White-tailed Eagles on the Isle of Mull
The Disputed King of British Raptors
Standing (from head to talon) at 1 metre tall and weighing-in at up to 7 kg, the White-tailed Eagle is the largest and heaviest bird of prey in the British Isles. Its huge bulk and long, broad wingspan of up to 2.5 metres make this disputed King of British raptors a truly majestic and awe-inspiring sight when seen perched or in flight. Despite its smaller size, many birdwatchers continue to refer to the Golden Eagle (Aquila chrysaetos) as the King of Britain’s birds of prey. As both species have the necessary requirements to have this crown of avian monarchy sit upon their head, it really is simply a matter of personal preference!
Adult birds possess a strikingly white tail, from which the bird derives its name, although immature birds have to endure several moults before their dark brown (almost black) tail feathers whiten.
The plumage of adult birds is mainly brown with a noticeably paler head and neck, the colour of which has a tendency to lighten further with age. The head colour of older birds can look conspicuously white and can be a useful pointer in helping to distinguish individual birds.
White-tailed Eagles have large heads, with adults sporting a huge, thick yellow beak, the colour of which can match its eyes, legs and feet. Immature birds take up to five years to reach maturity, during which time the bird’s eyes, tail and beak colour will progressively take on the appearance of adult plumage.
Adult’s White Tail
Fourth (Or Fifth?) Largest Eagle
The White-tailed Eagle is an awesome sight and presence on the Isle of Mull. Not only is it the largest bird of prey to be found anywhere in the British Isles, it can lay claim to being the fourth (or fifth?) largest eagle in the world, depending on which author you choose to believe!
1. South American Harpy Eagle Harpia harpyja (weight range: 4.0 – 9.0 kg)
2. Philippine Monkey-eating Eagle Pithecophaga jefferyi (4.7 – 8.0 kg)
3. Stellar’s Sea Eagle Haliaetus pelagicus (4.9 – 9.0 kg)
4. White-tailed Eagle Haliaetus albicilla (5.4 – 7.0 kg)
Reversed Sexual Dimorphism
As with other birds of prey, the female White-tailed Eagle is considerably larger (up to 36” in length, compared to 27”) and heavier than the male (up to 7 kg, compared to 5.4 kg) and tends to be the dominant partner in the relationship. This size difference allows a mated pair to hunt a wider variety of food items within their territory, females being capable of catching and killing larger and heavier prey.
This is certainly the case with White-tailed Eagles, although dimorphism in size is less pronounced in this large predator than in smaller birds of prey, e.g. Sparrowhawk Accipiter nisus, that hunt more active prey. Carrion, especially the remains of dead Red Deer and Sheep, may provide a significant proportion of the diet of some White-tailed Eagles during the Autumn and Winter, although there is never any shortage of live food for these birds on the Isle of Mull.
The flight patterns of a White-tailed Eagle are determined by its wing shape. White-tailed Eagles have long, broad, vulture-like wings that are ideal for gliding and soaring on up-draughts. It is not unusual to witness one of Mull’s White-tailed Eagles spiralling effortlessly on ‘lazy’ wings high above a hillside or sea cliff on the island. This is a great way for the bird to save energy while keeping an ‘eagle eye’ on potential hunting opportunities.
When a bird flaps its wings the energy it creates is released as heat. Birds, unlike mammals, are unable to perspire, so it makes sense for a White-tailed Eagle to minimise heat loss and prevent overheating by reducing the amount of time it spends in flapping flight. By searching for updraughts of warm air over hillsides and cliff faces, an eagle can conserve energy while remaining airborne for considerable lengths of time.