Owl

by Desmond Morris

  Preceding pages: Barn owl (Tito alba) with prey.

  Pels fishing owl (Scotopelia peli) with catfish prey.

  The diet of owls is varied but rodents in the shape of voles, mice and rats must account for the bulk of the food taken. In this respect the owls must be considered valuable pest-controllers and the farmers’ friend. Sadly, the refusal of old superstitions about owls to die away has meant that even today, in some regions, owls are still persecuted instead of treasured.

  Other foods taken include a variety of small birds and occasionally rabbits, fish, amphibians and reptiles. Bigger owls have no respect for their own kind and often prey on the smaller owl species. The largest of all owls have been known to prey on animals as big as foxes, small deer and dogs. The smallest prefer large insects, spiders and other invertebrates. Insects may be caught on the wing.

  When food becomes unusually plentiful, owls have been known to stock up a small larder for themselves. The surplus kills may be pushed into a tree-hole, in the crease or fork of a suitable branch, or sometimes even in the nest.

  PELLETS

  Owls that swallow their prey whole are faced with a problem. They may avoid having to perform laborious food preparation sequences but as a result are left with a stomach full of indigestible material, such as bones, beaks, claws, teeth, scales and insect skeletons. This unwanted material is gathered into a wet, slimy oblong pellet and is then regurgitated by the bird. Regurgitation is helped by the fact that owls somehow manage to form the pellet in a special way, enclosing the sharper objects inside an outer layer of smoother rejected material such as fur or feathers. The owl is then left with the soft parts of the prey that it can easily digest with the aid of proteolytic enzymes and stomach acids.

  Owl disgorging a pellet.

  These pellets, found on the ground near an owl roost or nest, are of enormous help to ornithologists. By collecting them from the woodland or forest floor and carefully dissecting them and analysing their undigested contents, it is possible to assess with great accuracy the feeding habits of the owls. The investigators are helped in this by the fact that owls have a rather narrow pyloric opening from the stomach into the intestines. This prevents all but the tiniest bones or other fragments from passing onwards from the stomach, so that the regurgitated pellet usually contains virtually the complete skeletons of the prey animals devoured the night before, facilitating the identification of the prey species.

  Burrowing owl with newly disgorged pellet.

  The analysis of owl pellets is such an excellent teaching device that there are even specialized companies set up solely to supply pellets to schools and other educational establishments. For example: Pellets Inc. boasts: ‘We take pride in providing the best barn owl pellets and the most complete line of support products available in the world . . . Because we collect, heat sterilize, sort, wrap, and ship every pellet we sell, we guarantee the highest quality and service. For 18 years the same Pellets Inc. employee has carefully hand-sorted and wrapped every pellet we sell.’2

  Contents of the pellet of a long-eared owl (Asio otus).

  The production of pellets by the owl follows a regular cycle. First, the owl hunts, kills and swallows its prey whole. The small corpse slides down the bird’s oesophagus and, since owls have no crop, straight into its glandular stomach, the proventriculus, where it is attacked by the digestive juices. It then moves on to the muscular stomach, or gizzard, where the digestible parts of the prey are passed on to the intestines for absorption, while the indigestible parts are compacted into the pellet. This pellet is then passed back up into the glandular stomach where it is stored for up to ten hours. During this phase of the cycle the owl cannot feed because the pellet is blocking the system. When the owl is ready to hunt again, it starts to show discomfort. It closes its eyes and stretches its neck upwards and forwards with its beak open. At this point the pellet drops out of its mouth and falls to the ground. Now the bird is ready to hunt once more and the cycle is complete.

  VOICE

  It has been said that owls are more often heard than seen, which may explain why some people fear them and others find them eerie and unearthly. Songbirds they are not. Even the traditional tu-whit tu-whoo that tawny owls are supposed to call out is too friendly and too kind to them. To listen to most owls crying in the night air, you might imagine you were standing outside a torture chamber. It is said that they hoot but in reality they are more likely to shriek, scream, screech and squawk. Others are known to growl, snore, buzz, cough or chime. Some sound like machines that need oiling, others like a man trying to start a car with a flat battery. Still others sound like a giant grasshopper or a cross between a barking terrier and a gibbon. Only the biggest owls produce smoother, softer sounds and even these are reminiscent of someone pretending to be a ghost to frighten a small child.

  Recordings of great horned owls (Bubo virginianus) are intriguing because they reveal that each bird has its own personal morse code of woo-woos which presumably means that individuals can easily identify one another even though they all have no more than two notes in their ‘song’, a long woooo and a short woo.

  One will go: woooo-woo-woo-woo-woooo-woooo

  While another calls: woooo-woo-woo woo-woo-woooo

  And a third calls: woooo-woo-woo-woo woooo-woooo-woooo

  These subtle differences will be enough for rival males, hooting to one another at night, to signal their territorial positions and to defend their hunting grounds. If one owl suddenly stops hooting at night, his territory will gradually be absorbed by neighbouring rivals.

  In the breeding season, the hooting of the males will attract females and help to bring them into reproductive condition. The vocalizations of owls may not echo musically through the woods like exquisite birdsong, but have precisely the same functions and are just as efficient.

  BREEDING

  Finding a mate can be a hazardous undertaking for an owl. As a territorial predator with powerful weapons, it is capable of defending its home against all comers. As males and females look the same as one another in almost all owl species, it is not easy to tell whether, at the start of the breeding season, an approaching bird is a member of the opposite sex seeking a mate or a rival of the same sex. Females are usually slightly bigger than males but this is not a good enough clue to establish the gender of another bird. More information is needed and this usually takes the form of differences in calls and in behaviour. Many owl species perform duets, with differing male and female calls being bounced back between one another. And the behaviour of an approaching female will also provide clues for a resident male. She will approach him in a manner that is appeasing and is neither too aggressive nor too frightened. Were she a rival male, the response to a male territory-owner would be either to fight or to flee. To appeal to a mate, the female must do neither of these things.

  During the more intimate stages of owl courtship there is a great deal of beak-snapping, body swaying, bowing, wing raising, head waving and feather ruffling, as the pair attempt to achieve the synchronized arousal that will eventually lead to the mating act. Occasionally, food presentation (well known in other birds) has been observed in courting owls, when the male breaks off his displays to swoop down for a quick kill and then flies back up and offers the corpse to his female as a special gift.

  The problems of choosing a partner are reduced by the fact that most owls are monogamous and therefore only face the difficult challenge of finding a suitable mate once in their lives. In many species the pair may not stay together during the non-breeding seasons of the year but even so, when the mating season arrives, they are only reacquainting themselves with one another, rather than starting from scratch.

  Owl nest with young of different sizes.

  For owls, finding a suitable nest-site is more important than building a nest. In terms of nest construction, owls are at the opposite end of the scale from weaver birds. Their nests are typically clumsy, rough-and-ready affairs, but their selection of a nest-site i
s done with great care. They search for a protected cavity, a safe corner of a deserted or ruined building, a tree hollow, a rock crevice or the abandoned nest of some other bird. Once they have found a suitable site and made it their own, the female will lay a clutch of white, almost spherical eggs. She will usually do all the incubating herself during the 21–35 days it takes to hatch the eggs. Throughout this time her mate will bring food to the nest for her. Once the eggs have hatched, both parents will bring food for them. When the chicks are tiny the parents dismember the prey before passing it to them, making it easier for them to swallow.

  Threat display by a great horned owl (Bubo virginianus) – a flightless youngster in defensive posture.

  The number of eggs varies enormously in different owls, but for most species three to four is the norm. The female usually allows several days between the laying of each egg, with the result that chicks are of different sizes. If food is plentiful all the chicks thrive but if it is scarce then only the larger chicks will survive. In bad times the younger chicks find it hard to compete for food and may starve to death in the nest, when they themselves will become food for the older chicks. This harsh breeding system ensures that the parents produce the appropriate number of chicks for a particular environment.

  Owl butterfly (Caligo eurilochus sulanus): a butter-fly imitating an owl’s face.

  Owls may be poor nest-builders, but they are superb nest-defenders. If an intruder, including one as large as an adult human, comes too close to an occupied nest, the parent owl may either perform a dramatic defensive display or carry out a savage attack. The display consists of ruffling all the feathers, spreading the wings wide and then rotating them forwards and downwards. This has the effect of making the owl suddenly look enormous. In this threatening position it may proceed to clatter its beak and perform hissing and other sinister noises, as if saying, one step nearer and I will attack. Its huge, brightly coloured eyes stare fixedly at the intruder, adding further intimidation. The threatening quality of these eyes has led to the evolution of eye-spot display markings on the wings of some moths and butterflies that mimic the appearance of an owl’s face. The most impressive example of this mimicry is found in the owl butterflies (genus Caligo).

  An additional defence strategy of some owls is to perform a distraction display. This consists of a parent bird flapping around near the nest as if it is badly injured and therefore easy prey. In this way it tries to draw the attention of the intruder away from the helpless chicks in the nest. When the distracted intruder is about to pounce, the apparently vulnerable adult bird quickly flies away to safety and, with luck, the chicks will be overlooked.

  On other occasions a full attack may be delivered in which the parent owl swoops low over the intruder’s head and tries to slash it with its razor-sharp claws. One famous bird photographer, Eric Hosking, lost his left eye to a tawny owl in just such an encounter. He later published an autobiography with the sardonic title of An Eye for a Bird.

  MOBBING

  One of the strangest aspects of the natural history of owls is the treatment they receive from other birds. If an owl for some reason makes the mistake of appearing in the open during daylight hours it can expect to be quickly surrounded and attacked by a swarm of angry daytime birds. These may be considerably smaller than the owl but feel they have safety in numbers.

  This mobbing behaviour has fascinated human observers since at least the sixth century BC. There is a beautiful Greek vase, a black-figure amphora, dating from that time, showing an owl tethered to a post underneath a tree. A flock of small birds fills the air around the captive owl and some of them have settled on the branches of the tree. These branches have been smeared with sticky bird-lime and, once they have landed there, the little birds are stuck fast and can easily be caught and killed for the pot. Even at this early date bird-catchers not only knew about the mobbing of owls but had learnt to exploit it.

  Mobbing of tethered owl by the Bucci Painter (last quarter of 6th century bc), a Greek Attic black-figure amphora.

  Two centuries later, in his Historia Animalium, first published in 350 BC, Aristotle reveals that this knowledge had not been lost, when he writes: ‘In the daytime all the other little birds flutter round the owl – a practice which is popularly termed “admiring him” – buffet him, and pluck out his feathers; in consequence of this habit, bird-catchers use the owl as a decoy for catching little birds of all kinds.’3 The oddity in this comment is the use of the word ‘admiring’ for the act of mob-bing. Because the Athenian owl was a revered symbol of wisdom, it was hard for a Greek author to view mobbing as the hatred of the owl by other birds. Instead it was more comfortable to suggest that ‘it was the wisdom of the owl that the small birds wondered at.’

  In Roman times Pliny mentions mobbing in his Natural History of AD 77–9. His description is a curious one but may have been based on an extreme case. He says: ‘It is a pretty sight to see the wit and dexterity of these Howlets [little owls] when they fight with other birds: for when they are overlaid and beset with a multitude of them, they lie upon their backs and with their feet make shift to resist them: for gathering themselves into a narrow compass, there is nothing in a manner to be seen of them, save only their bill and talons, which cover the whole body.’4

  The bestiaries of the thirteenth century often include a mobbed owl among their illustrations. There appears to have been a certain amount of cribbing from bestiary to bestiary, as the scene repeats itself with only minor variations. The owl is shown being pecked by three birds, the lower one being a magpie. The immobile owl is seen suffering this indignity stoically, in a stiffly upright posture. The pious bestiaries taught the lesson that the owl was being attacked because, as a bird that courted darkness, it had ‘rejected the light of Christ’.5 And to make the bird seem even more sinister one bestiary even claimed that the owl flew backwards.

  A little later, in the fifteenth century, five small birds are seen attacking another long-suffering owl, this time carved in wood on a misericord (the carved ledge below the seat) in Norwich Cathedral. Similar carvings can be seen in a number of other English churches of this period, from Yorkshire in the north right down to Somerset near the south coast.

  Owl mobbed by five birds, carved on a misericord at Norwich Cathedral, East Anglia, 1480.

  In the early sixteenth century Albrecht Dürer deserts his usually naturalistic style of rendering animals and gives us a desperate, wild-eyed owl with feathers raised and wings flapping as it is tormented by four angry birds, with their razor-sharp beaks and claws ready to attack. The following century Francis Barlow shows an owl mobbed at the entrance to its nest. His owl looks bewildered and the seven birds attack from all sides. Barlow describes his scene as symbolizing a sinner attacked by the righteous, feeling the need to give his picture a moral message despite the fact that it is a reasonably realistic portrayal.

  Albrecht Dürer, Owl in Combat with Day Birds, 1509–15, woodcut.

  A much more naturalistic seventeenth-century treatment of mobbing is to be found in a mosaic in Florence by Marcello Provenzale (1575–1639), where the artist depicts an owl harassed by a whole variety of precisely portrayed species, including the robin, goldfinch, greenfinch, chaffinch, sparrow and great tit. Ornithologically speaking, this mosaic is two centuries ahead of its time.

  Moving on to the present, these works of art have largely been replaced by photographs snatched by bird-watchers who are lucky enough to come upon one of these avian dramas. Thanks to their observations we also have much more detailed descriptions of exactly what happens when a stranded owl becomes the victim of mob violence.

  It is worth asking how the daytime birds develop this uncharacteristically bellicose habit. Small birds everywhere suffer from a lifelong fear of owls. It is an inborn fear that develops when they are only a few months old, regardless of whether they have ever met an owl. Confirming this is the fact that, as mentioned earlier, some moths and butterflies have evolved an owl-eye marking on th
eir wings that they can flash at small birds approaching too closely and which frightens them away.

  This inborn fear of owls generally results in a life-saving avoidance of these predatory birds, but occasionally – usually when they have companions near them – the tormented become the tormentors. Instead of fleeing, they stand their ground and confront the owl. Screeching an alarm call, they attract more and more small birds to the scene until the bird of prey is surrounded by a noisy, angry mob. They now start to harass the bigger bird, calling incessantly and loudly, twisting and jerking their bodies and even making mock attacks. Sometimes a particularly bold individual will risk a real attack, swooping in from behind the owl and striking at its plumage.

  A great horned owl mobbed by crows at Root River, Racine County, Wisconsin.

  This mobbing behaviour never occurs when the predator is actively hunting. It is most likely to occur when the owl is behaving in an odd way. If it is injured or sick it may sit quietly in an unusually visible position during daylight hours. A conspicuous stationary owl is a major target for mobbing. The small birds gather around and approach it remarkably closely, often as near as 3 metres (10 feet), and then start to display. The exact movements vary from species to species but in a typical finch such as the chaffinch the body is turned towards the owl, the crown-feathers are raised, the legs are bent, the wings are slightly raised, and the body is jerked quickly from side to side in a crouched posture, while the tail flicks up and down.

  Many species have been seen to indulge in this strange mob-bing display, including finches, tits, buntings, warblers, blackbirds, thrushes and even little hummingbirds. The hummingbirds become particularly hostile, buzzing around the big bird’s head, as close as 5 cm (2 inches) from its face, calling out and jabbing at its eyes. Some of the larger birds, like the blackbirds and thrushes, often risk a little dive-bombing, in which they swoop down on the owl from 9 metres (30 feet), heading straight for it, and then swerve aside only at the very last moment, when they are no more than a foot away. Sometimes they leap at it from behind and claw at its head feathers.