Books

Winged Sense

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MANY OF US take enough interest in birds to put out food for them in the garden and to enjoy spotting them on a country walk. But if you go for a walk with a real birder, you soon realize that what you took to be a place adapted by humans for their own use, with a few feathered creatures flying out of your way, is actually also a bird-world, whose many varieties of inhabitants are going about their business with the frenetic concentration of hedge-fund dealers. Professor Tim Birkhead, author of The Cambridge Encyclopaedia of Ornithology, is a birder of birders, and his new book, as its title implies, is an ambitious attempt to get inside birds’ headsto chart what we know of their senses, and even to some extent their emotions, and to understand how they perceive the world.

Like most good scientific books intended for the lay reader, Bird Sense includes the discoveries that have been made, and how they have been made, and brings the subject right up to date, with occasional appeals to enterprising students to tackle the many uncertainties that still remain. Indeed, this is as much a story of the ingenuity of human researchers as it is of the remarkable faculties of birds. These researchers attach radio transmitters to kiwis and geolocators to guillemots, put patches over the eyes of tits and jays to discover whether they see better with one eye than the other, check pigeons’ heart rates with electrocardiagraphs, pour bacon fat on the sea to test the sense of smell of black-footed albatrosses, and induce a male zebra finch to copulate with “a freeze-dried female” so that they can collect his semen.

Most of the more recent experiments, fortunately, seem to be less harmful to their quarry than those of old. The great bird artist John James Audubon shot ten Carolina parakeets (now extinct) and boiled them up for his cat to see if they were poisonous. “They probably were,” says Birkhead, given that they ate toxic cocklebur seeds, that seven cats had already died from eating parakeets, and that this latest cat “disappeared.” The eighteenth-century Jesuit priest Lazzaro Spallanzani, professor of natural sciences at the University of Pavia, removed the eyeballs of bats and was astonished to find they could still find their way through “subterranean pathways,” inspiring a Swiss surgeon to remove some more bats’ eyes, plug their ears with wax, and find that they “blundered helplessly into all obstacles.” But neither could explain why apparently silent bats needed their hearing to orientate themselves. It was not until the 1930s that an undergraduate at Harvard, Donald Griffin, using electronic equipment, proved that whereas we can only hear sounds with frequencies between two and twenty kilohertz, bats’ cries can reach one hundred and twenty kilohertz.

Griffin extended his experiments to the cave-dwelling oilbirds of Venezuela and established, again by sealing their ears with wax, that, like bats, they orientate themselves in darkness by uttering sounds, though in their case these are a series of rapid clicks, at a low frequency. Two later researchers in the 1970s pinned these down at two kilohertz, and by hanging plastic discs around the narrowest part of the cave, found that the birds could avoid the larger discs, but not the smaller ones. But most birds have a sense of hearing much like ours, though the great grey owl, with its enormous facial disc for amplification and its weirdly asymmetrical ears, the right ear high up and the left ear low down, can detect a mouse moving under snow. And even more unfortunately for mice, whose hearing only goes down to three kilohertz, they cannot hear a barn-owl’s wingbeats at a frequency of one kilohertz.

Birkhead gives a chapter to each of the five standard senses, plus a chapter for the birds’ extra magnetic sense and another for the still somewhat vexed question of birds’ emotions. Darwin was certain that birds as well as mammals experienced emotions, and Birkhead tends to agree with him. He cites the admirable social behaviour of guillemotshis own special studywhich preen their neighbours, protect each other’s eggs and young from predators and keep each other’s eggs warm if the parents are absent too long.  But his most impressive example of birds’ attachment to each other is the behavior of gannets:

As one member of the pair returns to its partner at the nest, the two birds stand upright, breast to breast with outstretched wings and skyward-pointing beaks. In a frenzy of excitement they clash their bills together, intermittently sweeping their head down over the neck of their partner, calling raucously all the time.

Studying gannets in northern England, the researcher Sarah Wanless witnessed a particularly astonishing example of this, when a female gannet returned after an absence of five weeks to the nest where the male had been caring for their chick. Their greeting ceremony lasted seventeen minutes. Beat that next time your partner returns from a long trip abroad!

In his chapter on Magnetic Sense, Birkhead describes climbing down a steep slope on an island off Wales, with the sea crashing on black basalt rocks far below, and hooking guillemots with a modified fishing pole, so as to attach geolocators to their legs. These tiny devices record the amount of daylight every ten minutes and thus, since the light varies in different places, reveal where the bird has been. In this case, it turned out, they had travelled from West Wales, in late summer after the breeding season, to the Bay of Biscay in the autumn, to north-west Scotland for the winter, then back via Biscay to their Welsh island in the spring. However, this little trip is nothing to that of the bar-tailed godwit which commutes from Alaska to New Zealand, 11,000 kilometers non-stop, in eight days.

Whether or not birds possess a magnetic sense remained controversial until the 1950s, when two German researchers, using an “orientation cage” open to the night sky, were able to observe that European robins “hopped or fluttered” in their usual south-westerly flight direction, using the stars as their compass, but that they did exactly the same even in complete darkness. The researchers then surrounded the cage with an electro-magnetic coil, altered the orientation of the magnetic field andpresto!the robins hopped in another direction.

I do not remember ever marking so many passages in any book I have reviewed, and this brief selection leaves out most of them: the touch receptors in birds’ beaks; their ultra-violet vision, enabling them to see colors we cannot see; the fact that sparrows’ testes are no bigger in the winter than a pinhead, but swell to the size of a baked bean in spring; the guillemot chicks that have learned their parents’ calls even before they emerge from the egg and the parents that can recognize their partners approaching from out at sea well before they’re visible to a human; and so on. Each chapter is illustrated with a handsome line drawing by Katrina van Grouw, a former bird curator at London’s Natural History Museum. This is a book to make one whistle, both at the sensational senses of birds and at the patient curiosity and cunning of those who study them.

John Spurling is a playwright and novelist. His latest novel, A Book of Liszts: Variations on the Theme of Franz Liszt, was published last year by Seagull Books.

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