in the Red-tailed Hawk nest at Philadelphia's Franklin Institute. A third egg had not hatched as of the last time I checked. The chicks seem to be doing well so far.

Below is a video of the chicks in the nest, courtesy of CamFan, who has a collection of short videos captured from the live webcam.




For more updates, including photos and video of the nest, check the Hawkwatch blog or the live webcam.

Update (4/22): And the third egg hatched.

Franklin Institute Hawks

A pair of Red-tailed Hawks appear ready to return to their nest at the Franklin Institute in Philadelphia.

Their webcam is officially up and running. Last year, it gave thousands of viewers a peek-a-boo view of a red-tailed-hawk couple nesting on a ledge of the Franklin Institute. They hatched three young that flapped their way into the hearts of fans worldwide.

More than 300,000 people clicked in to one of two Web sites, some again and again. Some people - occasionally a dozen or more at a time - lined up on the sidewalk below to watch, waving at their friends through the webcam.

The birds were a hit not because they are rare - red-tailed hawks are common - but because the webcam offered such an intimate view. The camera was less than two feet away, inside a window.

Over the last month or so, the adults have brought new material to the nest, making it bigger and redefining the bowl in which the female can lay her eggs.

If the hawks follow roughly the same timetable, eggs could appear within two weeks. Last year, the female laid them March 9, and the chicks hatched April 16 and 17.

Institute president and chief executive officer Dennis Wint said there was no guarantee the birds would nest there again, of course, but "their behavior would indicate that they are likely to do so."

It helps the birds' popularity that their nest is in such a prominent location. The average Red-tailed Hawk nest is more concealed than a window ledge in the downtown of a major city. The best way to follow the breeding season's progress is at the Hawkwatch blog and, of course, the nest webcam. It is wonderful that we have the opportunity for such an intimate view of nesting activities while causing minimal disturbance.

Dunlin Lose Weight to Avoid Peregrines

Dunlin at Surf Beach in California / Photo by Alan Vernon

A new paper based on studies from the Fraser River estuary in British Columbia finds that hunting pressure from Peregrine Falcons (Falco peregrinus) is changing the behavior of Pacific Dunlin (Calidris alpina pacifica). The peregrine population crashed forty years ago, primarily thanks to pesticides, and has since rebounded. All of those new falcons need to eat, and shorebirds are suitable prey because of their size, numbers, and penchant for foraging and loafing in open spaces. Shorebirds have to adopt evasive behaviors to avoid being eaten.

One way of avoiding predation is to move to areas without much raptor activity. Some dunlin do this by finding safer roosting locations away from where peregrines are hunting, even if it means flying longer distances to reach them. Others utilize a behavior known as "over-ocean flocking": during high tides, they fly out to sea and remain in flight there for an extended period of time. This behavior was rarely noted by observers prior to the late 1990s, but since then it has been observed regularly. According to daily observations from January 2006, "over-ocean flocking" occurred on 15 out of 17 days, and dunlin spent an average 2.8 hours over the ocean at a time.

Most shorebirds build up their body fat reserves to avoid starvation during the winter. The extra weight helps them survive when food supplies are low, but has the trade-off of making it more difficult to escape predators in flight. In the face of increased predation, shorebirds like dunlin will need to carry less body fat to avoid becoming prey. According to bird banding data, that is exactly what happened when peregrines rebounded. Data from the 1970s shows that dunlin mass builds quickly and remains high through November and December before falling in January and February. By the 1990s, the average autumn peak weight had fallen by about 4 grams, or about 7% of their historical peak body mass.

This study is a good example of how conservation actions to benefit one species can have unintended results elsewhere in the ecosystem. In this case, the increased presence of peregrines around coastal areas is forcing dunlin to change their feeding and roosting behaviors.

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Ydenberg, R., Dekker, D., Kaiser, G., Shepherd, P., Evans Ogden, L., Rickards, K., & Lank, D. (2010). Winter body mass and over-ocean flocking as components of danger management by Pacific dunlins BMC Ecology, 10 (1) DOI: 10.1186/1472-6785-10-1

Hawk Trapped in the Metro

According to the Washington Post, a hawk got trapped in an escalator at the Benning Road Metro station. The article speculates that the hawk was chasing after a pigeon, which seems like a reasonable explanation. It does not identify the hawk, but Red-tailed Hawk (pictured at right) would be a good candidate to be chasing a pigeon in an urban area. Other potential pigeon-chasers would include Peregrine Falcon and possibly Cooper's Hawk. There are other common hawks in DC, such as the Red-shouldered Hawk, but if it were one of those, the explanation for what the hawk was doing in the station might need adjustment.

The good news is that rescue crews responded quickly and freed the hawk by partially dismantling the escalator. Apparently it was released on the station.

There is a joke about Metro's escalators in here somewhere.

Peregrines for Falconers?

Speaking of migratory raptors, the state of Florida plans to allow falconers to capture Peregrine Falcons and use them for hunting.

The Florida Fish and Wildlife Conservation Commission (FWC) approved a rule allowing falconers to take peregrine falcons for the sport of falconry at its meeting in Clewiston on Wednesday.

The U.S. Fish and Wildlife Service determines the number of falcons that may be taken over a broad range of states. It is estimated fewer than five falcons will be allocated to Florida in 2010. Under the new rule, falconers must receive a permit for the take of peregrine falcons for falconry. The FWC will randomly select applications and issue permits annually based on the number allotted to the state. Priority for receiving a permit will be given to Florida residents.

Needless to say, the state's falconers are pleased with the decision. Apparently Audubon of Florida and Defenders of Wildlife both testified at the meeting, but neither have posted any statement on their websites about it. The FWC's press release only quotes this from Audubon:

"Audubon views the peregrine as an iconic species, and we have concerns regarding lack of monitoring in Florida to help ensure no future decline in this species," said Julie Wraithmell of Audubon of Florida. "We hope the Commission will help promote conservation of the species by funding monitoring projects."

There is precedent for taking wild raptors for use in falconry, which is regulated under U.S. and state laws. I would prefer more caution, though, in using such a recently-recovered species.

Some Good News for Migratory Raptors

Earlier this week, the U.S. House of Representatives passed H.R. 2062, the Migratory Bird Treaty Act Penalty and Enforcement Act, by a voice vote. (Search thomas.loc.gov for the bill number if the link does not work.) The bill increases penalties for killing birds protected under the Migratory Bird Treaty Act. Particularly egregious killings would be treated as felonies with a fine of up to $50,000 and/or imprisonment for up to two years.

A version of this bill has first introduced by Rep. Peter DeFazio (D-OR) two years ago in response to high-profile arrests of several roller pigeon racers. Roller pigeons have an unusual trait that leads them to tumble in midair; this in turn attracts the attention of raptors who see them as an easy meal. These particular individuals were caught because they admitted systematically killing raptors to undercover agents. The U.S. Fish and Wildlife estimates that roller pigeon clubs were responsible for the deaths of up to 3,000 raptors annually on the West Coast.

The bill still needs to pass the Senate, but since the terms do not seem particularly controversial, it ought to pass.

Post of the Month

Thanks to Bora Zivkovic for selecting my post on raptor talons as the PLoS ONE Blog Pick of the Month for November. Unlike many academic publications, the PLoS family of journals are open access and thus are easier for bloggers to read and write about on their websites. A pick of the month recognizes a post that does the best job of reporting on a PLoS ONE article for that blog's audience.

Welcome to anyone visiting from the EveryONE blog!

Also, welcome to readers from Greg Laden's Blog!

Golden Eagle Killing Goats

Golden Eagles are powerful hunters. Unlike Bald Eagles, which primarily hunt fish and often scavenge, Golden Eagles will take a variety of mammals and land birds. Last night, Lady Woodpecker tweeted this video of a Golden Eagle taking down a young goat.



There seem to be at least two different hunts combined in this video with the first and third segments showing the same hunt from different angles.

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.

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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