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Finicky eating habits are probably the greatest threat to any creature’s chances of survival in a changing environment, and if the panda is at one end of the spectrum of adaptability, the Tawny Owl must be close to the other extreme.
A tawny will cheerfully eat almost anything smaller than itself that creeps, wriggles, runs, flies or swims. Its most frequent prey are small rodents, though the balance of types on the menu varies to a certain extent with the cycle of seasons and vegetation, and with the amount of competition from weasels and stoats. One famous English study of the tawny population in an area of Oxfordshire established that bank voles, wood mice and a few shrews made up 60 per cent of their diet, and that the whole range of small mammals – including rats, moles and young rabbits – made up 95 per cent. (Owls usually leave shrews alone; they are too small to be worth much effort, and their defence mechanism is a foul taste.)
In the countryside smaller birds make up only 5 to 10 per cent of the tawny’s diet, but urban sparrows, starlings, thrushes, blackbirds, pigeons and sometimes even jays provide the great majority of the prey taken by those pioneer families that have responded to loss of woodland habitat by moving into our cities. (There is even one record of an impressively ambitious tawny taking a mallard duck off a city lake.) Tawnies will also routinely hunt on foot to catch beetles, slugs and snails, and particularly earthworms. Those living near water eat molluscs and crabs, and some have been filmed wading into the shallows to catch fish – anything from minnows and garden-pond goldfish up to small river-trout.
The Tawny Owl’s most characteristic method of hunting is to pick a look-out post in a tree at sunset and to wait patiently, watching and listening for the movement of prey below. When they locate a potential meal they judge its direction and range with great precision, then make a short, direct pounce or glide to hit it with their talons. Unlike Barn Owls, they do not make hunting patrols – flying back and forth over open ground, watching out for potential prey below them – but they may dive to make opportunistic kills if they happen to spot something that looks tasty during their short woodland flights from perch to perch. Their ability to locate their prey in darkness – at least, on ground that they know intimately – depends in equal measure upon their extraordinarily efficient eyes and ears.
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Animals whose survival depends mostly on seeing what is sneaking up on them, in order to avoid being eaten, generally have their eyes placed in the sides of narrow skulls so as to give the widest possible field of vision. Animals that depend on doing the sneaking up usually have their eyes placed forwards in broader skulls; this increases their binocular (i.e. overlapping, and thus three-dimensional) vision, so that they can judge accurately the distance to their potential dinner as well as its direction.
For example, a pigeon has a total visual field of as much as 340 degrees of the 360-degree compass, with a blind spot of only 20 degrees directly behind its head, but only another frontal ‘pie slice’ of about 24 degrees of the total is binocular. When it is looking in a given direction a Tawny Owl’s visual field is probably a bit less than half that of a pigeon, but up to 70 degrees of this is binocular vision. You and I also have wide-set eyes, and within our total visual field of around 180 degrees – roughly, from ear to ear – the natural area of binocular vision is about 90 degrees. But although we can also swivel our eyeballs from side to side to cover up to 140 degrees stereoscopically without moving our skulls, we cannot rotate our heads right round to look backwards. Owls can, so they still win hands down.
Despite our folklore, owls cannot see in total darkness, but their perception of ‘total’ darkness is nothing like our own – indeed, under the open sky there is never a complete absence of light. Owls’ eyes have evolved to exploit the faintest levels of illumination, and function well enough even on overcast, moonless nights that appear to us as pitch black. Various estimates have been published of the superiority of owls’ visual sensitivity over that of humans, but defining the exact terms of reference, and devising experiments so as to screen out the many possible variables, has proved extremely difficult. Extraordinary as it seems, scientists have proved that an average nocturnal owl’s ‘absolute visual threshold’ – the point below which it cannot even detect the presence of light – is only about 2.2 times lower than ours, and that some exceptional individual humans may even have a lower threshold than some individual owls. (Incidentally, by this measurement a cat performs better than both of us.) Of course, the practical question as far as the owl is concerned is not how superior its absolute visual sensitivity is when compared with ours under laboratory conditions, but how efficiently it can function at the levels of illumination that occur in its natural surroundings.
Experiments have been carried out with Barn Owls in adjustable indoor conditions of artificial darkness, in which the owls flew a course hampered by randomly hanging strips of light card. Mathematical interpretation of the results suggested to the researchers that in these conditions the owls’ efficient exploitation of low light might be roughly 100 times superior to our own, and to that of daytime birds. In the end, however, when illumination had been artificially reduced well below what we would call absolute darkness, the owls collided with the cards, and thereafter they sensibly refused to take any further part in the experiment. There is plentiful evidence that Tawny Owls sometimes collide with branches or even tree trunks on very overcast nights when they are below the thick woodland canopy – the darkest of all natural environments – and comparative studies suggest that they are more prone to collision injuries than daytime birds.
Again, a number of experiments have been carried out in the hope of quantifying the light levels at which an owl can locate, say, a mouse when compared with a human’s ability to see it. These produced the suggestion that an owl was some 300 times better at this trick than a scientist, but the methods used to reach this conclusion seemed to other researchers a bit hit-or-miss. Apart from the failure to ensure sufficiently consistent conditions and to provide controls, the question of motivation seems to have been ignored: just how badly did the scientist want to find the mouse, compared with the hungry owl? (Incidentally, in case the question of body warmth occurs to the ingenious reader, only dead mice were used in these tests; and anyway, a suggestion that owls can detect a source of warmth by infrared vision has since been disproved.)
Despite all these uncertainties, one analogy that has been quoted is that an owl could spot a mouse on a football field that was illuminated by a single candle. This may well be true – but only for a particular owl, hunting a mouse of a particular colour, on a particular football field, in particular conditions of ambient light and weather, and lit by a candle at a particular distance and angle. All in all, it is probably enough for us to know that (a) in darkness owls can make much better use of their eyes than we do; but (b) this has less to do with the absolute relative sensitivity of our eyes than with how accustomed we are to using them in extreme conditions, in conjunction with our other senses. Put simply, it isn’t so much that owls have very dramatically better visual equipment than we do; their advantage comes from their using it far more efficiently.
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Because even owls’ ability to see in darkness has its definite limits, hearing is actually at least as important to them as eyesight, and much more important when the darkness is at its deepest.
An owl’s highly developed ears are located in two vertical ‘trenches’ in its skull located just behind the edges of the facial disc. The shape of the facial disc gathers sound, because the dense ruff of feathers around the edges forms a dish, and there is some evidence that the consistency of the tiny face feathers also plays a part in this effect. The pitch or frequency of sounds is measured in units of kilohertz; human ears (when young – an important qualification) can detect sounds within a wide range of between 2 kHz and 20 kHz, but are most efficient at around 4 kHz. Tawny Owls’ hearing is also at its most acute at medium frequencies, between 3 a
nd 6 kHz. It has been estimated that within certain given frequencies an owl’s ears are ten times more efficient than ours, although this varies from species to species; for instance, Barn Owls have an optimum frequency of 7 to 8 kHz. Hearing in owls has been estimated to be perhaps 300 times more acute than that of daytime birds, but – as with their eyesight – its superiority is nothing like that great when compared with either humans or cats. (And as with eyesight, there are important qualifications to any headline figures, to take account of variables like background noise. Hunting by ear is obviously far more difficult on a windy night in the woods than on a still night in the middle of a cropped field.)
An owl’s range of aural perception coincides with the sort of noises that a rodent makes when rustling through grass or leaf-litter, and if the potential meal is foolish enough to squeak then the owl’s job becomes very much easier – as the pitch of a sound rises, the owl’s ability to pinpoint it in space improves. (Shrews are extremely quarrel some creatures with high-pitched voices, and their ‘sound discipline’ at night is stupidly lax; we might guess that it is only their vile taste that has stopped owls wiping them out long ago.)
In addition to acute hearing, the more nocturnal species of owls have a highly developed ability to process the information that reaches their ears, filtering out the distracting background clutter and drawing conclusions from the sounds that interest them. Their brains include a sort of ‘target acquisition computer’, which calculates the angle and range of whatever is making that attractive rustling or squeaking noise. This capacity for fine judgement is enhanced by the fact that some species – including Tawny Owls – have asymmetric ears, with one slightly higher than the other and oriented at a slightly different angle, so that even when the head is motionless a sound reaches each of the two ears after slightly different time delays. In tawnies this asymmetry only affects the soft outer flaps rather than the actual passage through the bone; in some other owl species the difference is more pronounced, and one ear trench is also noticeably larger than the other, giving the skull a slightly uneven shape.
As well as equipping them superbly for night-time hunting, their all-round vision and sophisticated auditory equipment naturally make owls, in their turn, extremely difficult to sneak up on. While it is far from true in other parts of the world, in Britain an able-bodied adult of the larger species of owls has very few natural predators to fear. (There is one report of a Tawny Owl being taken from its daytime roost by a buzzard, at a time when rabbits were particularly scarce in that area; but the owl fought to the death, inflicting such serious damage on its killer that this must presumably be a rare event – predators will always prefer to go for an easy kill.)
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Because Tawny Owls are so relatively plentiful in Britain, and so attached to permanent territories, ornithologists have been able to make some quite thorough studies of their way of life. This applies particularly to their rearing of families, during a time of the year when, necessarily, they stick close to home and follow repetitive patterns of behaviour.
Tawny cocks and hens first seek mates around the turn of the year, when they are perhaps eight months of age. By this time they have survived the initial shock of their first early winter in the territories that they established during the autumn. Courtship among all territorial carnivores must certainly be a nervous business, since their instinct is to challenge, drive off or attack all strangers. Tawny Owls show little if any difference in appearance between the genders, but are clearly able to recognize each other’s sex and individual identity by means of calls. (Some species of owls, though not tawnies, sing formal duets together at a distance before approaching one another.)
It is often said that cock tawnies give their familiar wavering hoots, which are answered by the hens with sharp ‘kee-wikk’ calls. This does happen, but the distinction is not rigid; both cocks and hens make both sounds, though not during the same conversation. Although Mumble might suddenly exclaim ‘kee-wikk!’ if she seemed to hear some distant hoot, her initial long-range calls were hoots; the hoot most often seems to be the question and the ‘kee-wikk’ the reply, from either sex. (This means that the traditional rendering of the call as ‘tu-whit … tu-whoo’ merges both question and reply, and puts them the wrong way round – it should be ‘tu-whoo … tu-whit’.)
If all goes well at what we might call the ‘e-mail and phone-conversation’ stage, the male flies closer to the female and, after a certain amount of eager chasing through the trees, the pair tentatively agree to meet. From that point on body language plays the major part in their exchanges. Once a male and a female Tawny Owl have, as it were, agreed to lay aside their weapons and discuss this situation like adults, they settle on a branch for face-to-face negotiations. Some courting cock birds have been seen bringing the hen a gift of food, to break the ice. The cock utters a variety of quiet grunts, croons and clucks while sidling forwards and back again, swaying and nodding, alternately raising his wings and puffing out his body feathers, and then sleeking himself down again. He will sometimes be allowed to get close enough to nibble at the hen bird’s beak.
With luck, this performance will eventually earn him a softly repeated invitation from the hen; the cock then mounts her from behind, and Nature takes its course. As with many bird species, while the drive to mate is strong the actual moment seems to be a fairly brief and mechanical affair, distinctly short of ecstasy. To us the process anyway looks frankly impossible, since it involves the alignment of two internal vents hidden by feathers, but it clearly works for them. However, when the climactic moment has passed the pair will then roost together, side by side. Their earlier mutual suspicion is forgotten; pressing closely together, they will spend long periods preening each other’s faces, heads and necks, in what certainly looks like highly pleasurable pair-bonding.
While we cannot tell whether or not they are invariably faithful, Tawny Owls are monogamous. There are differing opinions, but most ornithologists believe that tawnies mate for life, although the pair do not live together all year round. They often roost separately – notably during the autumn, when their annual brood of owlets have dispersed. Nevertheless, as soon as the pair-bond has formed they establish a joint territory for hunting, and thereafter they will share it for life, working together to expel any competitors. A famous study carried out in the early 1950s over some 1,300 acres in Oxfordshire (led by Britain’s doyen of Tawny Owl studies, Dr H. N. Southern) estimated the pair territories at between 32 and 50 acres – smaller in close woodland, larger in mixed woods and open country. Size is determined by a territory’s richness in prey animals: further north in Britain territories of up to 80 acres have been recorded (and a German study in much less prey-rich conifer forest found pair territories that seemed to cover several hundred acres). The boundaries of this hunting range seem hardly to change from year to year, since both cock and hen defend it fiercely from rival pairs and from hopeful juveniles seeking a first territory in autumn.
In England, mating pairs of tawnies establish a nest in February or March each year, and may well use the same few convenient sites in their territory year after year. As in their eating habits, tawny homesteaders are extremely easy-going. They can’t be bothered to build nests themselves, and usually prefer to move into existing holes in tree trunks or stumps – often courtesy of woodpeckers. Alternatively, they may take over the abandoned nests of other large birds such as sparrowhawks, jackdaws, magpies and crows, or even old squirrel dreys. Unlike Barn Owls, they will not tolerate humans close by, but they may choose to set up home in a suitable corner of a derelict building, and they are perfectly happy to use artificial nesting boxes (though these have to be of a different design from those provided for Barn Owls). In conifer plantations with plenty of ground litter, and in extreme areas of their geographical range such as the thinly wooded Scottish highlands, they may even nest on the ground. This casual attitude extends to the interior furnishing; tawnies make no effort to line a nesting hole with soft
materials, since the hatchlings will emerge from the egg well protected by thick, fluffy down.
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Usually the hen lays three to five white, spherical eggs over a number of days from about mid-March, and sits on them for just under a month until they hatch; during this incubation period she is fed by the cock bird. Depending upon region, climate, prey availability and other variables, the staggered hatching of the owlets may be completed during April or not until June. From the time when the last hatchling breaks out of the egg the cock bird needs to catch perhaps a couple of dozen prey animals each day, depending upon their size. Each of the owlets needs feeding several times daily; the cock also has to hunt for their mother, who remains in the nest with them, and to keep up his own strength for this exhausting effort. This often forces him to continue hunting from dusk until well into the next morning in order to satisfy his family’s endless demands (which is certainly no fun for him, but provides us with our few opportunities to see a Tawny Owl out and about by daylight).
The hatchlings remain in the nest with the hen bird for about three weeks before they achieve the strength and confidence to start creeping outside and moving about. They grow very fast, and their ravenous appetite increases with their size. At this point the demands of feeding the whole family become more than the most dedicated father can handle alone. Thereafter both parents have to take turns in a busy hunting rota throughout this fledging stage, and the fledglings remain dependent upon their parents for all feeding for up to another twelve weeks. To satisfy their voracious demands the parents have to provide well over a thousand rodents and small birds during this period. Among daytime raptors the parents will usually break up the prey and feed their chicks with bite-sized chunks. Initially Tawny Owl mothers will also roughly butcher the prey – at least taking the head off, to get the owlets started – but before long the babies’ own claws and beaks are strong enough for them to manage on their own when a meal is too big for swallowing whole.
The Owl Who Liked Sitting on Caesar Page 9