Bird Sense is arranged sense by sense, with a chapter on each of the main senses: seeing, hearing, smelling, touching, tasting, but also a chapter on the magnetic sense and the sense of emotions. In each chapter I have attempted to give a feeling for how the senses of birds have been investigated and what we currently know about those senses.
Here’s the beginning of Chapter 1 on ‘Seeing’:
As a child I once had a conversation with my Mum about what our dog could or could not see. On the basis of something I had heard or read, I told her that dogs could see only in black and white. My Mum was not impressed. ‘How could they possibly know that?’ she said: ‘We cannot look through a dog’s eyes, so how could anyone know?’
In fact there are several ways we can know what a dog, a bird or indeed any other organism can see, for example, either by looking at the structure of the eye and comparing it with other species or by behavioural tests. In the past, falconers unwittingly performed just such a test – not with falcons, but with shrikes.
This elegant little bird is used, not to attract the hawk as might be supposed, but to give notice of its approach. Its power of vision is perfectly marvellous, for it will detect and announce the presence of a hawk in the air long before the latter is discernible by human eye.
The elegant little bird is the great grey shrike and the trapping method an elaborate one, involving a turf hut in which the falconer is concealed; a live decoy falcon, a wooden decoy falcon, a live pigeon and – crucially – a great grey shrike (known also as a butcher bird) tethered outside its own miniature turf hut.
James E. Harting – falconer and ornithologist – saw this method in action during October 1877 near Valkenswaard, in the Netherlands, a traditional location for trapping migrating falcons. Here’s how he described it:
We take our seats on the chairs in the hut, and fill our pipes…. Suddenly our attention is attracted by one of the shrikes. He chatters and appears uneasy. He crouches and points…. He jumps off the roof of his hut, and prepares to take shelter within it. The falconer says there is a hawk in the air..
They watch and wait, but it turns out to be a buzzard and the falconer isn’t interested. But later:
Look! The butcher-bird is pointing again. There is something in the air. He chatters and quits his perch…. We look in the direction indicated, and strain our eyes, but see nothing. ‘You will see him presently’, says the falconer; ‘the butcher-bird can see much farther than we can.’ And so he can. Two or three minutes afterwards on the far distant horizon of that great plain [of Valkenswaard] a speck comes into view, no bigger than a skylark. It is a falcon.
As the raptor approaches, the nature of the shrike’s agitation informs the falconer of the species. Even more remarkably, the shrike’s behaviour also tells the falconer how the raptor is approaching: swiftly or slowly; high in the sky or low over the ground. The shrike – an invaluable asset – is kept safe from the raptors’ clutches by the provision of a little turf hut.
Other trapping methods employed shrikes as decoys, relying on the extraordinary visual acuity of the raptors to see them as potential prey. Expressions such as ‘eagle-eyed’ or ‘hawk-eyed’ attest to the fact that for a very long time we have known about the extraordinary vision falcons and other birds of prey.
One reason falcons see so well is because they have two visual hot spots at the back each eye – two foveas – rather than the one that humans have. The fovea is simply a tiny pit or depression on the retina at the back of the eye where blood vessels are absent (since they would interfere with the clarity of the image) and the density of photoreceptors – cells for detecting light – is greatest. As a result, the fovea is the point in our retina where the image is sharpest. The falcon’s two foveae contribute to its excellent vision.
Around half of all bird species examined so far have a single fovea, like us, and the question is whether shrikes have one or two. When I asked my academic colleagues that specialise in avian vision, no one knew. But one told me where to look: ‘Check Casey Wood’s Fundus Oculi’ he said. Remarkably, I knew of this obscurely titled book, published in 1917, although I had never looked inside it. Wood’s Fundus Oculi is a study of the retinas of birds, as viewed through an optician’s opthalmoscope. Its title – which guaranteed that it would never be a best seller – simply refers to the back of the eye.
Casey Albert Wood (1856-1942) was already one of my heroes. Professor of ophthamology at the University of Illinois between 1904 and 1925 and probably the most eminent eye specialist of his age, Wood was also fascinated by birds, bird books and the history of ornithology. Recognising, for example, the immense significance, of Frederick II’s thirteenth-century manuscript on falconry (and ornithology), Wood went to the Vatican Library, translated it and published it making this extremely rare manuscript much more widely available. He also discovered and purchased for his personal library, a unique hand-coloured copy of Willughby and Ray’s Ornithology (1678) that John Ray had presented to Samuel Pepys when Pepys was president of Britain’s Royal Society in the 1680s. Another major achievements was Casey Wood’s Introduction to the Literature on Vertebrate Biology, a remarkable reference book that I own and use on a regular basis, that lists all known zoology books (including those on birds) published before 1931.
Wood’s The Fundus Oculi of Birds grew out of his belief that a better understanding of the exceptional eyesight of birds would throw light on the biology and pathology of human vision. It was a stroke of genius and, employing the same equipment he used to examine the human retina, Wood described and catalogued the eyes of a wide range of living bird species. Such was his knowledge, that it was said that he could identify a bird simply from an image of its retina! 
My first opportunity to look at Wood’s Fundus Oculi occurred during a visit to the ornithological Blacker-Wood Library at McGill University, Montreal, that I visited while searching for material for my book The Wisdom of Birds (2009). Casey Wood had donated his huge personal library to the university in honour of his wife. I went with my colleague Bob Montgomerie, specifically to look at the Pepys’ Ornithology, and while there Eleanor MacLean – the librarian – asked if I’d also like to look at the Fundus Oculi. Stupidly, I declined, befuddled by its title and distracted by too many other more interesting old books.
Even if I had looked at it, there was no way I would have remembered whether Casey Wood had included shrikes in his survey, and when I later needed it, I discovered that the book was scarce in British libraries. I eventually found one, and there under ‘California Shrike Lanius ludovicianus gambeli’ now known as the Loggerhead Shrike, Wood writes:
There are two macular regions in the fundus of this bird. In other words, yes, there are two foveas (macular regions) on the back of the eye (the fundus) of the Loggerhead Shrike. Excellent! Just as I hoped, and as Wood says: ‘Birds with double foveae have exceptionally good eyesight.’
The human eye has long fascinated lovers, artists and physicians. The ancient Greeks dissected eyes, but struggled to understand how they worked, unclear as to whether they received or emanated light. The anatomical descriptions of the eye made by Galen – physician to the Roman gladiators during the second century AD – remained the standard until the Renaissance when there was renewed interest in the natural world, and in the wonder of vision, inspired by translations of Islamic manuscripts from the thirteenth and fourteenth centuries. The German polymath Johnann Kepler (1571- 1630) was among the first to create a theory of vision, later elaborated by Isaac Newton, René Descartes and many others. In 1684 Anton von Leeuwenhoek, pioneer microscopist got the first glimpse of what we now know to be light-sensitive cells – the so-called rods and cones –– in the retina. Two hundred years later, using a much better microscope and a very clever way of staining different types of cell different colours, Santiago Ramón y Cajal (1852-1934) provided a wonderfully detailed – and exquisitely illustrated – description of the way the cells of the retina connect to the brain, in a variety of animals, including birds.
Most birds – like the Batleur Eagle shown here (below right) – have excellent vision. Indeed, it was once thought that vision was pretty much the only sense birds have and one vision enthusiast, an ophthalmologist, defined birds as ‘A wing guided by an eye’. How wrong he was – the other senses of birds – as I show in Bird Sense – are very highly developed. Nevertheless, it remains true that birds have excellent vision. One of the most remarkable recent discoveries is that birds can see all the colours we can, plus some extra. Birds – like the two male Zebra Finches here (below left) – can see colours that we cannot and their view of the world is rather different from our own.
Birds rely on calls and song to communicate so it hardly surprising that their hearing is excellent. In fact, it is similar to our own. There are some notable exceptions however. Owls, like Barn Owl here, have exceptionally sensitive hearing and can hunt in total darkness simply by using their ears. Unlike us birds do not have an external ear and in most birds the ear opening is hidden beneath feathers of the head. The internal structure of a bird’s ear is similar to our own, and the drawing on the right from the 16th century – when this structure was discovered – shows a bird’s skull from behind, with the ear opening (A) and the inner ear (C) with what are called the semi-circular canals (numbered 1,2 and 3), responsible for a sense of balance, and the cochlear (e) responsible for detecting sound. The structure on the left is the bird’s single ear bone (much enlarged) from the middle ear, the columella. We have three inner ear bones, birds have a single one.
The sense of touch is greatly under appreciated in birds. The right hand picture above, shows the nether region of a bird called the Red-billed Buffalo Weaver. The bird – a male – is colour-ringed because it was one of our individually marked study individuals. We studied this species in Namibia, because – uniquely – the male has a false penis (visible in the picture), which it uses – employing a sense of touch. Even more uniquely for birds, the buffalo weaver experiences an orgasm!
Below is a is a Fiery-necked Nightjar in Zambia: look at those bristles around its mouth. Many birds have such bristles, but these are enormous and are almost certainly sensitive to touch and may have a role in feeding – but their exact role remains a mystery.
Many birds preen their partners or ‘friends’ – it is referred to allopreening. In the video below, a male Zebra Finch allopreens his partner. He is actually a bit rough, normally, allopreening is much more gentle than this, as illustrated in the other video clip of Guillemots.
It is hard to image birds having a sense of taste, but they do. In fact, if you think about it, it is hard to image how they might function without one. A sense of taste is essential for deciding what and what not to eat. The most detailed research on taste in birds has been conducted on Mallard ducks (shown here). It was first thought that as in ourselves, their taste buds would be on its tongue, but they not, they are in the lining of the mouth, including the bill tip, shown here.
Another sense we don’t associate with birds, but the evidence that birds have a sophisticated olfactory sense is overwhelming. The Kiwi (shown here) for example, has such a sensitive ‘nose’ it can detect earthworms (a favourite food) through 15 cm of soil – simply by smell. The Greater Honeyguide (right) – this is a chick about to fledge – has a well-developed sense of smell enabling it to locate the nests of wild bees: it feeds on wax and bee larvae. Albatrosses, like this Black-browed Albatross (left) in New Zealand, locate good feeding areas by detecting the odour released by plankton.
A handful of birds can echolocate, like bats. The Oilbird from South America shown here (above), roosts and nests in caves and can operate in complete darkness by listening to the echoes of the clicks it utters. I went to see these birds in Ecuador and I have to say, they were one of the most extraordinary, wonderful birds I have ever seen.
Once thought impossible, there is now increasing apparent that most birds – like the Sedge Warbler (shown here) – have a well-developed magnetic sense, essential for finding their way on migration. Because we do not possess this sense ourselves, it has been extraordinarily difficult to study, but our knowledge is increasing in leaps and bounds. Some of the recent discoveries that I describe in the book are almost unbelievable. As one of my colleagues said ‘You couldn’t make this stuff up!’
One of the topics we know least about is whether birds experience emotions such as pleasure or fear. Many birds, like the Atlantic Puffin and Common Guillemot (below), have long-term pair bonds, sometimes lasting years – what is the ‘glue’ that keeps them together? The greeting ceremonies of birds like these on being re-united after long periods of absence are so extravagant it is hard to imagine they don’t have emotions. The image immediately below is of a Long-tailed Sylph in Ecuador – how that species relates to emotions is revealed in the book.