Bird Sense (12 page)

I visited New Zealand while writing this book, and when I wasn’t chasing kiwi and kakapo I took a few days off to visit Fiordland on South Island. The weather was perfect and the scenery spectacular, but the most striking aspect of this area was its auditory desolation. I have rarely been anywhere so quiet. Peaceful, yes, but this was a melancholic silence. The birds that once inhabited the forests clothing the steep-sided valleys have all been killed by the predatory stoats and weasels that the early settlers foolishly introduced. Native birdsong is absent across mainland New Zealand and made me wonder whether the introduced dunnocks, blackbirds and thrushes sing more softly in New Zealand than in their native Europe due to the lack of competition.

The studies I have just described show clearly that habitat affects the types of songs birds use, and in a way consistent with what is known about sound attenuation. These studies, however, provide only indirect evidence that the birds themselves hear sounds differently in different habitats. Some nice evidence that they do comes from a study of the North American Carolina wren which uses song to defend its territory throughout much of the year. The presence or absence of leaves on the vegetation (in winter and summer, respectively) has a huge impact on the way songs sound. The wrens’ songs degrade over distance more rapidly when there are leaves on the vegetation than in winter when there are no leaves. When Marc Naguib broadcast undegraded or degraded songs at the same volume and from the same location, the wrens typically responded to the undegraded song by flying directly to the loudspeaker. When he broadcast degraded songs, however, the birds flew
over
the loudspeaker, as though they perceived the intruder to be further away. In other words, the wrens could tell the difference between degraded and undegraded song and adjusted their behaviour accordingly.
38

The audio equivalent of a microscope or high-speed camera is the sonograph, a machine that produces a picture of sound. Invented during the
1940
s by the Bell Telephone Laboratories in the United States, the sonograph was first used by W. (Bill) H. Thorpe in Cambridge to understand birdsong. Being able to ‘see’ sound, as a sonogram, transformed the study of birdsong. Of course, tape recorders had been around before this, but listening to a bird’s song, even if played at reduced speed, doesn’t really provide the same resolution or sense of understanding as an image. Only by transforming an auditory signal into a visual one did we really start to appreciate the full complexity of birdsong, and to speculate about how much of that complexity the bird actually hears, or can make sense of. As an undergraduate I completed a three-month-long project on the contact calls of golden-breasted waxbills and can still remember the distinctive acrid smell of the sonograph machine as it burned the sound image (the sonogram) on to the heat-sensitive paper.

If one listens to the song of the whippoorwill – a North American nightjar – it sounds, as its name implies, as though it consists of three notes, rendered in David Sibley’s
Guide to Birds
as: WHIP puwiw WEEW (i.e. whip-poor-will). If one then makes a sonogram of the call, it becomes clear – from this ‘slow-motion’ visualisation – that the call actually consists of
five
separate notes, not three. To the human ear the call is delivered so fast that the separation of the individual notes is obscured. When this was discovered by the ornithologist Hudson Ansley in the
1950
s it was not clear whether the whippoorwill itself hears three or five notes, for at that time little was known about birds’ hearing. However, as Ansley pointed out, if one looks at a sonogram of a mockingbird imitating a whippoorwill, it uses five rather than three notes, suggesting that this species at least can resolve the fine detail of the whippoorwill’s song.
39

Tests of human hearing indicate that our ability to resolve different sounds starts to break down as the interval between the sounds approaches one tenth of a second. Many birds’ songs, however, contain elements occurring at much shorter intervals than this and there is increasing evidence that birds are able to detect such differences. Indeed, this is the one aspect of hearing in which birds are much better than humans. It is as if they have the auditory equivalent of a slow-motion option in their brain, allowing them to hear details that are completely lost on us. This raises an interesting point: if we were able to hear birdsong exactly as a bird hears it, would we still consider it ‘beautiful’; would we still consider birdsong to be akin to music?

Striking evidence of the ability of birds to hear the fine details of song involves the so-called ‘sexy syllables’ in canary song. When a male canary sings in front of a female around the time she is laying eggs, her response is often to solicit copulation by crouching. Detailed analysis reveals that the part of the song that triggers this response is a succession of rapidly alternating high- and low-frequency elements (produced from the right and left sides of the syrinx – the bird’s voice box – respectively) at a rate of about seventeen times per second. To us, the burst of sexy syllables midway through the song sounds like a continuous trill, but the females hear the fine detail. Using a computer to create artificial songs, Eric Vallett manipulated different components of the sexy syllables, making them faster or slower by altering the interval between the syllables and then playing them to females. The female canaries had no difficulty distinguishing between the two songs, demonstrating their preference for the faster trill by crouching for copulation.
40

Driving through Ecuador’s monumental mountain scenery, we start to descend into a forested valley on a road so steep it feels like zooming in on Google Earth. Down, down, down, slithering and sliding on the rough track until, after forty-five minutes, we eventually stop in a cloud of dust beside a small ravine. It doesn’t look very promising: a crudely constructed bamboo scaffold supports a black plastic pipe emerging from a cleft in the rocks. Treading over plastic rubbish, boulders and dead leaves, we make our way gingerly up the sunless gorge. Within a few metres we turn a corner and are suddenly confronted by three oilbirds sitting on a low, muddy shelf. They are just as shocked by our intrusion as we are by their proximity. Without warning, they clatter into the air shrieking and clicking like demons. In fact, that’s just what they seem like, medieval birds more fitting for a Harry Potter film than the tropics. Their local name is
guácharos
– literally ‘one who cries and laments’, and is possibly onomatopoeic . . . likened by others to the sound of ripping silk. Their scientific name,
Steatornis
, which literally means ‘oil bird’, refers to the fact that in the past their very fatty chicks were rendered down for oil which was used in cooking.

The birds settle eventually on a ledge ten metres up and sit in close bodily contact. Like a cross between a hawk and a nightjar in appearance, ‘nighthawk’ might have been a good name for them, although they are far from hawk-like in their habits. They have huge, dark eyes; a walrus-moustache comprising twelve long bristles sweeping down from each corner of the mouth; an enormous hawk-like bill with distinctive oval nostrils; and perhaps most striking of all, rows of brilliant white spots decorating their russet plumage. There are three rows of spots, running along the wings, the tail and on the breast, and more on the top of their head – like a scattering of stardust. We stand stock still, rooted to the spot, partly in awe and partly in fear of disturbing these extraordinary birds. After fifteen minutes they seem to relax, closing their eyes and returning to the sleep from which we disturbed them. As our own eyes become accustomed to the gloom, and as theirs adjust to the light, we see more and more birds distributed on ledges and in small caves. Our guide tells us that there are about a hundred in total: all the more remarkable because this might be one of the few places in Ecuador where oilbirds live. But the birds are desperately vulnerable. The plastic water pipe running through the gorge comes from a newly built road only tens of metres above the birds.

Construction of this road is a vicious rent through the forested valley bottom, stretching wider and wider as it extends across the landscape, stripping away the forest on either side. Once the road is open I wonder how long the
guácharos
will last; it is hard to imagine them dozing through the day with a barrage of noisy trucks thundering overhead in a haze of diesel fumes. It is hard to imagine, too, when the trees have gone, how they’ll be able find enough fruit.

The oilbird is one of just a handful of birds that relies – like many bats – on hearing the echoes of its own voice to navigate in total darkness. It is well known that bats use echolocation to operate in the dark, but that particular discovery was protracted and hard-won.

The pioneer of bat senses, and much else, was Lazzaro Spallanzani (
1722
–
99
), Jesuit priest and professor of natural sciences at the University of Pavia, Italy. Endlessly curious about the natural world, Spallanzani was a brilliant observer and ingenious experimenter. Watching a captive barn owl he noticed that if the bird accidentally extinguished the candle that was lighting the room, the bird lost all ability to avoid collisions. Bats had no such problem. Placed in total darkness, the bats Spallanzani collected from a local cave ‘continued to fly around as before and never struck against obstacles, nor did they fall down as would have happened with a night bird [i.e. the owl]’. Two bats whose eyes Spallanzani covered with a dark hood also flew quite normally.

These phenomena induced me to perform another experiment which I considered decisive, namely to remove the eyes of a bat. Thus with a pair of scissors I removed completely the eye-balls in a bat . . . Thrown in the air the animal flew quickly, following the different subterranean pathways from one end to the other with the speed and sureness of an uninjured bat . . . My astonishment at this bat which absolutely could not see although deprived of its eyes is inexpressible.
41

Spallanzani wondered whether the bats possessed a sixth sense. He wrote to everyone who could help, offering a challenge: could anyone discover how blinded bats could ‘see’ in the dark? One of Spallanzani’s letters was read to the Geneva Natural History Society in September
1793
, where the Swiss surgeon and natural historian Charles Jurine was in the audience. Intrigued, Jurine decided to conduct his own experiments and began by repeating what Spallanzani had done, but adding an ingenious twist. As well as removing their eyes, he also plugged the bats’ ears with wax, and to his amazement found that they ‘blundered helplessly into all obstacles’.
42
The conclusion was extraordinary: bats needed to hear to be able to ‘see’.

Spallanzani learned of Jurine’s remarkable results the very next day and immediately started some new experiments of his own, deafening bats and confirming that they rely on reflected sound, but with no notion of where it came from. Puzzled, he said: ‘But how, if God loves me, can we explain or even conceive in this hypothesis of hearing?’ Given that the bats were silent, why were their ears so important in avoiding obstacles? The experiments gave the same results, time after time; the problem was that, unable to imagine that certain sounds might lie outside the range of human hearing, they simply did not make sense.

Georges Cuvier (
1769
–
1832
), the renowned and influential French anatomist, decided in
1795
, on the basis of little more than logic, that bats avoided obstacles through a sense of touch. Even though Spallanzani had earlier tested and comprehensively rejected the touch hypothesis, Cuvier’s idea became the accepted explanation and he was ‘lauded for having brought order out of a chaotic state of affairs left by Spallanzani and Jurine’. The reason why Cuvier carried the day was that, with no notion that bats might utter sounds inaudible to humans, Spallanzani’s and Jurine’s ideas seemed completely fanciful.
43