The Owl Who Liked Sitting on Caesar (11 page)

TAWNY OWLS ARE
found all over the great Eurasian land mass, from mainland Britain (though not in Ireland) right across to Siberia, China and Korea, down to Iran, the Himalayas, north-west India and Burma. The north–south limits of their range in the Western world are roughly from central Norway and Sweden down to the Atlas Mountains of Morocco. They must have arrived in Britain at least 8,000 years ago, before the English Channel finally broke through between the North Sea and the Atlantic Ocean and turned us into islanders – tawnies don’t have the aerial endurance for long flights over open water.

Adult tawnies measure up to 14–16 inches (35–40cm) long, though their characteristic squatting posture makes them look misleadingly shorter; they weigh between 13oz and 1lb 12oz (385–800g), and their wingspan varies between about 37 and 41 inches (94–104cm). The female – the hen bird – is usually about 5 per cent longer and up to 25 per cent heavier than the cock. Their coloration provides camouflage in their woodland habitat: barred and mottled shades of either brown or (less commonly) grey,
and off-white – the plumage tends to be grey on tawnies living in northern regions, and brown in the more temperate zones. Brown tawnies can be of either a ‘rufous’ chestnut, or (like Mumble) of more chocolate shades. All of them are highly vocal at night, particularly from October to January. Their familiar hooting has long led to them being regarded as the ‘generic owl’ of Europe, and a recording used as a radio sound-effect instantly sets the scene as ‘night-time in the countryside’.

The tawny’s concealing woodland habitat makes its numbers harder to count than those of the Barn Owl, but it is certainly Britain’s most numerous owl – indeed, it is our most common bird of prey. Estimates of the Tawny Owl population published over recent decades have varied surprisingly widely, between a low of about 20,000 breeding pairs and a high of 100,000, and one source even put the number of individuals at around 350,000. The honest answer to the question ‘How many Tawny Owls are there in Britain?’ is therefore ‘Nobody knows – but lots.’ Although the numbers have seemed to be in slight decline in both Scotland and south-west England, warming of the climate may now be extending their range a little to the north, and the national conservation agencies do not consider the overall population figures to be ‘of concern’. (In Europe as a whole their numbers have been estimated at anything between 900,000 and 2 million, and are believed to be increasing slightly. This is heartening – and perhaps surprising, given the indiscriminate attitude of hunters in many parts of southern Europe.)

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The introduction in Britain of chemical pesticides and seed-dressings in the 1950s coincided with an apparent collapse in Barn Owl numbers. Despite legal protection and public campaigns to install nesting boxes for them, their population is still estimated at only about 4,000 breeding pairs. However, while many people simply assume a direct causal link between the use of agricultural chemicals and the decline in Barn Owl numbers through poisoning of their food chain, in fact this remains stubbornly unproven.

The population had actually been dropping steadily for decades before the 1950s because of other aspects of modern farming. Modern metal barns are less hospitable to owls than the old wooden structures (and since – despite their name – about 40 per cent of Barn Owls nest in hollow trees, the much more recent loss of 22 million trees to Dutch elm disease has also had an effect). But food is more urgently important to any carnivore than shelter, and it is the Barn Owl’s hunting terrain that has suffered most.

The ploughing up of rough pastureland and field edges, the grubbing-up of hedgerows, the great decrease in the number of hay fields since the demise of the plough-horse, and the increase in the numbers of sheep (which crop the grass too close to shelter rodents) have all greatly reduced the habitat for the short-tailed voles that form an important part of the Barn Owl’s diet. Since farmers embraced both the combine harvester and modern grain storage
there has also been less food for rats and mice lying around in rick-fields and farmyards. The food chain of grain– rodents–owls must have been central to Barn Owl numbers ever since the invention of agriculture. Tellingly, the few studies that have been made of significant numbers of dead owls suggest that chemical poisoning is in fact very rarely the cause of death, while straightforward starvation is far more common. While there are certainly a number of different reasons for variations in owl numbers over time and place, and a complex inter-relationship between these factors, Tawny Owls have clearly shown themselves to be much more adaptable than Barn Owls, and since the 1950s they have survived in far greater numbers.

Ornithologists believe that this success may largely be explained by the fact that tawnies are birds of the woods rather than the fields, are more purely nocturnal, and are more ‘sedentary’ (home-loving) than Barn Owls, and these last two factors are strongly linked. If you live in thick, dark woodland and hunt almost entirely by night, then however good your eyes and ears are you also need to build up an intimate knowledge of your hunting territory. Owls’ eyes are wonderfully developed, but they are not a super-sense: they are superior to ours only in degree, not in nature. If you are a Tawny Owl, you need to be able to fly between the branches even on nights when it is too dark for you to see anything more than a vague difference between shades of black. You need to be able to find your way to favourite hunting perches close to the areas and the runs used
regularly by your whole range of potential prey, and to find your way home with a kill – whether to your safe roosting tree, or to the nest where your owlets are waiting impatiently. If you are going to exploit all of your home patch’s varied possibilities for seasonal shopping, you need to have a reliable bank of topographical and spatial memories of its whole extent.

Once you have acquired all this vital knowledge, by sometimes perilous trial and error, why would you choose to travel? So, once it has mated, a Tawny Owl may spend its whole adult life in an area sometimes measuring as little as 350 yards each side, and with the same mate. The Barn Owl does not have such a predictable breeding season as the Tawny, and cock birds may spread their favours more widely among several partners. They may maintain a small defended territory around a nest, but to catch their prey Barn Owls have to range widely on the wing over a much larger area of fields. This more opportunistic method of hunting must logically leave them more vulnerable to variations in the number of available prey animals, which can be dramatic, and can occur every few years. With a smaller hunting area that it has taken the trouble to get to know like the back of its foot, and with a permanent partner, the Tawny Owl can adapt to such changes much more successfully.

While British tawnies prefer to live in old broad-leaved or mixed woodland, they have also proved resilient in the face of the felling of forest for replacement with commercial conifer plantations, so long as these are broken
up with a reasonable number of clearings and rides. (Conifer forests in continental Europe support large numbers of tawnies.) Many British tawnies have also responded to the spread of urbanization by moving successfully into the suburbs, and even into city centres, where they may nest in nooks and crannies on the ledges of tall buildings.

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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.