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 determine the threshold for detecting lights of different colours. In this way the sensitivity of the bird across the spectrum, its spectral sensitivity, can be determined. If lights of different colours are paired it is possible to determine how close they can be before they can no longer be told apart. This will give an indication of how fine the colour vision of the bird is. It is also possible to introduce flickering lights so that the slowest and fastest flicker speeds can be detected. This can have useful applications in that it gives a clue to how slow or fast a flickering light might need to be to act as a warning or distraction.
With other ingenuity and interests it can be seen how this kind of psychophysical approach can be used with sounds rather than light. In these kinds of studies birds are not trained to respond when they see something but when they hear it. Systematic alterations of sound levels and frequency can provide insights into a bird’s absolute sensitivity to sound, its relative sensitivity to sound of different frequencies (audiograms), and how finely the birds can discriminate between sounds of different frequencies. It would also be possible to introduce sounds with different time patterns, trills etc., and investigations with these can give insight into the ability of birds to detect differences in bird songs.
Some investigators have even been able to modify the overall procedure to work with smell and taste to determine which chemical stimuli are more salient and can be detected at low concentrations. However, controlling the smell or taste compounds and presenting them in a uniform way can be very difficult, and perhaps only a few trials can be done each session.
Who, how, and what to measure?
There are many ways in which such training techniques can be used to investigate the senses of birds in a robust way. These allow comparisons between species. However, not all species will be readily amenable to such training, or investigators may not be able to invest the time necessary for the use of these training procedures in full. Some investigations have been able to use the basic principles of these training procedures to gain insights into birds’ senses albeit in a rather limited way compared with the full descriptions that might be aspired to. For example, the first investigations of whether birds could detect ultraviolet light was done using a modified procedure with hummingbirds, and some early work on the sense of smell in doves was carried out using modifications of this kind of approach.
Much depends on the time available to do the work and the motivation of the investigators to work with their species. Unfortunately, modern ways of funding science and pressure to produce papers often preclude long-term training projects of the kinds described here, but work is still being done. For example, recently there have been some valuable investigations of vision in diurnal birds of prey and in parrots, using just these kinds of procedures. Valuable insights into what diving birds might see underwater have come from training investigations of these kinds. All of these studies took many months and required the accumulation of many thousands of trials with the birds.
There remain plenty of opportunities to pose questions about the sensory capacities of most bird species. As the comparative database of species’ sensory thresholds grow, the results become of ever greater value.
Vision first emerged on Earth about 540 million years ago. The first eyes
