Bird Sense (8 page)

Rather than end this chapter on a sleepy note, I want to finish on something a bit more dynamic – the extraordinarily rapid flight of certain birds. Think of a descending swift; or the way a hummingbird zips from one bloom to another; or the way a sparrowhawk or sharp-shinned hawk hurtles among the branches after its prey. Such high-speed movements must require high-speed brain function, and I’ve often wondered how birds do it. Perhaps we shouldn’t be too surprised that birds have this ability since insects, whose brains are much smaller and whose vision is much less sharp, manage extremely well.

The closest we can come to imagining what it is like to process information as rapidly as a hummingbird or a hawk is the sensation of time slowing down that occurs during a near-death experience. Over the years I have had a few near-death experiences while doing fieldwork, and I imagine many readers will have had, like me, the same sensation during traffic accidents. As you slam on the brakes and slide inexorably towards another vehicle or a tree, it is as if your brain is taking in every detail and each second is drawn out until it feels ten times longer than it really is.

The bizarre thing is that, while this provides a convenient way for us to imagine what it is like to be a fast-moving bird, psychologists now realise that the sensation of time slowing down in near-death situations is an illusion. It is a quirk of our memory: scary events are remembered in great detail, so we perceive a slowing down of time only
after
the event. The hummingbird or accipiter, of course, experience events in real time.
47

2

Hearing

The great grey owl – with its enormous facial disc for sound collection. Thumbnail shows the highly asymmetric skull of a saw-whet owl, whose ears, like those of the great grey owl, are asymmetric.

It cannot be doubted that the faculty of hearing is highly developed in birds, not only the mere perception of sound, but also the power of distinguishing or understanding pitch, notes and melodies, or music.

From Alfred Newton,
1896
,
A
Dictionary of Birds
, A. & C. Black

This is a strange place: it is dark, wet and by British standards curiously remote. The horizon of the night sky is stained orange by the urban glow of Peterborough and Wisbech, while somewhat closer the floodlit chimneys of a brickworks belch fiery columns of smoke into the clouds. On the flat, featureless landscape I see the lights of an occasional car trundling along quiet country roads. The most bizarre aspect of this place, though, is the repetitive, toneless
crex crex
of corncrakes from the black meadows. One bird is fairly close, another more distant, but it is difficult to tell for the call has a curious ventriloquial quality: sometimes loud, sometime quiet, depending on which direction the birds are facing.

Hoping simultaneously to deter male corncrakes and attract a female with his mechanical rasping call, this bird – not much larger than a thrush – could live out an entire breeding season without ever being seen by a human. This is a bird whose presence is betrayed only by its voice.

Looking out across the Nene Washes I see there are houses whose bedroom lights are on and windows open. I imagine people lying in bed and hearing the corncrakes: do they recognise this reassuring ornithological renaissance for what it is?

Corncrakes once thrived here before the Washes were drained by ingenious Dutch engineers drafted in for the job. Back then this area was one huge wetland heaving with insects, birds and other wildlife. Even now, repaired and restructured by the Royal Society for the Protection of Birds (RSPB) and others, the Washes hold some special birds including spotted crakes, cranes, black-tailed godwits, ruffs and snipe.

As we plough our way through waist-high grass wet from an earlier downpour, the air becomes heavy with the scent of water mint. A corncrake calls nearby, or so it seems. ‘Here,’ says Rhys: ‘This is where we will put the net.’ We erect the
60
-ft mist net with hushed voices and muted head torches. Like some weird clockwork toy the corncrake continues to call, apparently oblivious of our efforts. Rhys, with a tape recorder crudely packaged in polythene bags to protect it from the wet, positions himself behind the net and in line with the bird, and I creep with my tape recorder into position between the bird and the net in case it overshoots as it tries to confront the auditory intruder and has to be called back.

Rhys is the RSPB’s corncrake champion, and for several years he has been overseeing the species’ reintroduction to this part of England. We are old friends, having first met at a student bird conference back in
1971
. Rhys’s recorder blasts out a crude, almost deafening echoey corncrake: recorded elsewhere during the day, there’s a skylark trilling between the booming
crexes
.

It is a continuous, relentless loop, much like the programme in the bird’s brain seems to be. I cannot imagine what’s running through the real bird’s head, but suddenly it stops calling; there’s a barely audible flutter overhead as it launches itself at the apparent intruder, and it is in the net. ‘Right!’ shouts Rhys and we spring into action to retrieve the bird. Reaching inside the folds of the net, I can see that the bird is already ringed. It is in fact one of several captive-reared corncrakes released earlier in the year. In the hand this is a beautiful russet and grey bird, whose laterally compressed body and wedge-shaped head are beautifully designed for pushing through the grass. Quickly checked and weighed, the bird is released and we walk back to the car.

Driving along a pockmarked road, dodging the enormous puddles, we stop and through the open windows listen again. ‘There’s one,’ Rhys says and we collect the net and walk out across the sodden fields towards the sound. The protocol is as before, with me between the bird and the net. On goes the tape, blasting its challenge across the flat, wet landscape. The territory owner continues to rasp. On and on the tape continues; on and on goes the bird: stalemate, I think. It is uncomfortable lying in the grass, the tips of the blades are tickling my nose, neck and face, but I dare not move. The bird stops. Has it given up, defeated by its much louder rival?

All at once I am aware of a sound in the grass, almost like the footfall of distant cattle. Then it stops. An illusion? I’m not sure. The rustling starts again and I realise the corncrake is walking towards me. Almost unbelievably just a few centimetres from my head, but totally invisible, he starts to call again. At point-blank range the full power of his
crex
is even louder than the tape. He is moving again, and very close. Against the glow in the night sky I can see the seed heads of the grass wobbling. All at once he is walking past my face: a flurry of wings and he’s in the air – and in the net.

‘Right!’ shouts Rhys, jolting me out of my reverie, and it is straight down to ringing. This bird is not ringed and is therefore entirely wild, evidence that the captive-reared birds are doing their job and successfully drawing in migrant concrakes. In the hand the bird is accommodating and patient. A few minutes of processing, the only real insult the glare of our head torches, and he is gently released back at exactly the spot where we first heard him. And a minute later, he’s found his tape loop and is off again in his relentless quest to attract a female.

I discover later that at close range the corncrake’s call registers about
100
decibels (dB). Putting that in context, normal conversational speech at the same distance is about
70
dB; a personal stereo at maximum volume is around
105
dB and an ambulance siren about
150
dB. Fifteen minutes of corncrake calls at this close range and I’d start to damage my ears.

Why, then, doesn’t it damage the corncrake’s ears? After all, the corncrake is even closer to its own call than we could ever be. The answer is that birds possess a reflex that reduces the sound of their own voice. This auditory reflex may be extreme in the case of the capercaillie, a turkey-sized game bird, in which the male performs a particularly noisy courtship display. The nineteenth-century ornithologist Alfred Newton wrote this about it: ‘It is well known that the cock for several seconds towards the end of his rutting ecstasy is completely deaf to any external sounds.’
1
According to the German ornithologists who investigated the underlying mechanism in the
1880
s, the male capercaillie’s temporary deafness is the result of the external ear being blocked by a flap of skin while he calls and for a few seconds afterwards. Subsequent studies of a range of bird species suggest that simply opening the mouth wide to call results in a change in tension on the eardrum, reducing the ability to hear.
2

Despite its mechanical, toneless quality, the corncrake’s call has the same function as the song of a passerine bird: a long-distance signal that says ‘keep out’ to other males, and ‘come in’ to females. Long-distance indeed, for the corncrake’s rasping call can be heard over one mile away. While this is fairly remarkable it isn’t the most extreme. The record for sound transmission goes to two birds whose deep, booming calls are sometimes audible to humans as far as two or three miles away.

The first of these is the European bittern, nicely described by Leonard Baldner, a fisherman-naturalist living on the Rhine in the mid-
1600
s. Baldner noted that the bittern’s boom was uttered with the head held high and the bill closed and that the bird has ‘guts with a long stomach five ells long’ (an ell is an old unit of measurement), referring to the bittern’s enlarged oesophagus which is employed in sound production.
3

The second is the kakapo, New Zealand’s flightless giant parrot, whose booming was familiar to the Maori at the time of the first European settlers: ‘At night . . . the birds come forth and collect at their . . . common meeting place or playground . . . having collected, every bird . . . goes through a strange performance by beating its wings on the ground and uttering its weird cry, at the same time forming a hole in the ground with its beak.’
4
Writing in
1903
Richard Henry said: ‘I think it likely that the males take up their places in these “bowers” [the bowls], distend their air sacs, and start their enchanting love songs; and that the females . . . love the music . . . and come up to see the show.’
5
By watching through a night-vision scope New Zealand’s kakapo hero, Don Merton (
1939
–
2011
), confirmed that males assume an almost spherical shape during their booming.
6
Unlike the corncrake, which, like most other birds, relies mainly on his syrinx (or voice box) to make his presence known, both the bittern and probably the kakapo, too, use their oesophagus, gulping down air and then releasing it in a booming belch.

Predominantly nocturnal, the corncrake, bittern and kakapo all live secretive lives amidst dense vegetation, relying on their loud calls to announce their presence and on their hearing to detect the presence of others.

Long-distance communication is not, of course, restricted to nocturnal birds; most small birds sing to advertise themselves to potential territory intruders and to potential partners, and benefit from having their song heard from as far away as possible. One of the loudest of all songbirds is the nightingale and I once spent an almost sleepless night in a small bed and breakfast on a wooded hillside in Italy being ‘serenaded’ (‘blasted’ would be a better term) by a male just one metre from my bedroom window. It was so loud I could feel his song resonating in my chest! Laboratory studies show that nightingales sing at around
90
dB.
7

If we want to know what a human can hear, we simply ask. To establish what birds can hear we have to ask in a rather different way. This is most often done by looking at their behavioural responses to sounds, typically using captive birds such as the zebra finch, canary and budgerigar as ‘models’ for other species. Studies of this kind involve training birds to perform a simple task like pecking a key in response to hearing a particular sound to get a food reward. If they (consistently) perform the task it is assumed they can hear the sound, or distinguish between different sounds (and vice versa).