BILL NAYLOR explains how these wonderful birds have uniquely adapted to life in the shadows
THE binocular vision of owls, together with the ability to blink their large eyes appealingly using their upper eyelid, gives them a curiously human quality. That’s probably the reason for their widespread popularity and explains why they figure so much in folklore.
Owls can be barred, mottled, spotted, dark or light-coloured. Many individuals become paler as they grow older. In a number of species there are colour phases, also known sometimes as morphs. Our own tawny owl (Strix aluca) is usually brown, but there is also a grey phase. Some species such as the scops, long-eared, short-eared and horned owls possess ear tufts, but these have nothing to do with hearing.
As with most birds of prey, the females are slightly larger than the male. Other sexual differences are few, but there are some exceptions. Female snowy owls (Nyctea scandiaca) are barred, while males are more of an unmarked white. When resting, owls are remarkably well camouflaged and often reveal themselves only when they move or open their eyes. Their feet have two toes facing forward and two facing the rear, including one toe that is reversible. Fishing owls, such as the mighty Pel’s fishing owl (Scotpelia peli) of tropical Africa, are adapted to seizing prey out of water – they have bare legs and feet with rough soles. In most other owls, the legs and feet are shielded by feathers, which is a precaution against bites by prey.
The characteristic owl’s fixed stare is due to their eyes being rigid in their sockets – they can’t roll their eyes as we do. However, they can move their head with more flexibility than any other bird. This is because they are equipped with 14 neck vertebrae, twice as many as humans. As a result they can turn their head by 270º and tilt it up or down by 90º. Owls’ eyes are often highly attractive. In the case of the spectacled owl (Pulsatrix perspicillata), the dark facial discs emphasise its eyes all the more.
In absolute darkness, owls can’t see (it’s impossible), but with their advanced hearing they can pinpoint their prey even when light is wholly absent. The great grey owl (Strix nebulosa) has learned to catch mice hidden beneath the snow, by listening for them and gauging their position with amazing accuracy.
Their broad wings and soft flight feathers allow owls to fly silently and to listen for prey without being heard themselves. Fishing owls are less silent, however, doubtless because their underwater prey will not be disturbed by the sound of their wingbeats.
Little and large
Although owls are birds of prey and the night time equivalent of hawks and eagles, their closest relatives are in fact the frogmouths, oilbird, potoos and nightjars, which comprise the order Caprimulgiformes.
Owls themselves fall into one of two families (see Family Tree). The barn owls can be recognised by their heart-shaped facial disc and long legs. The other family – typical owls – range in size from the elf owl (Micrathene whitneyi), which weighs 1½oz (42.5g), to the Eurasian eagle owl (Bubo bubo), which comes in at a thumping 10lb (4.54kg).
However, whether B. bubo is really the biggest owl of all is open to debate. Other species such as the great grey owl may equal it in size, but weigh slightly less. (A lot of their bulk is feathers.)
Besides the majestic giants which zoos tend to favour, there are many interesting medium- to small-sized owls. The 60-odd species of scops and screech owls occupied, until recently, one large genus, Otus, now usually treated as two – Megascops for the New World screech-owls and Otus for the scops owls of the Old World. Glaucidium contains some 20 species of the exquisite pygmy owls and owlets, mostly from the Americas.
The little owl (Athene noctua) shares its genus with the least nocturnal of all the owls, the burrowing owl (A. cunicularia). This New World species roosts and nests underground in animal burrows, which it modifies, and has even been known to take up residence on golf courses or alongside airport runways.
Owls don’t always shun the daylight, and many species will hunt at dawn and dusk. The largest of the North American owls, the great horned owl (Bubo virginianus), hunts by day or night. The little owl and the spectacled and other tropical owls actively sunbathe.
From prey to pellets
A wide variety of food is taken by owls. Where items are swallowed whole, the birds regurgitate pellets of undigested bones, fur and feathers. Captive owls need this roughage in their diet. Insects and earthworms are taken by small species such as the little owl, once sold in English markets for catching cockroaches.
Small animals are swallowed whole and even large owls catch small prey, which can be swallowed with less effort. Fishing owls will take crabs as well as fish grabbed from near the water’s surface.
Birds often feature as owl-prey, which explains why small songbirds recognise owls as predators and commonly mob them during daylight. The barn owl (Tyto alba) flies low over hedges to flush out smaller birds. Five per cent of an eagle owl’s diet consists of birds, which may include birds of prey – even the ferocious goshawk been taken!
Owls don’t build a typical nest. Instead, they take over the nests of crows or squirrels, as in the case of the long-eared owl (Asio otus), or nest on the ground, as do the short-eared owl (A. flammeus) and the snowy owl. The tiny elf owl nests in holes in cacti. Burrowing owls take animal dung into their nesting burrows, supposedly to confuse any predator’s sense of smell. (If that doesn’t work, they can also mimic the sound of a rattlesnake.)
Owl voices are varied. Individuals can sometimes be identified by their vocalisations and some tropical owls have quite song-like voices.
Their round, white eggs are usually incubated by the female. The young vary in size and, depending on the abundance of prey, a number of clutches can be laid.
Considering that the great majority of the owl species are nocturnal, they are well represented in aviculture and popular in zoos and bird gardens. They are long-lived in captivity – 22 years is recorded for a tawny owl and 68 years for the eagle owl.
Yet, sadly, it tends to be the same few species we seen in most collections, while many tropical owls are neglected. As denizens of wild wooded places, as the woods disappear, so do the owls.
The 217 species of owls (at the last taxonomic count) are divided into two families – the barn owls and the typical owls.
Family: Tytonidae (16 species of barn owls)
Family: Strigidae (201 species of typical owls)
Bill Naylor has worked with many species, including owls, in zoos and bird parks.