Have you ever watched a flock, or “charm”, of goldfinches (Carduelis carduelis) flit through the garden? With their wings flapping furiously, they move in a synchronised blur of crimson and gold. Each member of the flitting flock needs to work hard and expend enormous energy to stay aloft.
Yet on a warm, sunny day a buzzard – which is over 50 times heavier – can soar high above them in seemingly effortless fashion, as if held up by invisible threads. So why do these birds tackle flying so differently?
The answer is that the way a bird defies gravity is dictated by the shape and size of its wings. These fall broadly into four groups, each with its own specialism:
• wings with slotted tips for passive soaring over land
• long, slender wings for soaring over water
• elliptical wings for short bursts of speed
• narrow wings, long in comparison to body size, for prolonged, high-speed travel
Riding high on hot air
Wings in the first group tend to be broad and have large flight feathers called primaries, which separate like fingers to create slots. These slotted tips enable the bird to catch thermals (vertical columns of hot air), which makes a passive soaring technique possible.
Thermals are created when the sun warms the ground, which in turn warms the air directly above it. The warmer air close to the surface expands to become less dense than the surrounding air and rises, taking even large and heavy birds such as eagles, hawks and storks along for the ride.
Using these invisible elevators, golden eagles can climb several hundred metres without expending any energy. Once high in the skies, they are able to soar effortlessly around at an unhurried speed of 30mph, which is perfect for surveying their territories and spotting prey.
So, if you fancy catching sight of a golden eagle (Aquila chrysaetos) it’s best to choose a warm, sunny morning in preference to a cold, wet afternoon. They are much easier to spot against blue skies than against a background of grey crags.
Storks use passive soaring wings for a different reason – to navigate long migration routes. White storks (Ciconia ciconia) journey annually between Europe and Sub-Saharan Africa and, for many, the shortest route would take them over a large stretch of the Mediterranean Sea.
As air thermals don’t form over water, however, the storks would have to employ energetic wing-flapping to use such a route, a technique that burns 23 times more body fat than soaring over land. Instead, they cross over at the Strait of Gibraltar, where Europe and Africa are separated by a mere 7.7 nautical miles of ocean.
The storks rise as high as they can over the southern tip of Spain, after which they “surf” above the sea at height before once again picking up the thermals over land in north Africa.
Wings over water
The second group features birds that travel long distances across the oceans, such as gulls, gannets and albatrosses. Their wings are long and narrow and they employ a technique called active soaring. Wandering albatrosses (Diomedea exulans) use their enormous 3.5 metre wingspan to glide and soar out at sea for hours without rest or even a flap of their wings.
Rather than fly in a straight line, it seems the albatrosses simultaneously snake from side to side and swoop up and down, making best use of the wind to propel them over the waves. They gain height by flying into the wind at an angle. Then they turn and swoop along for up to 100 metres, losing height as they go, before changing tack again and repeating the manoeuvre.
One bird was recorded to have travelled nearly 4,000 miles in 12 days by repeating this zig-zag pattern. Covering huge distances in this way, albatrosses can search for patchily distributed food, such as fish and squid, with minimal energy expenditure.
Bursts of speed
Birds in the third category have an elliptical wing shape that’s advantageous for short bursts of high speed. The ability to move fast and suddenly can mean the difference between life and death when escaping from predators. Many of our garden songbirds and members of the crow family possess this wing shape.
The elliptical wings of the house sparrow (Passer domesticus) allow fast and explosive flights. Sparrows can quickly reach speeds close to 30mph as they power through the air using around 15 wing beats per second.
The sparrow’s wings share more similarities with helicopter rotary blades than with aeroplane wings. They allow a near-vertical take off that, followed by rapid acceleration, lets the sparrow speed quickly out of harm’s way. But it can’t keep up this frenetic pace for long and must quickly return to earth to refuel.
Going far on high-speed wings
Common swifts (Apus apus) encapsulate all that is flashy, fast and fabulous about the final category in which long, thin wings enable high-speed travel.
Swifts are famed for being the most aerial of all birds; it’s thought that fledglings don’t touch the ground again until they return to their birthplace to breed some four years later. They are also among the fastest fliers in the animal kingdom, with acrobatic and territorial display flights reaching speeds of up to 137mph.
In addition to being able to move at a considerable lick, birds furnished with high-speed wings – including ducks, falcons, terns and sandpipers – can also maintain a nippy pace over hundreds, or even thousands, of kilometres.
But the reigning champion in the distance category is surely the Arctic tern (Sterna paradisaea), which regularly travels between its Arctic breeding grounds and its winter home off the coast of Antarctica. This is a round trip of perhaps 25,000 miles – the highest annual mileage clocked by any bird.
UK viewers can discover more about how animals have conquered the sky by tuning in to Life in the Air, which begins on Sunday 3rd April at 17:00 BST on BBC One.
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