Wednesday , December 6 2023
The question of "how" birds migrate - that's one of many still up for grabs.

Book Review: Bird Sense: What It’s Like to Be a Bird by Tim Birkhead

Today I was watching a buzzard soar above the Morvan National Park in Burgundy. Tonight, I’m listening to one owl–maybe more–hooting gently in the valley below me that the raptor was dominating earlier, as the scene is very gently lit by a waxing, but still small, moon. They seem mysterious, unknowable – hard to track their lives and imagine what they would be like, but I’ve got a bit more of an idea after reading Tim Birkhead’s Bird Sense: What It’s Like to Be a Bird.

I know that the raptor has a preponderance of cone cells in its retina (“like low-speed colour film – high-definition and performing best in bright light”), and the owl a majority of rod cells (which “can be thought of as working like old-fashioned high-speed black and white film–capable of detecting low levels of light”). While humans have only one fovea–a spot in the back of the eye where images are sharpest, some birds, including raptors, shrikes, hummingbirds, kingfishers and swallows, have two. So that buzzard’s visual acuity, ability to see fine detail, is roughly twice my own, while I probably didn’t need the science to tell me the owl can see a lot better in the dark than I. (I do sometimes go walking in the forest at night without a torch, but it’s a case of walking by feel rather than sight.)

That might sound quite technical, but really Bird Sense is a highly readable book, that puts sometimes quite complicated science into terms entirely accessible to any interested lay person. The task of a mallard duck seeking food at the bottom of a muddy pond is likened to a human being given a morning bowl of muesli and milk to which has been added a handful of fine gravel:

“To understand how this is possible, first catch a duck. Then turn it over and open its beak so that you can examine its palate. The most striking feature is a series of grooves radiating around the curved tip, but you need to look beyond these at the outer edge of the bill. What you should be able to see now is a series of tiny holes or pores–some 30 of them. If you look on the lower jaw, you will find even more–about 180. Examining these pores with a magnifying glass, you will see that from each one protrudes the top of a cone-shaped structure called a ‘papilla’, inside which is a cluster of around 20 to 30 microscopic sensory nerve endings–these are the touch receptors–that connect to the brain.”

Migration is of course one of the great mysteries of bird life, and what stands out from Birkhead’s very clear explanation of the current state of knowledge is just how sketchy and uncertain it is. He begins the chapter with an account of his own work with guillemots on Skomer Island, off the Pembrokeshire coast, getting from new geolocating technology finally, in 2009, after decades of working with them, a detailed understanding of where they go when not nesting on the island. (In short south at the end of July for a few weeks in the Bay of Biscay, before flying 1,500km north to spend most of the winter off northwest Scotland.)

But knowledge of how birds navigate can clearly now be established by a process of patient work with different species, using the new technologies. The detail of “how” is still up for grabs.

Birkhead explains that it was only in the 1980s that it was generally agreed that birds possess a magnetic sense that allows them to read compass directions from the earth’s magnetic field. “Remarkably, birds also possess a magnetic map that allows them to identify their location–like a GPS system… Migratory birds are not unique in this respect: a magnetic sense has also been detected in non-migratory birds like the chicken, as well as mammals and butterflies, presumably to help them navigate over modest distances.”

He sets out three theories for the mechanism by which this is achieved: electromagnetic induction (possibly in fish but birds and mammals apparently lack the necessary receptors); the presence of magnetite, a form of iron oxide, minute crystals of which were found in pigeons’ eyes and nasal cavities; or, magnetic sense might be mediated by a chemical reaction, induced by light, which might mean birds actually “see” the magnetic field.

Of the last two, he says: “They do not seem to be alternatives, but, rather, two separate processes that might work in unison in the same animal: the chemical mechanism based in the eye provides the compass, while the magnetite receptors in the beak provide the map.” As the author continues, “The compass may detect the direction of the magnetic field while the map detects the strength… and by integrating both types of information birds can find their way home.” That something so basic, so fundamental to how a major part of the Earth’s ecosystems work, is still highly uncertain is a telling example of the current limits of human understanding.

There’s also further reminders of the limits of science—and its all-too-human frailties—in the “smell” chapter. Despite widespread common, folk knowledge about birds’ olfactory abilities, Birkhead explains there was an astonishing refusal among professional scientists to accept this. His tale of how this was finally corrected, singlehandedly, by Betsey Bang, a medical illustrator at John Hopkins University in the late 1950s, doing drawings for her husband’s papers. Her first paper on the subject was published in Nature in 1960.

This book even raises questions that you probably never asked, but will now wonder why you didn’t, like: why don’t birds have teeth? (The answer also relates to limits on the size of birds’ eyes, and therefore vision. “Flying birds are designed so that their weight is distributed in such a way that it does not interfere too much with flight. A heavy head is incompatible with flight and therefore sets an upper limit on eye size. Flight, and the need for large eyes, may also be responsible for the loss of teeth, which have been replaced by a powerful muscular stomach, the gizzard which birds use to grind up their food) near the centre of gravity in the abdomen.”

Even a humble city pigeon, or a passing flock of sparrows, won’t look quite the same as before, should you read  Bird Sense. And you’ll understand a little more about how little humans understand our world.

About Natalie Bennett

Natalie blogs at Philobiblon, on books, history and all things feminist. In her public life she's the leader of the Green Party of England and Wales.

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