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      FIGURE 2.1 The Greek philosopher Epicurus (left) and the Roman philosopher Sextus Empiricus (right). Their ideas laid the foundations for a comparative approach to studying the senses of animals. They viewed sensory capacities as linked to the environment in which an animal lives and to the tasks that it performs. Shaped by modern ideas about evolution, their approach to understanding the worlds of animals is manifest today in the science of sensory ecology. (Image of Sextus Empiricus from Wikimedia Commons, public domain.)

      It might not have been unreasonable to surmise that while birds experience a different world to that of humans, all bird species experience the world in the same way. Thus, it would have been possible to explain that bird species exhibited different behaviours simply because of differences in their anatomy and bodily structures. That is, it was only because of the differences in their bills, wings, legs, etc. that birds behaved differently, not because of any differences in the information that they had about the world. Sextus argued that this was not the case, and that while species might share the same environment the information that they had about it was different.

      Sextus argued that the anatomical and structural differences between species went hand-in-hand with differences in the information that each species received to control their behaviour. Having established this as a key idea, it can be seen that in essence sensory science, and especially sensory ecology, has for two millennia been filling in the details – an enterprise that has been boosted significantly by the evolutionary and ecological frameworks that emerged in the last 200 years.

      Sextus’ insights and their role in the evolution of fundamental ideas about the world have been profound. This is because they lead to a quite unsettling position. They inevitably lead to the question of what the ‘real’ world might be like. Is there such a thing, or are there many different worlds depending upon the information available to particular species? It was questioning in this way that laid the foundations of Scepticism – the idea that we must neither accept any idea as true nor any idea as false, but we must always question. Scepticism is based upon the recognition of how difficult it is to be sure of the world when it can be based only on the information that the senses provide. Scepticism, in turn, led to Empiricism and the system of enquiry that underpins the modern scientific approach to understanding the world through experimentation and hypothesis testing.

      Questions about senses: differences and dimensions

      Questions about the sensory world of birds are legion. Every reader will have his or her own set of questions. Questions may be focused around particular bird species, but with almost 11,000 species, specific answers are not always available. Some questions have straightforward answers, some answers will be nuanced, and many will not have an answer … yet. We might ask, for example, about how the ability to see details in a scene differs between species, or how visual fields might differ between species. If we know the answers to such questions, we might be able to account for a raptor’s ability to capture its prey, or why species differ in their vigilance behaviour. If we know something about hearing, we can ask about the sounds used by birds to advertise their presence in a territory and whether birds can accurately pinpoint a singing rival. If we know about smell and taste, we can ask about their role in helping birds to locate profitable foraging locations.

      Such questions lead to the three core challenges of sensory ecology:

      1. to measure sensory performance so that it is possible to properly compare like with like across species;

      2. to understand the anatomical and physiological differences that are responsible for differences in sensory capacities;

      3. to propose ideas about what drives these differences from an ecological and behavioural perspective.

      Devising ways of comparing sensory performance is a key challenge because we need to be confident that the same thing is being measured in different species. Despite differences in the size, general structure, behaviour, and ecology of bird species, it is necessary to get information about the same sensory capacities in each species. Furthermore, while it is possible to ask in general terms what a bird can see, hear, smell, taste, etc., each sensory capacity is highly complex (Figure 2.2).

      FIGURE 2.2 A photographic montage of bird species from a wide range of orders and families. Each species has a unique biology, and this montage could well be used to show diversity of bill structures and sizes and how they can be related to different diets and foraging techniques. However, aspects of the senses, especially vision, in all of these species have been investigated. These show that these species also differ in the information that their senses extract from the environment. Furthermore, it has been shown that these are intimately linked with both the ecology and the behaviour of these species. In short, the eyes and vision of these birds vary as much as their bills.

      The only way to characterise and quantify each sensory capacity is to subdivide it into particular dimensions. It is these subdivisions which can be measured and compared with confidence. For example, when investigating vision, comparisons across species are usually made with respect to a number of attributes. Prominent among these are resolution (the ability to detect detail), contrast sensitivity (the ability to detect differences in brightness), absolute sensitivity (a measure of the smallest amount of light that can be detected), relative sensitivity across the spectrum of light, colour discrimination (the ability to detect differences within the spectrum), and visual fields (a description of the space from which visual information can be extracted at any instant). Some of these dimensions of visual ability are also subdivided. For example, resolution and contrast sensitivity are found to differ when considering targets of high contrast (black and white) versus targets that contain lights of different colours (Figure 2.3).

      FIGURE 2.3 Four birds’ eyes. Clockwise from top left: Rock Dove Columba livia, Golden Eagle, Orange-breasted Waxbill Amandava subflava, Common Kestrel Falco tinnunculus. Although all are built to the same basic design, these eyes differ in many aspects of detail. This includes their overall size, the optical properties of their corneas and lenses, and the structures of their retinas. As a result, the vision of each of these species is unique, and each eye provides different information about the worlds in which these birds live.

      It will also be crucial to know how performance on these dimensions of vision is influenced by ambient (overall) light levels. This will be particularly important not only for comparing between species, but also for trying to understand how vision limits and constrains behaviour in natural environments. This is because in every natural environment, apart from underground or deep below the surface of water, light levels vary greatly through the daily cycle. When asking questions about what an animal can see, the answer will always depend upon how much light is around.

      Unfortunately, this means that the answer to a question about what a particular bird might see, and how it is related to particular behaviours, has to focus upon particular dimensions or capacities within vision, and even at what time of day the bird is active. General overall statements may be difficult to make, but as in so many things there is fascination in subtle detail and nuanced answers.

      Investigating all of these different aspects of vision is a tall order especially in light of how species can differ in terms of their behaviour, size, physical structure, etc. In essence it is necessary to find ways of asking animals questions and getting clear unambiguous answers about their senses. No matter what the birds may do, what they look like, or where

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