Read more: 鈥�Instant Expert 34: Bird senses鈥�
What does it feel like to be another species? In 1974, philosopher Thomas Nagel argued in his paper 鈥淲hat is it like to be a bat?鈥� that we can never truly know. He chose the bat because it is a mammal, like us, yet has the ability to echolocate 鈥� a sense he believed we would find impossible to imagine. Nagel was unduly pessimistic. Four decades on, we can now build remarkably comprehensive pictures of how other animals perceive the world. Birds are further removed from us than bats, and although we cannot know exactly what it is like to be one, we have learned enough about their senses to get a fair idea. Their vision is particularly well understood. So, how does a bird see the world?
Different ways of seeing
It is often said that the reason we identify with birds is because, like them, we are bipedal, basically monogamous and have a sensory system heavily dependent on vision. Birds have big eyes, and vision dominates our perception of what it is like to be a bird. Despite a superficial similarity, however, most birds see the world in different ways to humans.
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For a start, different bird species have three types of visual field, depending on the position of their eyes (see diagram). Those with eyes on the side of the head 鈥� blackbirds, robins and the like 鈥� have good lateral vision and some forward vision, but cannot see behind them. Birds with their eyes located high on the sides of the head, such as ducks and woodcock, also have good lateral vision 鈥� they may actually see two separate lateral images 鈥� and can see behind them, but not the tip of their own bill. Owls, with forward-facing eyes, have binocular vision as we do, not because this is especially important to them but because there is nowhere else to place their huge night-vision eyes.
Avian and human vision differs in other ways, too. Some birds, including European blue tits, budgerigars and zebra finches, can see ultraviolet, which we cannot. Others, especially raptors 鈥� hawks, falcons and eagles 鈥� can see to distances far greater than we can. One reason for this is that the light-sensitive layer at the back of our eyes, the retina, has one fovea, a sensitive spot where the image is sharpest. Raptors, in contrast, have two foveae in each eye, which is equivalent to a camera having both a telephoto and a macro lens (see diagram). Other birds, including seabirds such as shearwaters, have a fovea running in a strip across the retina, possibly allowing them to keep the horizon horizontal.
鈥淎 wing guided by an eye鈥� is how the ophthalmologist Andr茅 Rochon-Duvigneaud characterised birds in his handbook on vertebrate vision in 1943. Vision is certainly important for most birds 鈥� the nocturnal kiwi is the exception 鈥� but their other senses are also important, as we will see.
Left or right eyed?
The brains of vertebrates have two hemispheres, along with an arrangement of nerves that means information from the right side of the body is processed in the left hemisphere, and vice versa. In addition, certain cognitive functions are located in one or other hemisphere. Our ability to speak, for example, is controlled by Broca鈥檚 region in the left side of the brain. Body and brain sidedness, or lateralisation, was once thought unique to humans, but in the 1970s it was discovered that canaries use only the left side of their brain to control song production.
We now know that birds鈥� brains are more lateralised than our own. Intriguingly, in birds with eyes located on the sides of their head, this extends to using the left and right eyes for different tasks (Brain Research Bulletin, vol 76, p 235). In day-old chickens, for example, the right eye tends to be used for finding food while the left scans for predators. You might imagine this difference to be hard-wired 鈥� genetic 鈥� but it isn鈥檛. Just before a chick hatches, it has its right eye facing outwards, which means it receives some light through the shell. The left, inward-facing eye gets no light. However, if you gently turn the chick鈥檚 head in the shell so that the left eye gets most light, lateralisation is reversed.
鈥淚n day-old chicks, the right eye is used for finding food while the left scans for predators鈥�
Seeing with sound
Although oilbirds and swiftlets are not related, they have something extraordinary in common 鈥� both can echolocate. Unlike bats, they utter audible clicks, but the principle is the same: they use sound waves that bounce back off objects to 鈥渟ee鈥� in the dark. Low-frequency sounds are less effective for echolocation than the high-pitched calls of bats, but tests with oilbirds show that they are able to avoid objects larger than about 20 centimetres across. This is sufficient to allow them to find their nesting and roosting ledges in the totally dark caves they inhabit.
Nagel was wrong to think that humans cannot imagine what it feels like to echolocate. Blind people use both passive and active echolocation to find their way about, evinced most spectacularly by blind kids who go mountain biking. Interestingly, the clicks they use to navigate safely on two wheels are low-pitched, at about 2 kilohertz 鈥� not dissimilar from those of the oilbird. I am not blind, but over an eight-month period, while writing Bird Sense, I trained myself to echolocate and was impressed by my own abilities.
This article appeared in print under the headline 鈥淎 Bird鈥檚 Eye view鈥�