Think of this: vultures, amongst the heaviest of birds — some weighing in at 15 kg — are also the highest flying of all birds, reaching heights of 11,000 m, and expending scarcely a calorie in energy while up there. Yes, it takes a lot of mighty wing-beating to get off the ground, but once they’re in the “zone”, they can fly hundreds of kilometers with nary a wing-beat. Hummingbirds, the lightest of birds, averaging around 4 g, burn up fuel so fast that they have to be constantly refuelling on the wing, by taking in their own body weight in high-octane nectar. Otherwise, they might just drop dead. At night, they go into a stupor, with heart and respiratory rates dropping drastically. As aircraft engineers know, the lighter the aircraft, the easier it is to get off the ground and save fuel.
So what’s with these vultures and hummingbirds? All birds are really designed to be as light as possible — they have hollow bones — and have a respiratory system that ensures a constant supply of oxygen to their muscles, regardless of inhalation or exhalation of air. They’re also streamlined, with smooth outlines and aerofoil-shaped wings over which air passes as smoothly as possible, generating the maximum lift and keeping that enemy of flight — drag — at bay. For this, a few design tweaks are necessary. Vultures and pelicans must have broad wings in order to support their weight; the larger the surface area of the wing, the more lift it can generate. The problem happens at the wingtip. Due to the pressure differences at the top and bottom surface of the wing, eddies or mini tornadoes of air roll off the edge of the wing from the bottom to the top, creating turbulence and drag. So the birds (like others) have solved this problem by having splayed wingtips — with the feathers at the end — called the primary feathers — deeply slotted and upturned, so that the air can pass through them without getting too agitated and causing drag. To facilitate slow flight — say, while landing — many birds have a tuft of feathers called the “alula” or bastard wing, which pops up in front of their wings to smoothen the turbulent airflow as the airspeed decreases (you can clearly see this in blue-rock doves). Vultures also attain their economical fuel consumption figures by hitching rides on thermals, columns of warm air rising from the ground. Many nest on precarious cliff faces, so no laborious take-off is necessary.
The wings of birds like albatrosses and other sea-birds are very long and narrow which reduces the turbulence at the wingtips because of the low surface area. Albatrosses use another trick: they tack back and forth low over the waves over vast distances, taking advantage of the difference in wind-speed prevailing close to the waves and higher up (It’s slower near the surface of the waves due to friction).
The hummingbird has solved its problem by being light and by flying more like an insect. Normally, a bird’s wing generates lift only on the down-stroke; the upstroke is more a reloading move. But with insects (and the hummingbird) lift is created by both strokes. Even so, the hummingbird’s wings may blur at 20 beats per second, while its heart thrums at 1,000 beats per minute to get enough oxygen around, and it must hover to suck in enough fuel from maybe 1,000 to 2,000 blooms per day.
It has recently been discovered that even the peregrine falcon has a special tuft of feathers which pops up when it’s screaming down at 389 kmph to dampen turbulence that could send it out of control. Valves in the nostrils also ensure a steady supply of air — and not a gale blowing through them at that speed — and the eyes are protected by a nictitating membrane which slides over before impact with the prey. Owls are equipped with special sound-dampening feathers so they can approach prey soundlessly: I once observed a group of four barn owls demonstrate this in an aviary — it was truly eerie.
Ground-dwelling partridges, pheasants and junglefowl burn the carbohydrate glycogen in explosive bursts when panicked, which doesn’t need a supply of oxygen, but in which waste products build up quickly, tiring the bird. Long-distance endurance fliers like ducks and geese burn fat, which need a constant supply of oxygen to be provided by the blood — hence their meat is “dark”.
Flying techniques have also evolved to enable birds to escape predators, perhaps the most celebrated being the “murmuration” of starlings, wherein massive flocks give a breathtaking display of synchronised flying that completely bamboozles any predator. Tightly bunched, they twist and swirl like eddies of smoke, each bird keeping an eye on its seven closest neighbours, ensuring it maintains “safe distance”. On the lookout for the slightest change in direction, it and the others will follow suit immediately. Long-distance fliers, like ducks and geese fly in the famous V-formation, wherein the wake left by one bird’s wingtips are used by the bird following to assist lift, with a 10 per cent energy saving. To think we stuff these magnificent creatures in cages.
(Ranjit Lal is an author, environmentalist and bird watcher)
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