Bird Senses. Graham R. Martin

off by leaves as they are devoured by caterpillars, and we cannot interpret the echoes of an Oilbird’s call as it bounces from the wall of a light-tight cave.

      Even if we could look from the same perspective as the hunting Peregrine, we would not see and detect the world as it does. A view that is based upon the same vantage point as the bird’s cannot tell us what the Peregrine actually sees. Our eyes are not the eyes of a Peregrine, our ears are not those of an Oilbird, our fingertips are not able to detect what the Red Knot’s bill feels, and our tongues cannot taste what the Knot’s tongue tastes. Because of differences in the senses of different species, the information they provide of the same scene is different. In effect each species lives in a different secret world. Species may share the same environment, but the worlds that they inhabit are different.

      Our inability to experience the different worlds of birds is not because our senses are relatively impoverished; it is not because birds have ‘super-senses’. There are certainly examples of bird species that can out-perform us in particular ways, but equally our senses often out-perform those of particular birds. The problem is that our own senses are specialised for the conduct of particular actions just as much as the senses of animals are specialised for the control of their particular actions.

      Despite the popular view that humans are such an important species, it is clear that our eyes are not all-seeing, and our ears are not the detectors of every sound. All senses are selective of the information that they retrieve from the environment. One advantage that humans have over the birds, however, is that we can get valuable insights into their worlds. We can unlock some of their secrets through the application of science. In doing so we not only gain insights into the worlds of birds, but we also come to understand our own world from a fresh perspective.

      Sensory ecology

      Sensory ecology is the branch of science that enables us to understand how information controls animals’ interactions with their environments. In essence sensory ecology allows us to enter and understand the worlds of other species. When we apply the ideas of sensory ecology to birds, we move close to appreciating the world ‘through birds’ eyes’. We also come to understand the proper meaning of a ‘bird’s-eye view’.

      A bird’s-eye view is a much-used metaphor. It pops up everywhere, from highbrow literature, to journalism, to advertising. It is shorthand for appreciating the world from a fresh perspective, but often it is used to imply that we can actually see the world through the eyes of a bird. Sensory ecology casts doubt on that very notion. By going beyond just vision and eyes, a metaphorical bird’s-eye view throws light upon all senses and on how information from them is integrated in the conduct of particular tasks. In so doing it helps us delve deep into the problem of understanding ‘reality’ from a human perspective (Figure 1.2).

      FIGURE 1.2 A Golden Eagle Aquila chrysaetos stares into a camera lens. A White-tailed Eagle Haliaeetus albicilla flies over Paris carrying a camera as part of an advertising stunt to market a new camera. The advertisement carried the strapline, ‘The 100% eagle-eye view’. These are two species credited with the most acute vision in all animals, but what do they actually see? What is an eagle’s-eye view? Can it be captured in a single photographic image? (White-tailed Eagle from Sony Creative Commons; Golden Eagle by the author.)

      Sensory ecology tells us about the ways in which different animals gather information from their environments, about the factors that ultimately limit what can be detected, and the ways in which information from different senses is brought together to underpin life in different habitats. Sensory ecology reveals to us that there are very many ‘birds’-eye views’, many different sensory worlds, and that none is the same as ours. Our own sensory ecology is as specialised as those of Peregrines, Oilbirds, Knots, or Great Tits. We may wish to understand the world through their senses, but we can only ever experience one view of the world, our own. The best we can do as regards the worlds of other species is to know about them, we cannot ever experience and properly know their worlds.

      Sensory ecology can provide data on the sensory performance of other species. It can place these data in a comparative framework and tell us how species differ one from another. It is often tempting to use the terms ‘better’ or ‘worse’ when comparing the sensory capacities of species but, on the whole, these terms are not useful. This is because the senses of each species are adapted for the conduct of different tasks in different environments. There may be optimal solutions to different sensory challenges, but they are no better or worse than each other. They are fit for the purposes that they evolved to meet.

      Sensory ecology helps to determine why and how performance is limited by different environments and by the physiology of a species. Sensory ecology also indicates how information from one sense might be traded off against that provided by other senses for the optimal conduct of particular actions.

      Modern technology can help in the tasks of characterising the sensory performance of different species, but it must not be thought capable of providing simple answers. Experiencing the world as a Peregrine sees it cannot be achieved by strapping a video camera to its back. Such footage makes thrilling viewing and gives insights into the life of a Peregrine, but it cannot give us the real Peregrine’s-eye view. The photograph or film, viewed on a flat screen, still gives us only a view of the world that is two-dimensional and ultimately filtered by human eyes and brains. Understanding a bird’s-eye view has to be achieved by piecing together many different sources of information about vision, not to mention about how visual information interacts with information from other senses.

      Sensory ecology also shows us that we need to see the behaviours of birds in their proper context, the context of the actual challenges that birds face in their natural environments. For example, what is natural night-time really like? How does it change with habitat or with the annual cycle? We can also ask under what natural light conditions a Peregrine can best detect distant prey, and how changing light levels affect its abilities. Does our knowledge of these explain when and how it hunts? Similarly, we might ask questions about how soft and wet mud has to be for a Knot to be able to detect and identify a food item that is near the tip of its probing bill. We might also ask why some birds are particularly prone to flying into power lines. Is it due to limitations of their senses?

      To answer all of these questions, and very many more, requires the senses of birds to be seen in both an ecological and an evolutionary context. In other words, knowledge of senses gained through careful laboratory-based studies and field observations needs to be placed in the context of the wider world in which animals evolved and exist today.

      Key lessons from sensory ecology

      Sensory ecology is a multidisciplinary field of endeavour. It pieces together information from many sources gained through different techniques and relies a lot on comparisons between species. Often it is able to answer some specific questions about the behaviour of birds, but general principles are rather few. However, there are two key themes which shall be returned to a number of times in this book. These themes are the role of multiple information sources in the control of particular behaviours, and the reliance upon minimal information for the control of many actions. Examples of these themes will be discussed throughout the book, but it is worth introducing them in general terms now.

       Multiple information

      Sensory ecology suggests that information from one sense can often be considered to predominate in the control of the behaviour of a particular species. We might even be tempted to assume that birds are ‘all about vision’. However, information from other senses frequently comes into play, sometimes in unexpected ways. Information from different senses may often be traded off and used in complementary ways for the successful completion of a particular action.

      This is particularly seen to be the case when birds are active at low light levels, but it also applies to situations when birds execute tasks at high light levels. For example, a high-speed video will reveal the great precision with which a