Fly like an Eagle
Agile and Lithe
The White-tailed Eagle is never more impressive than when it is in flight, when its huge frame and enormous wingspan can be viewed to its best advantage. In flapping flight, a White-tailed Eagle may appear somewhat ponderous, yet it is remarkably agile for such a large bird. This is seen to its greatest effect during breathtaking aerial courtship displays, when potential mates will indulge in death defying cart-wheeling at incredible speeds.
White-tailed Eagles in the British Isles have a wing-span of around 2.5 metres (8 feet), varying between the smallest male to the largest female. There may be a south – north cline in this species that suggests that White-tailed Eagles in the northernmost part of their range measure consistently larger than their southern counterparts. Certainly, birds breeding in Greenland are thought to be among the largest White-tailed Eagles in the world, being noticeably bigger than White-tailed Eagles that nest in the British Isles in terms of weight and wing length. A female of the Greenlandic race Haliaetus albicilla groenlandicus has recently been found to have a wing-span measuring an incredible 2.9 metres – a real giant among birds!
A Lackadaisical ‘Vulture’
Although somewhat lackadaisical and leisurely-looking in flight, White-tailed Eagles normally fly at speeds of between 20 – 60 miles per hour, although dives have been recorded of up to 100 miles per hour. An apex predator (at the top of the food chain), the White-tailed Eagle forms a species pair with the North American Bald Eagle and share certain characteristics with vultures.
The flight profile of a White-tailed Eagle is very flat and vulturine in appearance, whereas the Golden Eagle soars almost Buzzard-like on up-tilted wings that are held at a dihedral angle (a shallow ‘V’ from the horizontal). Most encounters with eagles on the Isle of Mull (away from well-known nest sites) will be as a result of seeing birds that break the skyline, often along a mountain ridge. It is, therefore, important to recognise the different flight profiles of White-tailed and Golden Eagles, as often these birds will be far away from the observer or soaring high in the sky.
Featherweight – Heavyweight
It is likely that a White-tailed Eagle possesses as many as 7,000 vaned feathers in its plumage which, along with the additional weight of its down feathers, accounts for nearly 20% of its bulk. White-tailed Eagles do not replace these feathers annually, instead appear to be in continuous moult, renewing around half of their plumage each year. This shouldn’t come as any surprise given the size of the bird and the amount of energy birds require to initiate and complete their moult regimes.
It is sometimes possible to identify individual immature White-tailed Eagles on account of their moult sequence, as birds take up to five years to acquire adult plumage. However, this is can be a difficult task in the field when such birds are usually simply referred to as ‘immature’.
The photograph (right) shows a trio of such immature White-tailed Eagles sitting on rocks on the southern shore of the Sound of Mull. The strong wind has parted the birds’ feathers, revealing their white bases, giving them a particularly unusual appearance. For White-tailed Eagle read White-cheeked Eagle!
Outstretched and Horizontal
Flight is the obvious characteristic that separates birds from other animals. Due to the equally obvious physical demands associated with moving in air, flying is one of the most demanding adaptations found in the natural world. Ordinary flapping flight requires huge amounts of energy. Large birds, like the White-tailed Eagle, have difficulty carrying the necessary fuel and musculature to allow continuous flapping flight. To combat unnecessary energy usage and minimize heat loss, the White-tailed Eagle, wherever possible, will employ simpler and more advantageous methods of flying : gliding and soaring.
Gliding on outstretched and horizontal wings requires the minimum of effort and least metabolic energy consumption. The White-tailed Eagle has one of the highest glide ratios of any British bird, somewhere between 10:1 and 15:1 (speed of forward travel divided by speed of descent). Despite the unavoidable loss of height, gliding is important to birds, like eagles and other birds of prey. Combined with soaring flight, gliding is a means by which a White-tailed Eagle may exploit moving air currents.
Up, Up and Away
There is a powerful magnetism that draws the observer to an eagle soaring ever upwards in the Mull skies. It is hard to imagine a bird 1 metre high, with a wingspan of around 2.5 metres and weighing up to 7 kilos vanishing into the ether, yet that happens every day on the Isle of Mull, as White-tailed Eagles are lost in the clouds. Latching on to rising currents of warm air, known as thermals, White-tailed Eagles gain height, then glide cross-country until they detect a fresh thermal and start to climb again. Thermal soaring may be a way used by White-tailed Eagles for commuting, although it is put to good use, in accordance with their excellent visual acuity, when hunting.
The Isle of Mull is predominantly upland terrain, where air rises and is deflected by the slopes and rock faces of mountains. This offers birds of prey another opportunity for soaring and gliding, albeit at a lower elevation. White-tailed Eagles possess separated wing tip primary feathers. When these outer wing feathers show reduced width at their tips they are said to be emarginated. These are special adaptations for flight, which permit what is a large, bulky and potentially cumbersome bird the ability to take off from the ground or a tree. These feather structures also help eagles to take advantage of even the narrowest and weakest thermals to maximum effect.