Diurnal Raptors – The Wing
Quite an important bit of kit if you’re a bird, and one of Natures wonders
The main groups are:
Primaries: The main flight feathers which provide power and there are 10 on each wing.
Secondaries: Provide lift and number between 10-25 per wing, according to species.
Alula: Also known as “bastard wing”. A group of feathers on the leading edge of the wing corresponding to the bird’s thumb.
Coverts: On the upper wing and lower wing, providing lift.
Hard to imagine, but feathers actually developed from scales! Their basic structure is a hollow quill with vanes on each side consisting of hundreds of barbs. These barbs ‘zip up’ the feather and hold everything in place.
A bird can move each feather individually, altering the wing shape and amount of lift to suit its circumstances. The alula, for example, can be extended to increase lift and thereby reduce stall speed on landing – it acts as a leading edge slat does in an aircraft.
The shape of a wing is a good guide to the type of environment in which a species of bird operates.
Birds having broad wings include vultures, eagles and buzzards, all of which generally like to soar over open country. Under the right conditions they can soar for hours on end with barely a wing flap, being held aloft by wind currents, hot air thermals and the lift generated by their wing shape. To assist in generating lift and reducing drag, these birds will spread their primary feathers apart while soaring.
Short-winged birds are inhabitants of closed country and include the likes of the hawk family. Our sparrowhawk and goshawk chase after, rather than stoop on, their quarry and rocket after it in woodland. Bearing in mind the fact that the prey will try like stink to evade the deadly attentions of the raptor, a high-speed tail chase will take place. Having a short wing not only enables the hawk to manoeuvre much better in the confines of a wood but also reduces the risk of damage to itself: long wings would snag on obstacles. Such agility is vital to their survival and sparrowhawks, in hot pursuit of quarry, are often seen to go through gaps in hedges which simply aren’t there.
The last group are the long-winged birds: the falcons. Falcons are capable of a certain amount of soaring but will invariably have to give a few flaps of their wings to maintain height. This is because their wing shape and high wing loading (aspect of bodyweight vs lift generated by the wing area) are designed for high speed stoops. The feathers on a falcon are tightly packed together to assist in generating speed by reducing wind resistance. While all bids of prey can, and do, stoop on prey, none do it with the speed, grace and devastating effect of the falcon family.
The one bird which uses its wings differently to the rest of its family is the kestrel. A true falcon, it can perform a perfectly adequate stoop like its cousins. It also likes to still hunt from a perch, as does every other raptor (it uses up no energy). However, if there is no perch handy the kestrel will make its own by hovering, at various heights, over its hunting ground. This it does by turning into the wind: in a strong wind it will seemingly hang in the air and in calmer air it will need to flap its wings to keep station. It can hover due to the fact that it can alter the angle of attack of each of its flight feathers, especially its alula which is almost always extended to provide maximum lift.
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