A rustle, squeak, twitch of the grass; an owl, alert on a stump nearby, tenses and raises its head. It bobs and sways from side to side, confidently pinpointing the all important signs of its next meal. A sudden, silent pounce and another vole has gone. Short-tailed voles are the main food of the short-eared owl almost everywhere in Britain and although the owl takes them by watching from a low perch and then pouncing, it is much more familiar when hunting in the daytime, in a low, wavering flight over open ground.
As a winter visitor, the short-eared owl may turn up almost anywhere. But, in order to breed, it needs large areas of open ground, free from human disturbance, with a rich supply of food in the form of small mammals.
Voles can breed rapidly, with several litters a year, however their population tends to fluctuate in cycles of three or four years. When they are scarce, the owls too are scarce, they do not breed or lay fewer eggs. In good vole-years owls capitalise on the abundance of food and many pairs can be found nesting in close proximity, each laying more eggs and rearing many young. With a change of fortunes in its prey, the short-eared owl population is through necessity mobile and fluctuating. Whereas other owls may occupy a fixed territory for many years, and know it intimately, the short-eared owl is transient and may have to move great distances to settle where food is abundant, perhaps staying only for a season or two.
Food of course, is the vital factor and the short-eared owl is perfectly equipped to catch small mammals in moderately long vegetation in open spaces. It has long, broad wings which give a high degree of manoeuvrability, yet enable it to cover huge moorland areas with little effort on its low, gliding flight. Short-eared owls have small, stiff erectile feathers on the head, often inconspicuous, that give a false clue to its name. Its 'true' ears are large and sensitive, hidden beneath the facial disc, and equally its keen vision make it the perfect vole-detector. Even in flight the vivid yellow eyes, catch the attention, so sensitive to changes in light it is often possible to watch the pupils contract and dilate independently as the bird turns its head. The expression is one of keen alertness. The absence of voles in Ireland is the major reason why short-eared owls do not nest there whereas, elsewhere in Britain short-tailed voles comprise more than 60% of their annual diet.
Inch by inch, silently and efficiently, the ground is searched in a manner reminiscent of the hen harrier, perhaps its chief competitor (where and when harriers are not persecuted, but that is a tale for another blog) on the moors. Silent, buoyant, quartering the ground with the impression of strength and power in its steady wingbeats, this bird produces a thrill of expectation as it appears over some distant ridge and approaches across the moor. Then a hollow, rhythmic sound, a deep, far carrying "boo-boo-boo-boo-boo" in a long weird sequence, slowly forces itself onto the awareness of the observer, until it dominates the open ground. High overhead another owl, sails majestically across the sky, giving its resonant, unearthly, unbirdlike song. Suddenly, as if shot, it crumbles, loses height, then claps its wings rapidly together beneath its body, just as abruptly, it then resumes its steady onward passage. The rhythmic, booming hoots begin to fade into the emptiness of the moor.
We are just back from an exciting journey into the deep Arctic and once I have processed the images I will post daily updates of this extraordinary trip.