Question
I filmed a bird taking off from the ground with my new video camera. When I played it back in slow motion I saw the bird crouch down, then begin to open its wings and take off into the air with a combination of a thrust from its legs and a powerful wing beat. This got me thinking. First, are birds so amazingly light that they can do this? The average chicken I eat does not seem so light. And secondly, given that I weigh 75 kilograms, how big would my wings have to be for me to take off like a bird? Or is wing size not the only issue?

Polly Wadworth , Berlin, Germany

Answers
Birds have many adaptations to keep their weight to a minimum, such as hollow bones and light feathers. A sparrow weighs only about 25 grams. To maintain steady flight its wings must be large enough to generate a lift equal to its weight as it moves through the air. The wings of most birds are large enough to do this.

The chickens we eat are an exception, because they have been artificially selected to grow to a weight many times that of their wild ancestors. Because selection has had much less effect on their wing size, they are unable to fly. Laying hens however are lighter and can take off, fly short distances and make accurate landings.

Small to medium-sized birds use thrust from their leg muscles to jump into the air as they begin to fly, but this method does not work for larger birds. So some make long taxiing runs, while others exploit thermals to get airborne.

The mathematical relationship between a bird's weight and the size of its wings is straightforward and also applies to insects and aircraft. In principle, a person weighing 75 kg would need a wing area of about 1.25 square metres to achieve steady flight. If these wings had the same proportions as those of a typical bird, each would be nearly 2 metres long.

Attaching these huge wings to your back would not be the end of the problem, because you would need some means of flapping them. The muscles required would be enormous, and their extra weight would require still larger wings, and so on. So it is easy to see why the heaviest bird capable of flapping flight weighs only about 10 kg, and why humans have to rely on technology to take to the air.

Patrick Green , Edinburgh, UK

I also weigh 75 kg and my paraglider, with a wing area of about 26 square metres, gets me aloft perfectly well. It gives me a glide ratio of 1 metre of descent for every 8 metres travelled, which is probably better than a pigeon could achieve by gliding and certainly better than a chicken. It also has enough performance to easily gain height in updrafts or thermals.

However, my paraglider flies at about 30 kilometres per hour, which is slower than most birds. So probably a better model would be a hang-glider. They fly at 50 km/h or faster which is closer to the speed of a typical bird. Because the hang-glider wing moves through the air faster than a paraglider wing it generates more lift, so it can make do with a smaller surface area ­ typically about 15 square metres.

The problem though, when it comes to taking off like a bird by leaping from the ground, as opposed to running off a hill with a paraglider or hang-glider, is that you couldn't provide nearly enough muscle power.

Peter Condick , Leeds, UK

Even chickens, which can fly only a few metres, have the best meat in the huge muscles of their breast. These are attached to a massive keeled sternum to provide the power to flap their wings. They have little fat, hollow bones, lightweight bills rather than jaws with teeth, and no residual claws on their forelimbs. Special shoulder joints allow them to raise their wings close to vertical for take off.

For you to fly, some of these adaptations would help. Your wing size would depend on your flight speed. To soar from a cliff into a 20 km/h wind you would only need a little forward velocity to get airborne using wings like those of a hang-glider with a 7-metre wingspan. But to take off from the ground you would either have to run at 30 km/h or have muscles that could flap your wings.

The problem is one of scale. As your size increases, your mass gets bigger in proportion to your surface area. So if your wings were proportionally the same size as those of a small bird, they would have much less area relative to your weight and therefore wouldn't provide enough lift. That's why the hovering hummingbird is small, while the much larger albatross keeps landings and take-offs to a minimum.

The maximum weight of any living bird is around 12 kg. Of course this raises the question, how did animals like archaeopteryx and the huge pterodactyls fly, unless the environmental conditions were very different from today's?

Nick Butterworth , Northwich, Cheshire, UK