thresholds tend to be normally distributed when the results for individuals are collected together. This means that the absolute threshold for a species is best described statistically as a mean value with variation around it based on a sample of individuals. Between-individual differences are often particularly notable if age comes into play. A change in sensitivity is usually part of the ageing process in birds, as much as in humans. Therefore, to compare species it is necessary to refer to mean differences, and to be aware that individuals will have sensitivities below or above this mean.
A simple analogy is to consider what the answer might be if you asked how fast humans can run. Every able-bodied person can run, but there is no surprise that the running speeds of people will differ, often markedly so. The answer is that there will be a mean speed for human running performance but around the mean there will be a wide range of speeds. It would then make sense to compare the average running speeds of humans with another animal species, and while there might be a difference in the averages there may well be some overlap in the performances of individuals of the two species.
It is the same with absolute sensory thresholds: differences between individuals do occur. Also, an individual’s thresholds can change owing to a range of factors, including their motivation to participate in the investigation. So, although absolute thresholds are both important and interesting, and will feature a number of times in this book, it will always be necessary to express caution, or at least bear in mind that sensory thresholds are mean values based on a sample of individuals representing a particular species.
Relative sensitivity within a sense
Clearly, the sound and light that an animal is able to detect can vary in their total energy. We can see very bright as well as dim lights, hear very loud sounds as well as very quiet sounds. The ability to detect these stimuli indicates that there is a wide dynamic range to sensory performance. Absolute thresholds measure only the ability to detect the lowest amount of energy for a particular type of stimulus or sensory dimension.
As well as the wide dynamic range, most types of natural stimuli can also vary along a number of dimensions. For example, the vibrations of air molecules that we detect as sounds can have different frequencies, and it is these different frequencies that humans describe as sounds of different pitch. Light also varies in frequency, though we more often describe it in terms of its wavelength. Our visual system detects the different wavelengths as lights of different colours.
In each animal species there are likely to be different absolute thresholds for each frequency of sound or each wavelength of light. This means that a complete description of a bird’s vision or hearing requires knowledge of thresholds across a wide range of stimulus frequencies. For light these are presented as spectral sensitivity functions, while for sounds they are presented as audiograms (Box 2.2).
Box 2.2 Audiograms and spectral sensitivity curves
Light and sound have an important feature in common. Both can be described as waves. These disturbances are comparable to the peaks and troughs of a wave that we might watch travelling across the surface of a pond.
The distance between two peaks of a sound wave is referred to as the wavelength of that sound. Since the speed of travel of sound or light through a particular medium is constant (although sounds travel four times faster through water than through air, and even faster through solids such as metals and rocks) the wavelength bears a simple relationship to the frequency with which the waves pass by. It is this frequency that is usually used to describe one of the key properties of sounds. The unit of frequency used is hertz (Hz), the number of wave cycles per second, but sounds are commonly described in kilohertz, 1000 wave cycles per second.
The frequencies of different sounds vary, and it is these frequency variations that ears detect. We refer to sounds of high and low frequency as being of high and low pitch.
The relative heights of the peaks and troughs in a wave can also vary so that two sounds of the same frequency can contain different amounts of energy. These differences are detected as louder or quieter sounds.
The full spectrum of sounds in nature is remarkably broad, but any animal’s ears can detect only a portion of all possible sound frequencies. Across a range of frequencies an ear detects all sounds but sounds outside that frequency range are simply not detected. Knowledge of the upper and lower limits of the hearing range of a species is an important part of describing their biology. Different animals can hear sounds across markedly different frequency ranges, but no species can detect all possible sounds. Some species detect sounds in the highest frequency ranges, other in the lower ranges, and for others hearing is in a mid-range of frequencies.
Within the frequency range of an animal’s hearing, sensitivity to sounds is not the same across all frequencies. Some sound frequencies can be detected at lower intensities (the sound waves have low amplitude), while others can be heard only at higher intensities (waves of high amplitude). When hearing sensitivity has been determined across the range of frequencies that an animal can hear, an audiogram can be constructed for that particular species.
At a glance audiograms convey a lot of information about the average basic hearing abilities of a species. They define the upper and lower limits and also show which frequencies can be detected at high amplitude and which can be detected at low amplitude. Audiograms generally show a broadly ‘U-shaped’ function. This indicates that there is lower sensitivity (high-amplitude sounds are needed for their detection) to both higher- and lower-frequency sounds, and that the highest sensitivity (sounds of low amplitude can be detected) occurs in a middle range of frequencies. However, audiograms are typically not symmetrical, with rapid changes in sensitivity as a function of frequency in some parts of the hearing range, and less rapid changes in others.
The diagram here shows the average audiogram for Atlantic Canaries Serinus canaria (the common cage bird). It shows that maximum sensitivity to sound occurs at relatively high frequencies, the sounds of notes at the higher end of the piano keyboard, but above those frequencies sensitivity drops dramatically. At the low-frequency end, it looks as though a Canary will not hear the lowest notes of a piano, or at least they would have to be relatively loud to be detected.
If audiograms are available for a number of species, then differences in their hearing can be comprehended readily by comparing audiograms. This makes it possible to get a clear and rapid understanding of how the hearing of species differ one from another. Because of this, audiograms have become a valuable tool for characterising and comparing hearing across species throughout the animal kingdom.
Audiograms are also used as a clinical tool in humans. They are used to characterise different types of hearing loss by comparing an individual’s audiogram with a normal or average audiogram for humans. An individual’s audiogram is likely to change with age. Hearing loss at higher frequencies usually occurs with increasing age, and this readily shows up in an audiogram. This can be used to define the best characteristics of a hearing aid that can be recommended for a particular person.
Diagrams showing the differential sensitivity to light of different wavelengths are known as spectral sensitivity curves. Like sound, light can also be described as a wave phenomenon. However, rather than referring to the frequency of the wave it is usually more convenient to describe light by reference to its wavelength. The distance between the peaks and troughs in light waves is very short compared with sound waves.
The actual wavelength of light is too small to comprehend readily. However, by using the nanometre as the unit of measurement the numbers become manageable. A nanometre (nm) is one metre divided by 1,000,000,000, and using this unit it is possible
