Читать книгу Bird Senses - Graham R. Martin - Страница 27

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 placed in the field of view), and resolution changes profoundly as light levels change (Box 2.3).

Box 2.3 Resolution and light levels

The effect of light levels on spatial resolution is significant. Acuity is often measured or estimated at high daytime light levels, but in a few bird species acuity has been measured across almost the full range of naturally occurring light levels (See Box 2.4 for a discussion of how natural light levels vary depending upon the elevation of the sun and moon). In some species acuity has been determined across the narrower range of the light levels that occur in daytime, and in some species across the daylight–twilight range.

This graph brings together data for a number of species and shows some key points about the effect of light levels on acuity. In all species shown here acuity has been determined using behavioural training techniques and gratings, the kind of technique depicted in Figure 2.7.

Although the maximum acuities of these different species are significantly different (for example, the maximum acuity of the eagle is 60 times higher than that of the dove) it is also clear that in all species acuity decreases considerably as natural light levels fall. The only species in which there is not a steep decline in acuity is the Western Barn Owl Tyto alba, but even in Barn Owls acuity shows a significant fall as light levels decrease to the lower ranges of night-time. In some species the decline is particularly steep, and this may be a significant reason why some daytime birds, such a doves, go to roost as light levels fall. If we wish to pose questions about how vision is used to control natural behaviours in any species, it is necessary to bear in mind that natural light levels have important effects on visual abilities, especially acuity.

The change in resolution with light level is significant. Unfortunately, this makes it very difficult for an observer to determine exactly what visual information could be available to a bird at any one moment. For example, we may have knowledge of what a bird can detect at one particular light level, but it will not be the same at another light level. Furthermore, colour vision is an important mechanism that enhances spatial resolution for a wide range of natural targets. However, colour vision functions only at high (daytime) light levels. As light levels drop to those of natural twilight and below, colour vision no longer functions and so an important source of information is lost. Furthermore, even if we discount the contribution of colour vision and just consider the ability to discriminate detail in black and white, spatial resolution decreases markedly with light levels.

At any one location naturally occurring light levels may change over a range of at least a million-fold (10⁶) between maximum sunlight and moonlight. It is remarkable that our eyes can function across this whole natural range of light levels. Even more remarkable is that on moonless nights the range is extended downwards by a further 100-fold, and if we take into account how the presence of clouds and tree canopies can further reduce ambient light, then the total range of light levels in which an eye can function varies by a factor of 10¹¹. That is a huge dynamic range for any detector (Box 2.4).

Box 2.4 Natural illumination sources and light levels

During the day (when the sun is above the horizon), under clear skies light levels vary by about 100-fold from sunrise to full overhead noonday sun. The coming and going of cloud cover can extend this range to 1000-fold. During the night, however, light levels can be much more variable. This is because the main source of light, sunlight reflected by the moon onto the earth, varies not just with the elevation of the moon, but also with the moon’s phase, which changes on a monthly cycle. This results in night-time light levels that can vary by 1 million-fold between sunset and starlight on a moonless night. On top of this there is a further potential 10-fold in variability brought by cloud cover.

This figure captures much of the huge range over which light levels naturally vary and why. It shows how natural levels of illumination at the earth’s surface depend upon the elevation of the sun and moon, and upon the phase of the moon. The amount of light from these sources changes continually over the daily cycle, and over a very large range. (Note that the scale of illumination is logarithmic, which means that light levels change over 10-fold between each digit of the scale.)

Although this basic pattern occurs around the globe, it differs in detail with latitude and time of year. The basic unobscured sun curve shown here is for latitude 50° (approximately southern England) at the time of the summer solstice. Nearer the equator the rate of change of illumination levels each day is more rapid and at higher latitudes it is slower, which means that twilight (the period of transition between daytime and night-time) is shorter or longer than shown in the figure. Close to the poles light levels hardly change on a daily cycle but they do change systematically from day to day. At the equator the daily pattern and amplitude of light-level changes hardly varies across the year. Clearly, the pattern of natural light levels is significantly different across the globe and across the year, and it is within these patterns that the vision of animals has evolved.

A vegetation canopy, of course, reduces illumination at ground level. The exact amount of reduction depends upon the density of trees and their species, but a good rule of thumb is that a woodland canopy in leaf will reduce light levels by about 100-fold. This means that all of the light levels shown in the diagram are about 100 times (2 log units) lower inside a wood compared with outside it. As discussed later (Chapter 9), this can be significant when considering the challenges faced by nocturnally active birds living in open habitats as opposed to closed canopy woodland habitats.

While an eye can operate throughout this light range, the information that it can provide changes very markedly. As explained in the next chapter these changes are not trivial, the ability to see details in a scene can be dramatically different depending on whether we are considering day or night, bright sunlight or moonlight. This means that whenever we consider what a bird might be able to see in a particular location and how it can use that information to guide its behaviour, it is always necessary to consider the ambient light levels. We need to ask about the time of day, and to consider whether colour vision might be available, or not.