5 - The art of flying, how birds do it

As we already mentioned, the colours are beautiful to look at, but the feathers are primarily intended for flying. Flying requires the most effort of all forms of locomotion. The entire bird’s body is adapted to the art of flying. A very low body weight is a requirement, which is why all the bones in the skeleton are hollow, for example. The beak also has many of these hollows. Flying consumes a lot of energy and requires a lot of food and oxygen. The lungs are connected to an extensive system of air sacs. These run throughout the entire body, including cavities in the bones. The air in the air sacs is regularly refreshed and guided past the lungs in a complex manner. This guarantees a much greater availability of oxygen than if the bird had to rely solely on its lung capacity. This is very important for the energy the bird needs to fly. The mechanics of flying are another matter that I will not go into further. What I do find important are the different ways of flying, such as flapping flight, gliding flight, soaring flight and “hovering”. Flapping flight and hovering are the two that require the most energy. In the other two, the birds make use of air movements, which allows them to save a lot of energy. Usually, a combination of the four basic techniques is used.

Flapping flight:
Here we distinguish between take-off – which is necessary to rise quickly when flying away – and gliding flight – which ensures normal horizontal flight. The purpose of a wing is to generate the necessary upward force with as little resistance as possible. This means that a wing is curved on top and slightly hollow underneath. The pressure difference that arises in the airflow above and below the wing provides the “lift” needed to carry the bird against gravity. Above the wing, the air travels a longer distance than below it. This results in an upward force. We can achieve the same effect by gently blowing on a sheet of paper held with both hands at the corners about twenty centimetres from the mouth. The paper moves upwards. The wing acts like an oar. The downward strokes provide sufficient upward and forward force to carry the bird. The lift can be increased or decreased by increasing or decreasing the angle of the wing. This is used during take-off and landing. When braking, the wings are spread and lowered.

The shape of the wings often meets the environmental requirements of a particular species very well. The short, rounded wings of a Pheasant are suitable for fast and powerful ascents between the bushes and trees, but they cannot cover long distances. Long, narrow wings enable the Gannet to stay in the air for a long time without using much energy.

However, the bird has great difficulty taking off. You could say that a Pheasant is as helpless on the ocean as a Gannet is in the undergrowth. Landing is an art in itself. A bird does this seemingly effortlessly. It must approach at sufficient speed to remain in the air until the last moment.

Meanwhile, it has selected a twig and, flying carefully between other twigs, it lands softly and precisely on its chosen resting place. Landing is a perfect example of coordination between the nervous system and the flight muscles. A wide variety of muscle movements are performed in a very short time. One moment the bird is flying at full speed, the next moment everything is focused on slowing down. The wings then act as a parachute; the tail is lowered and spread; the legs are pointed forward and even the torso seems to be flattened to achieve the largest possible surface area, resulting in the greatest possible air resistance. And if, at the last moment, a gust of wind causes the twig to change position, the bird will react immediately and take to the air with the help of its uplifting flight feathers. It then turns on its axis by flapping one wing and braking with the other. This is followed by another landing, because it is not easily deterred.

Gliding flight:
Flapping flight requires a lot of energy, unlike gliding flight, which requires a minimum of energy. In fact, the bird falls downwards under the influence of gravity while gliding. The pressure difference above and below the wing causes the bird to be carried forward, but friction reduces its speed. Birds can maintain their altitude by occasionally flapping their wings or by utilising upward air currents. Good gliders can cover a considerable distance without taking any action. Larger birds of prey glide a hundred metres with a loss of altitude of ten metres. The same applies to the Stork. An Albatross can cover twice that distance.

Soaring:
Soaring birds use upward air currents. Not only thermals (warm rising air), but also rising air near rocks, sand dunes, mountains, forest edges and buildings are utilised. Different types of ground heat up differently. For example, a sandy plain, a freshly ploughed field and a motorway will give off more heat than a forest or a meadow. Birds make good use of this difference in rising heat (thermals). With minimal effort, birds are able to exploit air currents, allowing them to remain airborne for hours. We all know that on a summer’s day, birds of prey use this to move upwards in a rising spiral while gliding. The ascent continues until the upward force is equal to the force of gravity. At high altitudes, birds continue in gliding flight and slowly descend to a lower level, where they search for a new thermal cylinder that will provide the necessary lift again. Typical thermal birds have large wings with “fingers” at the end.

Hovering:
This is a highly specialised form of flying. Like all of us, the Common Kestrel is known for its hovering, but terns and Buzzard also master this to a greater or lesser extent. The birds hover against the wind. The air movement generated by the wings and the wind keep each other in balance. This results in the bird standing still, but in reality it is flying as fast as the wind is blowing.