Bird Sense (16 page)

Just over a century later, the Comte de Buffon wrote that the bony tip of the green woodpecker’s tongue ‘is covered with a scaly horn beset with small hooks bent back, and that it may be capable both to hold and pierce its prey, it is naturally moistened with a viscous fluid that distils from two excretory ducts . . .’.
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The idea that woodpeckers impale prey on their tongue persisted and was reinforced in the
1950
s by the pioneering wildlife film-maker Heinz Sielmann, who wrote that the great spotted woodpecker’s ‘harpoon-like tongue is particularly suited to . . . spearing insect larvae and pupae’. Re-analysis of Sielmann’s footage, however, showed that larvae are
not
pierced, but simply adhere to the sticky saliva at the end of the tongue. Exactly the same behaviour was seen in a study of a Guadeloupe woodpecker from the Lesser Antilles kept in captivity for a couple of weeks. On extending its long tongue into a cavity, the bird could tell immediately – using either touch or taste – when it had made contact with a prey item, and detailed anatomical studies confirm that the tongue tip is rich in touch sensors (we don’t know about taste buds, but I bet they are there). In turn, the insect larva was hardly passive on sensing the woodpecker’s tongue, and either retreated or grasped on to the sides of its hole with its legs, making it difficult for the woodpecker to dislodge it. Through a combination of sticky saliva, a barbed surface and an extraordinarily prehensile tip – but no piercing – the Guadeloupe woodpecker was able to extract its reluctant prey.
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I am in the swamps in a little-known part of northern Florida, on the Choctawhatchee River. This is red-neck country – similar to that of the
1970
s film
Deliverance
. Resting quietly in my kayak, I watch spellbound as four pileated woodpeckers chase each other noisily through the trees. The late afternoon light filtering through the olive-green leaves of the buttress-rooted cypresses is perfect and the birds seem to be enjoying themselves. They flip heavily from tree to tree, hammering and calling but offering only the occasional tantalising flash of their beautiful red, black and white plumage. I have never had such wonderful close encounters with this species before, but this isn’t what I am looking for. Instead, I am with a small group of ornithologists hoping to glimpse the pileated’s enormous cousin, the ivory-billed woodpecker.

Thought to have been extirpated in the second half of the twentieth century, a controversial sighting on the Pearl River in southern Louisiana in
1999
suggested that at least one ivory-billed woodpecker had survived. There have been several subsequent reported sightings in remote swamps, including some on the Choctawhatchee, but so far at least no video evidence – now regarded as essential proof of the bird’s existence.
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The ivorybill – known also as the Lord God Bird – has an enormous chisel-shaped beak. It finds its prey by searching trees in which huge beetle larvae lie hidden beneath the bark. Once a larva is located – almost certainly by the sound of its jaws chewing on its woody diet – the woodpecker hacks at and levers out a hand-size chunk of bark, exposing the larva’s retreat. Imagine how much effort this would require with a hammer and chisel and you have some sense of the bird’s enormous strength. As the larva wriggles away the ivorybill flashes out its extraordinary long tongue and snares it. This slick operation is one of sensory contrasts: a bill as insensitive as steel, a tongue more tactile than your fingertips.

The ivorybill’s power is legendary. In
1794
, Alexander Wilson, a Scottish weaver who emigrated to North America, and who later became one of the founders of American ornithology, shot an ivorybill in North Carolina. The bird was only slightly injured and Wilson decided to keep it. As he carried the bird back to town on his horse, it cried like a baby, surprising ‘every one within hearing, particularly the females, who hurried to the doors and windows with looks of alarm and anxiety’. Checking into the Wilmington Hotel, Wilson left the bird in his room while he went to take care of his horse. When he returned less than an hour later he found the bed ‘covered with large pieces of plaster, the lath was exposed for at least fifteen inches square, and a hole, large enough to admit a fist, opened to the weather boards; so that in less than another hour he would certainly have succeeded in making his way through’. Wilson caught the bird and ‘tied a string round his leg, and fastening it to the table, again left him, this time to search for something it might eat. As I reascended the stairs, I heard him again hard at work, and on entering had the mortification to perceive that he had almost entirely ruined the mahogany table to which he was fastened, and on which he had wreaked his whole vengeance.’ The bird refused all sustenance and, to Wilson’s regret, died three days later.
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Ivorybills nest in a cavity four or five feet deep, sculpted from the live tissue of the bald cypress, among the hardest of trees. That bill, once revered as an Indian amulet, is an extraordinarily powerful tool, mounted on a seriously reinforced skull. John James Audubon dissected the head of an ivorybill and described its seven-inch [
18
cm] tongue in detail, which, like that of other woodpeckers, is armed with an exquisitely sensitive tip.
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Audubon also provides a first account of the ivorybill’s foraging technique:

Then, having discovered an insect or larva in a chink of the bark, [it] is enabled by suddenly protruding its tongue, covered with thick mucus, and having a strong slender sharp point furnished with small reversed prickles, to seize it and draw it into the mouth. These prickles are of special use in drawing from its retreat in the wood those large larvae, often two or three inches in length; but it does not appear probable that the bristly point is ever used to transfix an object, otherwise how should the object be again set free, without tearing off the prickles, which are extremely delicate and not capable of being bent in every direction?
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The skin of birds and mammals alike is sensitive to both touch and temperature. This sensitivity is especially important when birds are incubating eggs or brooding chicks, not only to ensure that their eggs and chicks are suitably warmed, but also to avoid stepping on them or crushing them. The heating device is the brood patch, an area of skin from which the feathers are lost some days or weeks before incubation starts, and whose blood supply is increased.

In some birds the brood patch plays a vital role in determining how many eggs the female lays. In the
1670
s the naturalist Martin Lister conducted a simple experiment on the swallows nesting near his house – with entirely unexpected results. As each egg was laid, he removed it, only to find that instead of laying a normal clutch of five eggs, the female swallow went on to lay no fewer than nineteen eggs. Why they should limit themselves to five when they could so obviously lay more was a mystery that was solved only later. Subsequent tests with other species gave similar results, including a house sparrow that laid
50
eggs (instead of
4
or
5
), and a northern flicker that, instead of laying a normal clutch of
5
to
8
eggs, laid
71
eggs in
73
days! There are some species, however, like the lapwing, where removing eggs makes absolutely no difference to their final number of eggs laid. On the basis of this, ornithologists have categorised birds as either determinate (e.g. the lapwing) or indeterminate layers, although they have no idea why such a difference exists. The point, however, is that in the indeterminate layers, like the swallow, sparrow and flicker, egg laying is regulated through the brood patch. If eggs are removed as they are laid, there is no tactile stimulation of the brood patch and no message to the brain to limit egg laying. If the eggs are
not
removed, touch sensors in the brood patch detect their presence in the nest and then, via a complex hormonal process, allow only the ‘right’ number of ova to develop in the ovary.
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Once the clutch is complete it is crucial that the eggs are maintained at an appropriate temperature if the embryo within each egg is to develop normally. Successful incubation does not demand a constant temperature, but simply that it does not fall too low or get too high. Incubating birds often leave the nest to feed, during which time the eggs cool, but embryos are far more tolerant of brief periods of cooling than they are of over-heating. The eggs of most species are incubated at around
30
–
38
o
C and the incubating bird achieves this largely through its behaviour. Experiments in which eggs are artificially cooled or heated show that birds adjust their incubation posture – and in particular the contact between the brood patch and the eggs – to regulate the temperature of their eggs. This is true regardless of whether eggs are cooled – when the parent responds by transferring more heat to the eggs – or heated – when the parent responds by incubating more closely to draw off excess heat from the eggs.

On casual inspection the brood patch looks to be little more than a slightly vulgar patch of overly pink skin, but it is a remarkably sensitive and sophisticated organ. Birds regulate the temperature of their eggs by increasing or decreasing blood flow to the brood patch. What’s more, the contact between the eggs and the brood patch triggers the release of the hormone prolactin from the pituitary gland beneath the brain, which in turn keeps the bird incubating. If the clutch of an incubating bird is removed, prolactin secretion plummets – tactile stimulation is crucial to this process as was demonstrated in a clever experiment in which the brood patch of incubating mallard ducks was anaesthetised. Even though the birds continued to incubate, because they were unable to feel the eggs their prolactin levels dropped, exactly as if the eggs had been removed.
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The only birds whose eggs are not incubated by body heat are brush turkeys, malleefowl, talegallas, maleo and scrubfowl (known collectively as ‘megapodes’ on account of their large feet, which they use for digging). Instead, they place their eggs in a mound of fermenting vegetation or warm volcanic soil (depending on the species), which they maintain at a temperature of around
33
o
C. In the mound-building species, such as the Australian brush turkey, the male cares for the mound, typically for months on end, opening it up to allow excess heat to escape, or adding more material if the mound is too cool. Darryl Jones, who has studied mound builders for years, told me: ‘How they monitor mound temperatures is not yet fully understood. Most likely, both males and females possess a temperature sensor in the palate or tongue, as all species have been seen to regularly take a beak-full of substrate while working on the mound.’
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In birds that incubate their own eggs, the chicks must be sensitive both to each other (where there are several) and to their parents. The South American finfoot, also known as the sungrebe (a species I searched for unsuccessfully in Ecuador) provides an extraordinary example of the need for parents and chicks to be aware of each other through the sense of touch. This secretive, little-known bird nests in dense vegetation along slow-moving rivers and hatches its clutch of two or three eggs after just ten days of incubation. Blind, naked and pretty helpless at hatching, the chicks are more like those of a passerine than a non-passerine bird. Remarkably, the male sungrebe carries his two chicks around in a special pouch of skin under each wing. He can even fly with the chicks. The Mexican ornithologist Miguel Alvarez del Toro, who discovered this, described how, on flushing a male from a nest he had been watching, he saw the male fly with ‘two tiny heads sticking out from the plumage of the sides under the wings’. Surprisingly, the female does not have the pouches, nor do the males of the other two closely related finfoot species, whose chicks are much more developed at hatching. The male sungrebe’s pouches constitute the most extraordinary adaptation, and they beg the question of what touch receptors are employed by the newly hatched chicks to ensure that they are in the right place, and by the adult male sungrebe to know that the chicks are absolutely secure before he takes flight.
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In the case of some brood parasitic birds, the tactile sensitivity of recently hatched chicks has a more sinister aspect. The greater honeyguide is a tropical brood parasite whose nestling disposes of its nest mates in a particular grisly manner. On hatching, the honeyguide has its eyes closed, but is armed with a needle-like structure on its downward pointing bill tip. It is this that it uses to kill the host young, allowing it to acquire all the food its foster parents bring back to the nest. Seeing this evil-looking device for the first time, I assumed that the honeyguide chick would simply pierce the skull or body of the host chicks, but this is not what happens. Using infra-red video cameras located inside little bee-eater host nests, Claire Spottiswoode watched as the honeyguide chick used its sharp beak to grasp a young bee-eater, and, like a pitbull terrier, simply shake it to death. If the host chick is tough, it can take several sessions, between which the honeyguide chick pauses to catch its breath before starting again. Because its eyes are not yet open, and it is dark inside the bee-eater nest cavity, the honeyguide chick presumably uses both movement (touch) and temperature to tell it whether more shaking is necessary. Once the host chick is dead the honeyguide ceases to respond to it, and the unfortunate parents remove it from the nest.
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