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.
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