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

150 Years of Evolutionary Theory

On this day in 1859, Charles Darwin's On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life was published for the first time. Even before Darwin's work, other scientists had speculated that existing species could be transformed into other species. Darwin drew from these concepts, but he advanced the idea that species are continually adapting in response to natural selection. Despite its vilification by creationists, evolution by natural selection remains the guiding concept of the biological sciences. Many bloggers are more qualified than I to discuss the implications of Darwin's work and how evolutionary theory has developed since 1859, so I will leave more detailed discussions to them.

Birders, however, should celebrate this anniversary as we are in a particularly strong position to appreciate the results of evolution. We do not necessarily think consciously about evolution when we go birding; in my own experience, I think about Darwin very rarely while I have binoculars around my neck. But among birds, we can observe the diversity of forms and behaviors produced by natural selection. Warbler species, for example, evolved to fill many specific ecological niches. One warbler species forages mainly among dead leaf clusters; another breeds only in jack pine forests of a certain age; yet another warbler with bark-like black-and-white streaking specializes in picking invertebrates off the trunks and limbs of trees. Whether we realize it consciously or not, these evolutionary adaptations help us to identify birds. That buzzy, insect-like call is likely to mean different sparrow species in a grassy field and a saltmarsh; the same goes for a trilled song heard in a swamp and a suburb. Without some awareness of these species' adaptations to ecological conditions, birding by ear and birding by GISS would be a lot more difficult.

Since On the Origin of Species is old enough to be in the public domain, there are many complete digital copies of this book available online. Try Literature.org, TalkOrigins, Bartleby, or Project Gutenberg if you want to read it.

For other evolution-related reading on this blog, see my review of Pilgrim on the Great Bird Continent, an article on the evolution of wood warblers, the announcement of a new crossbill species in Idaho, an analysis of the evolution of waterfowl genitals, and a more recent post on new research into birds of the Galápagos.

A Washed-up Skate

At the second site we visited during Saturday's trip to Barnegat Inlet, there was a skate washed up on the beach. The tracks and dropping around the skate indicate that some birds had already discovered the skate's remains. The skate has a bloody patch on its right side, presumably from scavengers picking at it. While some openings on this skate are probably natural (the eyes and mouth need openings), others are more likely the work of scavengers. I am not entirely sure which holes fit in which category.

The photo above is a close-up of the skate's nose, eyes, and mouth.

Another point that I am not sure about is which species of skate was on the beach. There are six species of skate present in the Atlantic off the northeastern United States. Its translucent head suggests that this may be a Clearnose Skate, but I am not certain that this feature is diagnostic, especially for an animal that is getting picked on. Clearnose Skate is a southern species; according to the NOAA site New Jersey is at the northern end of this animal's range.


I did not realize this at the time, but it is possible to imagine a human face in the skate's remains if you view them from a certain perspective.

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Added to Macro Monday.

Birds at Barnegat Inlet

Yesterday morning I went with my parents and sister to meet up with my uncle and his wife at Barnegat Lighthouse State Park. During winter months, this site offers some of the best coastal birding in the state. One of its major advantages as a birding site is that it allows relatively close approach. Birds that winter there seem used to being around large numbers of people and tend not to flush as quickly as at other sites. This allows close viewing of birds that we do not see very often.

It is the most reliable site in the state for finding northern waterfowl such as Common Eiders and Harlequin Ducks. Both of these species, as well as scoters, were present in modete numbers yesterday. I only saw one Harlequin Duck myself, but there was a larger flock farther down the beach. Other wintering waterfowl such as Long-tailed Ducks and Buffleheads were nowhere near their winter peak numbers. In addition to the headline ducks, there was a flock of Brant next to the jetty.

Several shorebird species were present along the jetty. A small flock of Black-bellied Plovers were hiding in the beach grass. Flocks of American Oystercatchers flew past us over in the inlet. On the jetty itself, there were Ruddy Turnstones, Dunlin, and Purple Sandpipers. The largest group of shorebirds that I noted was a flock of Dunlin numbering around 100. Above is a Dunlin and a Ruddy Turnstone loafing on the same rock. I posted other photos of Dunlin and Turnstones at my Flickr account.

Gulls, though abundant, should not be neglected. This handsome immature Herring Gull started its long call just as I took its photograph.

Walking from the lighthouse to the far end of the jetty is always exciting. I am lured farther and farther out by the prospect or seeing new birds or having a better view of birds I have seen distantly or in poor light. Plus there are constantly new birds – sandpipers, sparrows, etc. – popping up from between the jetty's rocks. On the way back I am more tired, more worried about my footing, and have seen most of the bird species the jetty has to offer. At least the return trip offers a marvelous view:

In the afternoon we walked along the "High Bar" beach to the west of the lighthouse. Most of the birds were the same as at the jetty – small flocks of Brant, Black-bellied Plovers, and Dunlin, with a few other water birds in the mix. One new addition was a hatch-year Merlin perched at the top of a bare tree. Not bad at all for a fall day!

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Christmas Bird Counts Coming Soon!

The first of this year's Christmas Bird Counts are less than a month away. The counts will run from Monday, December 14th, to Tuesday, January 5th. Most will occur on the weekends of December 19-20th and 26-27th, with a few clustered around New Year's Day.

Christmas Bird Counts give birders the opportunity to participate in a long-running citizen science project. By late December, most migratory birds have settled into their winter ranges. CBCs provide observational data to assess where those ranges are, how big the winter populations are, and whether there are any changes over the years. Most counts have been running for a few decades; some have been running for much longer.

You can find a schedule for New Jersey's CBCs here. Similar schedules may be available for other states or regions. You can also find the times and locations for CBCs in other areas at the National Audubon Society site. This year, I will be participating in at least two counts in New Jersey.

Loose Feathers #213

Emperor Penguins / Photo by lin padgham

Bird and birding news

• A study of the fossil record argues that there were six genera and nine species of moas (much lower than other estimates) and that the North and South Islands of New Zealand have been geographically isolated for 20-30 million years.
• The Puerto Rican Nightjar's range is much than previously estimated; in all this species has 1,400-2,000 individuals ranging throughout southern Puerto Rico.
  
• Satellite transmitters on penguins, albatrosses, and other animals enabled the creation of an atlas of species and migration corridors in the Patagonian Sea.
• A birder is collecting birds that hit buildings in downtown Chicago to document the dead ones and give stunned birds a chance to recover. She has also gotten many buildings to shut off lights during migration.
  
• Surveys over the summer found 201 to 208 endangered Snowy Plovers on Oregon beaches, up from 50 in 1990. The goal is to bring the population up to 250.
  
• So far 17 Madagascar Pochard ducklings have been raised successfully in captivity. This almost doubles the known world population of the species.
• When two Common Ravens got electrocuted on a power line in Fairbanks, Alaska, observers saw dozens of other ravens flock to the scene.
  
• Tourists hoping to see Emperor Penguins in Antarctica got stuck in the ice.
  

Birds in the blogosphere

• Vermont Center for Ecostudies: Look What the Cat Dragged In 
• Bell Tower Birding: The Splitting of Birds
• Stokes Birding Blog: Cackling Goose Subspecies in the East 
• Birdfreak: Snipe Hunting: American Folklore 
• Urban Hawks: Great Horned Owl Still in Central Park
  

Environment and biodiversity

• Botanists worry that solar and wind development in the Mojave could wipe out rare species before they can even be identified. Since the 1920s, the area has averaged 25 new plant discoveries per decade.
• Maryland regulators approved a 20-turbine wind farm on top of Backbone Mountain at the western end of the state. Another company already has a permit for building 28 turbines on the same mountain.
• Meanwhile, the governors of Delaware, Maryland, and Virginia have formed a coalition to encourage wind energy development off the Atlantic coast.
• Oceans, one of the earth's largest carbon sinks, are losing their ability to absorb more carbon due to the extreme amount of emissions. On average, oceans absorb about one quarter of all carbon emissions.
  
• The U.S. Senate will not consider climate legislation until the spring.
• Carbon offset programs for air travel soothe consciences but lack the standards or monitoring to ensure that they work.
• Grizzly bears in the Pacific Northwest are highly dependent on salmon runs. Poor salmon numbers in recent years have led to declines in the numbers of adults and cubs found on aerial surveys.
• Carbon dioxide and methane are not the only compounds that cause climate change.
• 2008 was another record year for carbon emissions; humans emitted 8.7 billion tons of carbon dioxide. The rate of growth is also higher than it was in the 1990s. Much of the rate increase is due to more emissions in the developing world; however, these emissions largely serve consumers in richer nations.
• A new study of ancient lake sediments argues against several hypotheses for the extinction of large fauna in North America, including climate change, a meteor, and the Clovis people. Humans are not entirely off the hook yet, however.
• Coastal habitats such salt marshes and mangroves can sequester 50 times as much carbon as tropical forests. This finding emphasizes the need for protecting and restoring these fragile habitats.
  

Carnivals and newsletters

• The Moth and Me #8
  

SkyWatch: Black and White Reflections

Sometimes it helps to look down if you want to see the sky. Here is the color version.

These photos were taken in the Princeton Institute Woods. Stony Brook, the stream in the images, is crossed by a narrow footbridge that connects to the D&R Canal. The bridge is labeled a "swing bridge" on trail maps, but there does not seem to be any swing mechanism involved. However, it does bounce as you walk across.

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Two New eBird Projects

At the end of last week, eBird announced two projects designed to improve the quality of its database. The first, and most important for continental and regional range information, is the County Birding Project. One problem with the current eBird database is that some counties, especially ones near metropolitan areas, get strong coverage from birders while other counties are barely covered at all. Within counties, population centers are often well-covered while outlying areas are not. There are suggestions at the link for surveys to improve county coverage, both within counties and regionally.

To assist in the project, eBird added a new checklist type for random location counts. Random locations are locations at fixed intervals along a driving (or cycling) route.

The county birding announcement links to a spreadsheet with county checklist totals from across the country. As one might expect, few counties in New Jersey are lacking in checklists. The county with the lowest number of checklists, Salem, has 449. The next lowest is Warren, with 949. Even nearby states are pretty well covered compared to other areas of the country. For example, South Dakota has 22 counties in single digits. Also, Nate's Century Club could fill some gaps.

The second announcement was for a Site Survey Project. This concept is not all that new, since eBird has long encouraged users to bird the same locations regularly to compile the most useful data. The new part is that users can register sites where they expect to bird at least once per week and participate in a discussion group about site surveys.