Science: How falcons stomach the chase - New research into birds of prey separates the fast movers from the browsers. It's a question of guts. Malcolm Smith reports

If it's live entertainment you are after in the scorching midday of a Majorcan summer, it is hard to beat Eleonora's falcons. These small Mediterranean birds of prey - cousins of the peregrine - dive and twist in the air, accelerating after swifts and swallows, tumbling on to more sluggish moths, in a display of enormous agility. All to catch a lunchtime meal.

Falcons are high-speed chase and attack merchants. Not for them the easy gorging on a roadside rabbit corpse which can content a buzzard for hours. Or the lazy loafing on refuse tips, searching for meat scraps, which can keep a black kite's body and soul together day after day.

In order to feed themselves, birds of prey have evolved in a variety of ways. It is not all down to characteristics such as wing design and body shape. The efficiency of their digestion also, it seems, plays a major role, differing substantially between attackers, such as the falcons and sparrowhawk, and searchers, such as buzzards and kites.

New research, by Nigel Barton and David Houston of Glasgow University's Applied Ornithology Unit, has overturned the view that all birds of prey - because almost all of them are meat eaters - have roughly similar digestive efficiencies.

Drs Barton and Houston used feeding trials to compare the digestive efficiency of a peregrine, a specialist, attacking hunter, with that of a buzzard, a generalist, searching hunter. Both were fed two diets: pigeon, which is fat-rich and therefore provides a lot of energy, and rabbit, which has little fat (but more protein) and provides less energy.

The buzzards blossomed on both diets, maintaining their body weight. But the peregrines thrived only on the pigeon diet. With less energy-rich rabbit, peregrines lost more than 5 per cent of their body weight during the eight-day trial. Eventually, they might not have survived.

Their relative digestive efficiency (calculated by weighing food in and faeces out) proved different. Of 10 species of birds of prey examined, peregrines, at around 75 per cent, had the lowest digestive efficiency while buzzards, at up to 82 per cent, had the highest.

Other searching hunters, such as red kite and tawny owl, had a digestive efficiency as high as the buzzards. Tawny owls feed on a range of prey including small mammals, birds, reptiles and invertebrates. Like buzzards, they tend to stand around, perched on a post or branch, then drop on to their prey from above. Stealth and opportunism are their hallmarks.

More akin to the speedy peregrine, the woodland and garden-flying sparrowhawk catches much of its prey - small birds - in fast chases. The barn owl, though slower, also hunts on the wing rather than from a perch. The digestive efficiency of both sparrowhawk and barn owl is at the low end of the range.

These differences in bowel function are the result of varying lengths of small intestine. Drs Barton and Houston found that the small intestine of a typical searcher such as the red kite (never known for its agility and speed) was around 130cm long. The small intestine of the female peregrine, on the other hand, is some 95cm, while in the smaller males it could be as short as 72cm.

Assuming that, length for length, small intestines in different birds of prey are equally efficient at absorbing nutrients from food, then digestive efficiency is greater in those species with the most guts, at least anatomically if not behaviourally.

But why do the faster fliers have the shortest guts and therefore the least ability to extract goodness from their food?

It is all down to specialisation, say Drs Barton and Houston. Active chasers such as peregrines and sparrowhawks feed on birds. The success of an attack depends partly on surprise, but mainly on speed and agility. Although prey is abundant, several small birds need to be caught each day to fuel this energy-sapping behaviour. They need to consume up to a quarter of their body weight each day in food.

Presumably, such speed merchants have a small gut and a rapid throughput of digested food so that they can return quickly to their optimal flying weight after a meal. An overweight peregrine, merlin or sparrowhawk would never succeed in the chase. It is a question of trade-offs. For fast fliers, ability to capture prey is more important than squeezing every bit of goodness out of it after it is eaten.

The more easy-come, easy-go birds of prey such as buzzards and kites need to eat less than one-tenth of their bodyweight in prey each day. And they don't even catch all of this. Carrion eating is a speciality.

Such birds of prey have no weight problems to worry about. They digest more of their food more slowly, allowing them to make the most of a wide range of prey of much more variable nutritional quality.

Falconers seem to know all this already. The comparative food value of meat from birds and mammals and the benefits of feeding different falconry birds different prey is something determined by trial and error - and falconers have had plenty of time to work it out.

As a sport falconry probably originated in the Far East, perhaps as many as 4,000 years ago, but the first indisputable written evidence of falconry is from Japan in AD244. Widely practised in Britain before the Norman Conquest (the Bayeux tapestry depicts soldiers carrying hawks), falconry flourished in the Middle Ages.

Falconers may long have known what to feed a peregrine to optimise its breathtaking displays of speed. But little did they realise that those gut-wrenching dives are the result of especially speedy digestion.

(Photographs omitted)

Start your day with The Independent, sign up for daily news emails
Arts and Entertainment
Lou Reed distorted the truth about his upbringing, and since his death in 2013, biographers and memoirists have added to the myths
musicThe truth about Lou Reed's upbringing beyond the biographers' and memoirists' myths
News
people
News
Ed Miliband received a warm welcome in Chester
election 2015
Life and Style
Apple CEO Tim Cook announces the Apple Watch during an Apple special even
fashionIs the Apple Watch for you? Well, it depends if you want it for the fitness tech, or for the style
ebooks
ebooksA special investigation by Andy McSmith
  • Get to the point
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: Project Implementation Executive

£18000 - £23000 per annum: Recruitment Genius: They work with major vehicle ma...

Recruitment Genius: Chiropractic Assistant

£16500 per annum: Recruitment Genius: A Chiropractic Assistant is needed in a ...

Recruitment Genius: Digital Account Executive - Midlands

£18000 - £26000 per annum: Recruitment Genius: They work with major vehicle ma...

Recruitment Genius: Web Developer

£28000 - £30000 per annum: Recruitment Genius: This company provides coaching ...

Day In a Page

NHS struggling to monitor the safety and efficacy of its services outsourced to private providers

Who's monitoring the outsourced NHS services?

A report finds that private firms are not being properly assessed for their quality of care
Zac Goldsmith: 'I'll trigger a by-election over Heathrow'

Zac Goldsmith: 'I'll trigger a by-election over Heathrow'

The Tory MP said he did not want to stand again unless his party's manifesto ruled out a third runway. But he's doing so. Watch this space
How do Greek voters feel about Syriza's backtracking on its anti-austerity pledge?

How do Greeks feel about Syriza?

Five voters from different backgrounds tell us what they expect from Syriza's charismatic leader Alexis Tsipras
From Iraq to Libya and Syria: The wars that come back to haunt us

The wars that come back to haunt us

David Cameron should not escape blame for his role in conflicts that are still raging, argues Patrick Cockburn
Sam Baker and Lauren Laverne: Too busy to surf? Head to The Pool

Too busy to surf? Head to The Pool

A new website is trying to declutter the internet to help busy women. Holly Williams meets the founders
Heston Blumenthal to cook up a spice odyssey for British astronaut manning the International Space Station

UK's Major Tum to blast off on a spice odyssey

Nothing but the best for British astronaut as chef Heston Blumenthal cooks up his rations
John Harrison's 'longitude' clock sets new record - 300 years on

‘Longitude’ clock sets new record - 300 years on

Greenwich horologists celebrate as it keeps to within a second of real time over a 100-day test
Fears in the US of being outgunned in the vital propaganda wars by Russia, China - and even Isis - have prompted a rethink on overseas broadcasters

Let the propaganda wars begin - again

'Accurate, objective, comprehensive': that was Voice of America's creed, but now its masters want it to promote US policy, reports Rupert Cornwell
Why Japan's incredible long-distance runners will never win the London Marathon

Japan's incredible long-distance runners

Every year, Japanese long-distance runners post some of the world's fastest times – yet, come next weekend, not a single elite competitor from the country will be at the London Marathon
Why does Tom Drury remain the greatest writer you've never heard of?

Tom Drury: The quiet American

His debut was considered one of the finest novels of the past 50 years, and he is every bit the equal of his contemporaries, Jonathan Franzen, Dave Eggers and David Foster Wallace
You should judge a person by how they peel a potato

You should judge a person by how they peel a potato

Dave Hax's domestic tips are reminiscent of George Orwell's tea routine. The world might need revolution, but we like to sweat the small stuff, says DJ Taylor
Beige is back: The drab car colours of the 1970s are proving popular again

Beige to the future

Flares and flounce are back on catwalks but a revival in ’70s car paintjobs was a stack-heeled step too far – until now
Bill Granger recipes: Our chef's dishes highlight the delicate essence of fresh cheeses

Bill Granger cooks with fresh cheeses

More delicate on the palate, milder, fresh cheeses can also be kinder to the waistline
Aston Villa vs Liverpool: 'This FA Cup run has been wonderful,' says veteran Shay Given

Shay Given: 'This FA Cup run has been wonderful'

The Villa keeper has been overlooked for a long time and has unhappy memories of the national stadium – but he is savouring his chance to play at Wembley
Timeless drama of Championship race in league of its own - Michael Calvin

Michael Calvin's Last Word

Timeless drama of Championship race in league of its own