The Owl Who Liked Sitting on Caesar (17 page)

WHILE IT WAS
Mumble’s looks and behaviour that captivated me, acquiring an elementary knowledge of the owner’s handbook still seemed in order. I never became even an amateur ornithologist, but I still felt an inborn need to understand what I was looking at. Though my grasp of the scientific principles of the internal combustion engine is shaky, I still want to know roughly what’s under the bonnet, and how the major moving parts interact. Consequently, this chapter may be regarded as a layman’s brief inspection tour around Mumble, with the manual in one hand. What I discovered while doing even some basic reading was impressive.

* * *

The Tawny Owl is rather charmingly described as ‘portly’ in the 1943
Handbook of British Birds
, and the first surprise was the enormous difference between Mumble’s external appearance and the ‘cutaway diagram’ (see the drawing on
here
). She was a miracle of concealed compression; although her normal comfortable squat made her look like
a relaxed bundle of feathers about 10 inches tall not counting the tail, her skeleton, if stretched out from head to talons, would have been at least half as long again. The most striking revelation was the length of her snaky, S-shaped neck, which was made up of about twice as many cervical vertebrae as a human’s. When she was at rest her neck was completely invisible, hidden by a sort of telescopic effect of the deceptively thick, loose ruff of plumage between the ball of her head and the bulk of her body. In repose her neck was arranged like a swan’s, and its concealed length obviously explained her ability to twist her head around to such extreme angles. It also contributed to a trick that she demonstrated far less often, but which I found so amusing that I was sometimes tempted to tease her so that she would keep doing it.

It is only in recent decades that human engineers have come up with a really efficient gyroscopic stabilizer for incorporation in tank turrets (bear with me – there is a point to this). If you watch film of a modern battle tank rolling fast across rough ground during a firing exercise, you will notice the uncanny way in which the gun remains aimed steadily at a fixed point even though the tank’s hull below it is rolling and pitching in all directions. Achieving this took half a century and cost many millions, so imagine my delight when I discovered that Mumble could do it automatically.

One day I happened to lift her off her tray-perch while she was concentrating on something she had spotted outside the window. As she stepped backwards on to my hand and I lowered my arm, her head remained immobile – I don’t mean in relation to her body, but
at exactly the same height and angle in mid-air
as when she had been up on the perch. As my hand and her body descended, her neck simply deployed upwards out of her shoulders, the feather ruff stretching and growing narrower, so that her head remained fixed in all three dimensions while her body dropped several inches below it. This looked so weird that I instinctively raised my hand again – her head simply stayed where it was while her neck steadily disappeared, telescoping smoothly down into her shoulders, with the feathery ruff fattening out around it again.

The skeleton of a typical tawny owl.

I need hardly add that I immediately fell prey to a mischievous temptation to play owl yo-yo – lowering and raising my hand, and watching her head remain fixed in mid-air while her neck extended and contracted below it. I did this several times, giggling foolishly, until Mumble got fed up with this childish game and took off.

* * *

With a little imagination, it is possible to figure out from a drawing how a bird’s skeleton evolved from that of a reptile all those eons ago in the Upper Jurassic. After some 160 million years it still recognizably has a head, a neck, a torso, a pelvis, a tail and four limbs, all more or less where we would expect them to be. However, a simple outline drawing of the skeleton does not tell us the half of it.

When I got stuck into the textbooks, I learned that Mumble’s body was designed – logically enough – with the
primary object of achieving a high power-to-weight ratio. Having once been an aviation journalist, I couldn’t help seeing her as Nature’s equivalent of an aircraft that has much of its structure built from drilled-out aluminium and moulded carbon fibre, to reconcile strength with light weight. To provide the thrust to get it into the air and flying at operational speed, this structure was powered by a high-revving engine (her heart and lungs), consuming large quantities of fuel (oxygenated blood) circulating at high speed.

When I picked her up she seemed too light for her apparent bulk, and this was not only because most of the visible Mumble was made of feathers and the air trapped between them. Her lightness was also due to the fact that many of her bones – parts of her skull, vertebrae, breastbone, the humerus or ‘main wing spar’, her ribs, pelvis and legs – were actually partly hollow, with air spaces inside. You would think that this made them dangerously fragile, but they had an internal bracing of little bone struts across the hollow shafts. The arrangement of her innards also showed some emphasis on saving weight: the designer had left out some of the squishy bits and liquid that we have, and in some cases of paired organs only one of them was fully developed and functional.

Many of the bones of her body – her massive breastbone, the shoulder blades, lumbar vertebrae, ribs and pelvic girdle – were fused together at their extremities, so as to create a rigid box structure protecting her internal organs. The most prominent part of this was the big keel of
her sternum or breastbone, to which the powerful flight muscles were anchored; this was braced to her shoulders by two strong bones, to prevent the pull of the muscles collapsing the box of her torso.

Inside this protective fuselage structure, the powerplant that enabled Mumble to generate the extraordinary muscular activity necessary for flying was a heart that was – relative to our body sizes – much larger than mine, and beating much faster. At rest, this amazing engine beat about 300 times a minute (four times the human rate), and it pumped – again, relatively speaking – about seven times as much blood, at much higher pressure.

The ‘carburettor’ that mixed the fuel from blood and oxygen was a pair of lungs that were relatively smaller than mine, but that were linked to an extensive secondary air-circulation system; this got large quantities of usable oxygen into her bloodstream quickly, while also contributing to the buoyancy of her body. Birds have a network of (usually nine) internal air sacs – simplified extensions or annexes to the lungs. These act rather like bellows, moving fresh, oxygen-rich air through the lungs quickly without it getting mixed up with the stale ‘exhaust’ air already inside the system. Eight of a Tawny Owl’s air sacs are arranged in pairs down the sides of the breast and belly, and the ninth, shaped roughly like an upside-down triangle, is mounted centrally at the top. This last sac has pipe-like pouches at the top corners that extend out along the inside of the humerus bones; when Mumble breathed in, air passed through her lungs and air sacs and right up inside her wing structure.

Unlike mammals, birds have no diaphragm muscle, and they breathe by expanding their ribcage to draw in air. (This is why it is vital, when holding a bird, not to constrict its torso – if the ribs can’t move, the bird can’t breathe.) It is almost impossible to see an owl breathing, unless you catch a glimpse of the slight, regular movement of the lower back feathers between the folded wings.

* * *

While I was reading up on owl anatomy, I often found myself glancing back and forth between a book in my hand and Mumble sitting modestly beside me, while I muttered things like ‘Damn me, sweetheart! There’s a lot more to you than meets the eye, isn’t there?’ One of the first things to fascinate me was an explanation of the inner workings of her own eyes. Behind her innocent gaze and friendly blink, there was a great deal going on that I had not previously understood.

Owls’ eyes have much the same basic components as our own. Behind the cornea (the transparent outer cover), the adjustable muscular ring of the iris (the coloured bit of our eyes) regulates the passage of light through the pupil or central hole (the black bit). In bright light the pupil shrinks, in low light it opens out, and this regulates the amount of light passing through the pupil on to a lens behind it. The lens focuses the light-images of what we see on to the retina, the screen at the back of the eyeball; the retina is connected by a bundle of optical nerves to the visual cortex of the brain, which interprets the images.

Despite the basic similarities, the arrangement of these components in an owl’s eye differs significantly from our own. The most obvious difference is that its eyeball is not spherical at all, but a sort of truncated cone, supported by a ring of small bone plates forming a short, tapered tube. We might perhaps compare the shape of the eye roughly with that of a light bulb, or – less prosaically – with an Apollo manned space capsule. The light bulb/capsule’s narrower, tubular end represents the cornea, iris and pupil, with a very large, thick lens at its base. Behind the lens the owl’s eye swells out in a blunt conical shape inside the skull, and at the wide end the broad, curved retina at the back represents the Apollo’s heatshield. The front-to-back depth of Mumble’s eyes was actually greater than that of my own, despite my much larger head. Because of its elongated shape the eye cannot swivel around in its socket – there is simply no room inside the skull to accommodate any movement. Mumble’s eyes remained fixed forwards and pointing very slightly apart, but her long, flexible neck compensated for this by allowing a huge range of head movement.

There are two types of photoreceptor cells in the retina at the back of the eye. Rod cells are sensitive to light intensity, and cone cells are associated with both colour vision and resolving power – the ability to distinguish fine details. The photoreceptors of daytime birds are about 80 per cent cones, which pick up colours all the way across the spectrum from red through yellow, green and blue to ultraviolet. By contrast, the owl’s large retina, backed up by
a reflective layer, is particularly rich in closely packed rod cells, which allow the eye to function well over a very wide range of light levels. The rod cells of Mumble’s retina were packed more than five times as densely as those in my eyes, and the image projected on to her retina was 2.7 times as bright as that created on mine. Some experts believe that Tawny Owls may actually have the best low-light perception of any vertebrate creature, and that in this regard a tawny’s eyes have evolved to the limit of theoretical perfection.

The cost of this high sensitivity to light is a reduction in acuity or resolving power, which is only about 20 per cent as good as that of some daytime raptors. At light levels at the very bottom end of their visual range owls can make out only the coarsest contrasts, though such darkness only occurs under the woodland canopy; if they can see the sky, this always provides enough light for them to see things with reasonable clarity. Compared with our own eyes, those of owls have far fewer cone cells relative to rods, and their exact ability to distinguish colours has been a matter of debate. However, it has been established that Tawny Owls’ eyes are, at the very least, sensitive to the yellow, green and, to a lesser extent, blue wavelengths.

An owl’s superlative ability to adjust its eyes to compensate for a wide range of light levels also works the other way round. The idea that they are blind in strong sunlight is entirely mistaken; they can see better by day than by night. When Wellington, my Little Owl, was in the dimmest light his irises were narrow yellow rings surrounding big
black discs; when he was out and about by day his pupils shrank to black dots in the middle of big yellow discs, but he showed no inclination at all to stay out of the sunlight.

Neither did Mumble – far from it – but because the whole outer surface of her eyes was very dark their adjustment to light levels was virtually invisible. Her eyes were big, slightly protruding, and appeared to be a glassy black. It was usually impossible to tell the central pupil from the surrounding iris; but very occasionally, if sunlight struck her eye at a particular oblique angle, I could distinguish the faintest of colour differences between the black-brown outer ring and the jet-black centre.

The rims of her eyelids were coral-pink, and the eyelids themselves – top and bottom – were covered with what looked like the palest grey-buff suede. Both lids closed, but the lower lids seemed to close upwards further than the upper lids dropped. She closed her eyes when she was sleeping, or wanted to protect them – noticeably, when she was either eating or grooming herself. (When she was young I formed the impression that blinking was also a sort of greeting, so I used to blink slowly back at her.) When she wanted to wipe her eyes, or to protect them without closing them, she instantly closed her ‘second eyelids’ – the nictating membranes, which slid diagonally across from the top inner to the bottom outer corners (owls also close these when just about to strike their prey). In owls, unlike some other birds, these membranes are not transparent, and in tawnies they have a milky-blue appearance except for a narrow transparent edge. If I happened to look at Mumble while she was
flicking them across, I got the startling impression that her eyes had changed colour instantaneously from shiny black to a blind-looking and rather sinister translucent grey-blue.