Bird Senses. Graham R. Martin

be highly motivated to respond when they come to a training session. Also, it is best to give a bird a training or testing session every day, no time off at weekends.

      Depending on the species and on the individual birds, this initial training may be completed in a couple of weeks, but it can take a couple of months. The investigator has to be alert to the possibility that the birds will learn to solve the problem posed by using a cue other than the one that the investigator intends. When birds are being trained to make these kinds of visual discriminations they can be particularly alert to sound cues which might also reliably indicate where to go to get food. So, for example, a click or rumble associated with changing the positions of the panels between left and right could well be learned, and the bird may then ignore the orientation of the stripes. This is perhaps not too surprising. After all, from the bird’s point of view the task is to learn how to get its food reliably, and any cue that is consistent is as good as another, and sounds are very important to birds. So if the investigator introduces more than one reliable cue the bird could latch on to any of those that are available.

      Once this initial training has settled down and the investigator can be sure that the bird is responding only to stripes of the correct orientation, the actual investigation of sensory abilities can begin. The first stage is to start presenting the birds with pairs of stripes of the same width but to change stripe width between trials. On some trials stripes will be very wide and the difference between the panels’ orientations will be obvious to the bird. However, if the stripes of the grating pattern are made very narrow the task will be difficult, as the fine stripe pattern cannot be determined and the panel looks a uniform grey.

      The result is that on some trials the birds may make a mistake, and errors will start to systematically creep in. If the stripes are so narrow that they are below the birds’ threshold and they truly cannot ever tell the panels apart, then random behaviour would be expected, and the birds would get the task correct on only 50% of the trials. At intermediate stripe widths, however, the birds may get the discrimination correct on 70% or 80% of the trials. Over time, by mixing up trials with different stripe widths, a bird’s motivation to keep responding even on difficult trials can be maintained.

       The uncertain threshold

      When enough trials are accumulated a relatively stable relationship between error rate and stripe width will emerge. This relationship is called a psychophysical function, and the point at which a bird gets 75% of the trials correct is considered the threshold of sensory performance (Figure 2.8). So, after all the training and testing, it is this 75% correct stripe width that the investigator is trying to determine, not what the bird can see 100% of the time.

      FIGURE 2.8 An example of a psychophysical function of a Harris’s Hawk. Performance over a large number of trials has been accumulated on a two-choice task using the kind of setup shown in Figure 2.7. The average percentage ‘correct’ performance for panels showing stripes of different widths is accumulated. The black dots on the graph show the bird’s actual performance for different stripe widths and the red curve is the line of best fit to those data points. With wide stripes the bird is correct on nearly every trial, while with very narrow stripes its choices are random (50% correct). It is in the region between these two extremes that the threshold of the bird’s acuity lies. The region where the birds is 75% correct is usually taken as the threshold performance. In this example 75% correct would occur with stripes that are about 0.7 minutes of arc wide. Similar data are gathered for a number of birds using the same technique, and these are averaged to give an acuity estimate for the species. In the case of Harris’s Hawk, the published average acuity is 1 minute of arc (29 cycles per degree).

      The important point to note is that the threshold is defined statistically. It is the stripe width at which on average the birds get the choice right 75% of the time (Figure 2.8). This statistical definition reflects the nature of sensory thresholds. The limit of performance can vary from moment to moment and from day to day, depending on the physiological state of the observer and on their motivation. Keeping up a bird’s motivation is very important because these investigations can last many months and the bird needs to be fit, healthy, and equally motivated throughout that period.

      Once a threshold for visual acuity has been determined for a group of birds, an average value can be calculated as representative of that species. Individuals will differ, and so in any sample of birds there will be individuals above and below the average determined for the species. But it is usually the average value that is used in any comparisons between species; the best performance is no more representative of the species than the worst.

       Elaborating the task

      Determining an average acuity threshold is likely to be just the beginning of a much longer series of investigations. With well-trained and motivated birds, and highly motivated investigators, it is possible to repeat the investigations at different light levels or repeat them changing the contrast between the stripes. It is usual to start the investigation at high light levels, equivalent to daytime, and use grating patterns that contrast very highly. But many of the more interesting questions about what information is available to birds in their natural environments occur when light levels are not those of midday and when patterns are not highly contrasting. Most real-world tasks involve detecting objects that differ by shades of grey.

      By systematically altering both light levels and contrast it is possible to investigate how a bird’s acuity will differ between daylight, twilight and night-time, and how its ability to see details changes as contrast reduces. Actually, getting birds to continue working at very low light levels and at low contrasts takes a lot of ingenuity. This is because when tasks start to get difficult the bird may simply stop responding and hunker down until the task gets easier. Therefore, getting runs of thresholds for a wide range of light levels and contrasts can be very difficult. As a consequence, most studies tend to report acuity at a single light level and contrast, or within a narrow range. One further problem of working at low light levels is that the birds must be allowed to ‘dark adapt’ to the required low light after being placed into the apparatus, and the light levels need to stay the same throughout a session. This can add greatly to the time a session takes and decrease the motivation of the bird to respond.

       A disconcerting experience

      If conducted properly, the procedures described above will produce robust results that allow comparisons between species whose sensory thresholds have been measured in similar ways. Should the investigation call for it, it is quite possible to put a human in the same situation as the birds and for them to do exactly the same task. This has been done a number of times, and I can personally attest that measuring one’s own threshold in this way is an exacting task, especially when the patterns presented are close to threshold.

      There is a range of stripe widths that can be considered ‘disconcerting’. When looking hard and comparing the patterns in this range, it seems impossible to make a decision – but if you look and respond quickly then you may be correct more times that you are wrong! It seems like guessing, but clearly there is a little information that makes it more than a guess. This is when visual discrimination is around the threshold level, in the 75% correct zone. It is no wonder that individuals, humans as well as birds, can lose motivation and even stop responding when faced with a choice close to threshold.

       Measuring other sensory dimensions

      It is quite easy to imagine how the whole procedure described above can be adapted to investigate many other sensory questions. Instead of striped patterns, the stimulus panels could be uniformly lit with white light paired with an unlit panel, or with lights of different colours. Using these and adjusting light levels, it is possible to measure the minimum amount of light that can be reliably detected; that is, the absolute visual threshold can be determined. That too is a disconcerting task close to threshold.

      Alternatively, it is possible to