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in nature.

      Finding out about our own senses has proved to be a difficult task, requiring careful procedures and controls. It is more difficult to apply those techniques to ask the same questions in similar ways of non-human animals. It seems relatively easy to ask another person what they can see, hear, smell, or feel, but even then we want to know just how carefully controlled were the stimulus conditions. We also need to know whether the person was fully attentive to the task, especially if the task became increasingly difficult as the stimulus got closer to the limit of a sense’s abilities. Did the person really try to see the finest detail? Hear the faintest sound? Or did they give up on the task before reaching the limits of their performance?

       Animal psychophysics

      The branch of science that investigates questions about the relationship between stimulus conditions and how they are perceived became known as ‘psychophysics’. The term was coined 160 years ago by Gustav Fechner, who is credited as being the first experimental psychologist. The techniques developed in his early studies in psychophysics put in place a set of procedures for characterising sensory performance that persist to the present day. However, it was not until the development of ideas about ‘conditional’ responses by Ivan Pavlov in Russia, and later by B. F. Skinner in America, that researchers found ways of establishing ‘animal psychophysics’. Using these techniques, it became possible to combine carefully controlled stimulus conditions with consistent and reliable behavioural responses to ask an animal about what it can see, hear, taste, smell, etc.

      The pioneering work in animal psychophysics used the techniques of operant conditioning developed by Skinner to train animals to make specific responses when a particular stimulus or set of stimuli is presented. Remarkably, it was found that animals could be trained to respond to a very wide range of arbitrary stimuli that really had no place in the natural world in which the animals had evolved. Once the animal has learned to respond in this way to a novel stimulus it is then possible to systematically make changes to a stimulus and establish the limits of performance. Hence it becomes possible to answer such questions as how sensitive an owl’s vision is, or how sensitive a pigeon is to the smell of particular compounds, and to be confident that answers are reliable and comparable between individuals and across species.

      Properly controlled animal psychophysics studies involving trained animals are the benchmark for understanding animals’ senses. However, not all animals can be trained easily using operant conditioning and a great deal has been learned from less systematic, often field-based, studies. Such studies often give only an indication of what a bird might be able to detect, and they often lack accurate quantification. For example, a lot has been learned in recent years about the use of the senses of smell and taste by birds. Knowledge is now being gained about which smells are more salient or important to different species, but it has not yet been possible to determine just how sensitive birds are to particular compounds.

       Two-choice discrimination tasks

      The most rigorous technique used in psychophysical investigations of birds usually involves a small number of individuals of a particular species being trained initially to make a choice between two quite different stimuli which are presented simultaneously. That is, it involves a ‘two-choice discrimination’ and trains the bird to indicate that it can differentiate between two stimuli. It is then a matter of altering those stimuli so that they become more similar until a point is reached where they cannot be told apart. This then defines the limit of a particular sensory ability.

      The technique is perhaps most easily described by the example of an investigation of the limit of visual resolution. In this the investigator is seeking to determine the smallest spatial detail that a bird can see. Initially the bird is presented with two panels placed side by side, one showing a pattern of black and white stripes (a grating) oriented vertically, the other showing the same pattern oriented horizontally. The panels will be of the same size and brightness; all that differs is the orientation of the stripes. The birds will have already become used to taking small food items from a hopper or some other device in the area where the stimulus panels are displayed. Today the patterns might be shown on a computer screen or back-projected onto a panel, but previously stripe patterns on photographic negatives or printed on cards were used (Figure 2.7).

      FIGURE 2.7 A schematic drawing of a setup used to determine visual acuity in a Harris’s Hawk Parabuteo unicinctus. The bird is given a two-choice task. It has been trained to wait on the perch until a pair of patterns is shown on the panels, which are 10 m away. As soon as the bird leaves the perch the patterns are switched off and the bird flies to the perch of its choice. If it flies to the perch in front of the ‘correct’ panel it receives a small food reward. The bird is observed remotely so that humans cannot influence its behaviour. Many such trials are conducted and the width of the stripes on the panels is altered in a random sequence. This allows the performance of the bird with respect to the width of the stripes on the test gratings to be measured, and a threshold stripe width can be determined (see Figure 2.8).

      By the time the investigation gets to this point, the person running the investigation will have got to know the birds well, but will not be in direct contact with the birds. The birds’ behaviour will be remotely observed, so that the investigator cannot inadvertently influence the birds’ choices. The way the investigator influences the bird is through the presentation of food items which are paired with the presentation of only one type of stimulus panel, for example, the one with horizontal stripes. If the bird is hungry it will usually learn rapidly to move towards, or peck at, the panel with the horizontal stripes, as long as it reliably receives a food item each time it does so.

      I have done vision investigations with doves who will peck directly at the panels for grain, owls that will peck at a bar beneath a panel to be rewarded by a small piece of meat, and with Great Cormorants who will swim up to a panel underwater in return for a small fish dropped to them through the surface. Some species can be trained to make their choice of panel from a fixed distance and then fly, swim, or walk towards the panel along a runway or swim-way. Owls seem particularly amenable to running, while diurnal birds of prey will readily fly from a start perch and land on a perch placed just in front of a panel.

      In instances where flying, running, or swimming is involved, the bird must not be able to change its choice once it has passed a certain point. This ensures that the bird’s decision is always made a fixed distance from the panel. The use of a fixed distance is important for calculating the visual size of the object as seen by the bird and hence for defining its best performance. Depending on the species, it might be better for the bird to make a choice between relatively widely striped gratings at a long distance and fly to the panel (for example, a hawk), rather than choose between finer stripes at a closer distance (for example, a Budgerigar Melopsittacus undulatus). It is a matter of working with the bird’s natural behaviours to enable it to perform the choice task readily and hence reveal the ultimate limits of its vision.

      Once this link between a target panel and the reinforcing food has become established, it is then a matter of starting to manipulate by small increments the task that the bird faces. The left–right positions of the panels will be changed randomly between trials, the light levels of the panels will be changed randomly over a wide range of light levels, and the widths of the stripes in the grating pairs will also be varied between trials. The point of these variations is to ensure that the bird is responding exclusively to the orientation of the stripes. In other words, that the bird has really learned that it is the orientation of the stripes that it should be attending to.

       Training and testing

      This type of simple discrimination is likely to be learned robustly by most birds. Once trained, birds will make the correct choice at least 90% of the times that a pair of stimulus panels is presented. In many cases mistakes are never made, the birds being consistently 100% correct. By ensuring that the birds gain most of their daily food intake in these sessions, and that the sessions take place at the

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