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Saturday
Dec212013

book review: bird sense by tim birkhead

In this concise guide, Tim Birkhead’s goal is not only to explain the biology, anatomy and physiology of bird sensation, but also to enable us to perceive what it feels like to be a bird – to be snuffling the humid undergrowth like a kiwi, or to sniff rain falling 100 km away like a flamingo.

The author gives a lucid description of bird senses, allotting a chapter to each: seeing, hearing, touch, taste, smell, magnetic sense and emotions, and traces the observational and experimental history that led to the current understanding. He relates clever, insightful means of testing hypotheses and debunking or confirming folk anecdotes. An important point is that our conclusions of how birds sense are constrained by the limitations and biases of our own human senses. Notable in this respect, was the discovery of birds’ ability to detect ultraviolet light.

In some cases, as for the eye, the avian sense organ is anatomically distinct from that of humans. Birds of prey, with their extraordinary vision, have two foveae (where the image is sharpest) whereas humans have one. Birds have a nictitating membrane under the eyelid, which cleans and protects the eye. They also have an unusual structure, the pecten, which projects into the posterior chamber and contains a mass of blood vessels able to provide oxygen and other nutrients to the eye. As opposed to the human eye, which has a richly vascularized retina, the bird’s eye is largely bereft of blood vessels, other than those of the pecten.

Birkhead sidesteps for a moment, to remind us that the brain, of course, is the ultimate mediator of all sensation: long, fine, neuronal axons link the sense receptors to the brain. However, the brain is not just a passive control center. For example, the center of the avian brain that controls acquisition and production of song in male birds shrinks at the end of the breeding season and grows again the next spring.

a Sanderling probes for foodUntil recently, birds were not thought to have senses for touch, taste and smell. The development of higher power microscopes and finer dissection techniques revealed that birds have touch receptors within pits in their beaks. Using these sensitive bill tips, birds like sandpipers, woodcock and snipe detect prey such as worms or mollusks either by touching them directly or detecting their vibrations, or by noticing pressure changes in sand or mud.

Complex studies have also elucidated the sense of taste in birds. Taste buds were found to be located at the back of the tongue and throat and in the palate. Birds are able to respond to the same four categories as humans: sweet, sour, bitter and salt. Research has shown that hummingbirds can detect differences in the amount of sugar in nectar, fruit-eating birds can differentiate ripe and unripe fruit, and sandpipers can taste the presence of worms in sand.

At the end of the chapter on taste, the author takes an odd, almost reverse track in discussing the five New Guinea birds that are toxic to humans and he goes on to query the relation of plumage color to palatability.

If a book on bird physiology can possibly have a climax, Birkhead pulls it off in the chapter on smell. He begins by dethroning our beloved John James Audubon, at least on his merits as a scientific logician. Audubon believed vultures had no sense of smell, as they were unable to detect carcasses he had hidden in secluded spots, like a dead tree cavity. His conclusions even received plaudits in the scientific journals of the time. Later it was determined that vultures only respond to fresh carcasses, whereas Audubon had supplied only rotting carrion.

Contrariwise, it was Betsy Bang, a Hopkins trained medical illustrator, but only an amateur ornithologist, who did the fine dissections delineating the labyrinthine maze of bird nasal cavities. She also measured, with a ruler, the olfactory bulb size of many birds and developed an index of size corresponding to olfaction’s relative importance, the kiwi coming out on top. However, in subsequent decades, this straightforward reasoning did not always hold true. Recent, the advent of 3D reconstruction by means of high resolution scanning and tomography has shown the volume of the olfactory bulb to be a much better measure of olfactory sensibility.

Topping off this chapter on taste is a bombastic statement only a pretentious birder would declare: “Apart from the publication of Jane Austen’s Pride and Prejudice and the ongoing Napoleonic War, the most significant event of 1813 was Europe’s discovery of the kiwi." Indeed, having very poor eyesight, the kiwi is very dependent on foraging by snuffling into the ground for earthworms.

As humans have no innate magnetic sense, scientists were at ground zero when contemplating this capability in birds. Actually, they were in negative territory, since it is only recently that it has even been speculated that birds possess this special property. Some of the first studies were done by Steve Emlen implementing his Emlen funnel: "It consists of a blotting paper funnel about 10 cm in diameter, with an ink pad at the bottom, and a domed wire mesh top, through which birds can see the sky. As the bird hops, the ink on its feet leaves a trace on the blotting paper which provides an index both of the direction and intensity of the migration." (A drawing of this apparatus would have enhanced the reader’s visualization.) In further searching for the Holy Grail of a “magnetic compass,” robins were put in a cage surrounded by huge electromagnetic coils. Shifting of the magnetic fields altered the direction of the robins’ hopping.

Alas, there is no “magnetic organ” as such. “Magnetic sensations are different, because unlike light and sound, they can pass through tissue: this means it is possible for a bird to detect magnetic fields via chemical reactions inside individual cells and through its entire body.” Ingenuous studies have substantiated a visually induced chemical reaction as the mechanism of magnetic field detection. On a grander scale, we have seen how the new geolocators and satellite trackers have been utilized to study bird migration and navigation.

Lastly, emotions – still largely a conjecture and somewhat of a romantic frontier of investigation – perhaps mocked by many, but any devout birder will tell you that the behavior she/she has seen suggests otherwise.

Although each chapter has an introductory page with a few drawings and diagrams, more would have provided further clarification. Birkhead’s writing is clear and flows at an even pace. Anatomy and biology come alive and are not fact-ridden and textbook like. At only 209 pages, it will provide pleasant and enlightening reading for a few dark winter evenings.

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