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 they live, it is useful to know some basic things. What is the finest detail that they can see? What is the frequency range of sounds that they can hear? How finely can the animal discriminate between lights of different colour, and sounds of different frequency? Which airborne molecules can be detected by their olfactory systems? How fine is a bird’s ability to discriminate the frequency and amplitude of movements of an object touching its bill tip?
Answers to such questions help put broad limits on what an animal can detect and therefore what can influence its behaviour. Unfortunately, even these questions have been answered in relatively few species, and rarely are answers available to all such questions in a single species. However, we do know enough for some general principles to have been established and some rules of thumb to have been generated.
Describing sensory performance
How can sensory performance be quantified? In essence we want to ask some specific questions. We want to ask a Barn Owl and a Starling what it can see and hear, a Sanderling what it can feel at its bill tip and taste on its tongue, a Storm Petrel what it can smell (Figure 2.4).
FIGURE 2.4 Four bird species which differ markedly in their senses and how they use them to gain information that is used to guide their behaviour, especially their foraging. Clockwise from top left: Common Starling Sturnus vulgaris, Barn owl Tyto alba, Leach’s Storm Petrel Oceanodroma leucorhoa, Sanderling Calidris alba. These birds differ in their vision, hearing, sense of smell, and sense of touch. Furthermore, each species relies upon a different primary sense to guide its behaviour. They also differ in how they combine and complement information gained through different senses. In short, each bird lives in a different sensory world. (Photo of Starling by Pam P. Parsons [West Bay Dorset, via Flickr as Pam P Photos], Barn Owl by Graham White [CC BY-NC-SA 2.0], Storm Petrel by C. Schlawe [public domain], Sanderling by J. J. Harrison [https://www.jjharrison.com.au, CC BY-SA 3.0].)
Sensory science is mainly concerned with these kinds of questions, with revealing sensory capacities and discovering the mechanisms that underpin them. Sensory ecology, however, takes this information further and is more interested in revealing what sensory capacities are in play in a particular situation, or in finding out how an animal uses the information that it has available to guide its key behaviours.
The aim of sensory scientists has been to manipulate just one or two parameters of a stimulus at a time, and if possible to use those manipulations to determine the limits of sensory performance. If this can be done in a comparable way across a range of species then we should be able to say with some confidence that this species is more sensitive than that one, or this species is able to gain information over a wider range of parameters. These are often the kinds of things that birdwatchers, journalists, and TV documentary makers are keen to know about.
I am often asked whether this or that species is ‘better’ or ‘worse’ than humans. But the biologically relevant question is whether the species is better or worse than the species that it is competing with. What is important to a sensory ecologist is using this information to understand what information an animal has available to guide its behaviour in real-world tasks, and understanding how information from different senses might be integrated.
It does not matter whether an animal’s senses are ‘better’ or ‘worse’ than humans. Humans are, after all, just one species, adapted to living in certain types of environments through the conduct of particular behaviours, so comparison with humans may not be important. The desire to compare another species’ sensory performance with humans is born of an anthropocentric view of the world. It is a viewpoint which provides a strong pull. I try to resist it, but we shall not be able to escape from it completely in this book.
Measuring senses in a similar way in different species does have great value for comparative studies and also for helping to understand the basic mechanisms that underlie a sense. For example, if differences are found between species in their ability to see detail, and systematic differences are also found in the structure of the eyes’ optics or retinas, then it is possible to start piecing together an understanding of basic mechanisms. This in turn allows the possibility of being able to predict what another species might be able to see just from knowledge of its eye structure.
Sensory thresholds
Limits of sensory performance are defined as absolute thresholds. These are measures of the minimum amount of a particular stimulus that can be detected. In the case of sound, this will be the smallest amount of air disturbance that the hearing system can detect. In vision, it is the lowest number of photons received per unit time that can be detected. In the chemical senses (taste and olfaction), it is the lowest concentration of molecules of a particular substance that can be detected. In mechanical senses (for example, touch sensitivity of a bird’s bill) it is the smallest displacement of the detecting surface.
Absolute thresholds are particularly interesting because they allow simple and direct questions to be posed. Which species has the most sensitive hearing? Which species has the most sensitive eyes? However, comparing thresholds is not as straightforward as one might hope, and answers are rarely clear-cut. This is because thresholds differ not just between species but between individuals.
Like all measures of performance,
