Bird Senses. Graham R. Martin

it seems natural to use humans as the base comparator species, seeing and hearing outside the range of humans is nothing special, it is just different. Indeed, we might well ask why human hearing is stuck in the middle and does not reach these ultra- and infra- ranges. Many of the mammals that we share our everyday lives with hear sounds that are well outside the range that we can detect (Figure 2.5).

      FIGURE 2.5 The spectrums of light and sound, showing the general range of frequencies and wavelengths that humans and other animals are able to detect. Infrasounds have frequencies below those that humans can hear, ultrasounds have frequencies higher than those that can be detected by humans. However, there are many animals species which can detect sounds within the infrasound and ultrasound frequency ranges. Similarly the electromagnetic, light, spectrum has a range of wavelengths that humans detect as spectral colours (the colours of the rainbow) from violets to reds, but some animals can detect light at shorter wavelengths, in the ultraviolet part of the spectrum, and some can detect light in the infrared part of the spectrum.

      Costs and trade-offs in senses

      Humans are all too keen to think that they ‘know’ the world, that the world is as they perceive it through their senses. However, even the above brief mention of infrasounds and ultrasounds, of infrared and ultraviolet light, tells us that the world contains far more information than we can directly receive. By definition these are sources of information about the world that we cannot directly access. These are sounds that we cannot hear, lights that we cannot see, but other species can. This means that it is arrogant to believe that humans should be the comparator of all things – but it is equally true that no organism can fulfil that role. Put simply, it is not possible for any one species to be able to detect everything that is going on in its environment. It is not possible for any one species to ‘know’ comprehensively how the world actually is.

       Trade-offs between senses

      The world of any one species is no more important or special than that of another. All sensory worlds have equal importance. They have been shaped by natural selection to extract information for the efficient conduct of the life of each species. For individual species there will be important constraints on how their sensory organs can perform. This is because there are costs and trade-offs in sensory capacities within a sense, and also between senses. The trading off of information between senses is something that will be discussed a number of times in this book. One particularly dramatic example is found in some ducks which, unlike most other birds, have comprehensive vision of the world about their head. This is only possible, however, because their foraging has become controlled by touch and taste information, so that they do not need to see where their bill is. This has resulted in a trade-off between vision, touch, and taste that has given rise to particular diets and behaviours.

      An important constraint on how animals detect their environments comes from the metabolic costs of operating different sensory systems. Vision is particularly costly. Not only are eyes demanding of support and protection in the skull, but their actual running costs are high. There is a rapid and constant turnover of materials and large amounts of neural processing are necessary to extract information from visual input, and neural processing is demanding of energy. Eye size is a fundamental factor in both visual resolution (the amount of detail that can be extracted from a scene) and sensitivity (the minimum amount of light necessary for the extraction of information). As a general rule the larger the eye, the higher its sensitivity and resolution.

      The eyes of most birds are small, but there are plenty of species that have large eyes – for example owls, albatrosses, raptors, hornbills, and penguins. In all of these species larger size would seem to be the result of natural selection for either high sensitivity or high resolution, or even both. But not all nocturnal species have large eyes. We might reasonably predict that large eyes could have easily evolved in kiwi species, because they are flightless and weight should not be a problem – but in fact the eyes of a kiwi are similar in size to those of a small passerine. The answer to this apparent paradox lies in the fact that kiwi conduct many tasks guided by information derived from non-visual senses, most notably smell, hearing and tactile cues from the bill tips. This is another striking example of how information from one sense can be traded off or complemented by information from another. In the case of kiwi, the result of these trade-offs is that their sensory world, their reality, is far removed from those of other nocturnal birds.

       Trade-offs within a sense

      When attempting to understand the behaviour of particular species the above examples of complementarity or trade-offs between the information received from different sensory systems are particularly important and fascinating. However, significant trade-offs also occur within a sensory system. In fact, compromises and trade-offs within a sense should be considered the norm. This reflects the simple truth that within a particular sensory system it is not possible to collect all of the information that is potentially available.

      Trade-off within a sense is clearly seen in the relationship between visual resolution and sensitivity. At the very limit, both resolution and sensitivity are determined by the quantal nature of light and by noise within the nervous system. This trade-off is evident most dramatically in the fact that resolution always decreases as higher sensitivity is gained. We experience this every day of our lives: as light levels naturally fall, and our eyes become more sensitive to dim light, we accept that there is less detail in a scene. But the detail has not gone away, it is still there, it is just not detected by our eyes, which are adapted to detect the reducing number of light quanta in the environment.

      Our loss of spatial details with decreasing light levels is not just a quirk of our vision. It is because it is a fundamental constraint on any vision system, including cameras. It is the very physical nature of light, its quantal nature, that precludes high visual resolution at low light levels. An eye that has evolved to achieve high sensitivity is unable to detect fine spatial information at low light levels, but neither can a highly sensitive eye readily achieve high resolution when there is a lot of ambient light. Life is full of compromises, and that is certainly true both within and between different sensory systems. Natural selection has worked on these trade-offs and fundamental constraints to shape sensory information optimally for the conduct of different tasks, in different environments, by different species (Figure 2.6).

      FIGURE 2.6 The trade-off between high sensitivity and high resolution is found in all vision and imaging devices, including cameras and eyes. It arises from fundamental constraints imposed by the quantal nature of light. The trade-off is exemplified here by two bird species. In Short-toed Snake Eagles Circaetus gallicus vision has evolved to provide high resolution but low sensitivity, while in Tawny Owls Strix aluco high sensitivity is achieved but resolution is low. Hence, while the vision of a Tawny Owl is suitable for activity at low light levels it has low resolution and cannot detect fine details. On the other hand, eagles achieve high resolution but have low sensitivity and so they tend to cease activity as light levels fall towards dusk. (Photo of Short-toed Snake Eagle by A. Román Muñoz Gallego, University of Malaga.)

      A unique property of vision

      The trade-off between resolution and sensitivity exemplifies a unique and important aspect of vision. Within the range of light levels in which vision normally operates the information that it can extract varies markedly with the amount of the stimulus (light) that is available.

      All other senses effectively provide the same range of information within their normal operating ranges. Across a wide range of sound levels, or concentrations of chemical compounds, or physical components of a touch stimulus, the sensory systems are able to extract more or less the same information. In vision this is not the case. This is because the information that vision provides is based primarily upon spatial resolution (the ability to see details in a scene or to say exactly where something is