Bird Senses. Graham R. Martin
textbooks show arrays of different wing and bill types along with discussion of their different attributes and functions. Such discussions make it clear that there is not a single optimal wing or bill type. A single wing type cannot fly at all velocities, or support birds of different weights, or carry out all kinds of manoeuvres. A single bill cannot be an all-purpose tool for extracting and handling many types of food. What is optimal depends upon the task. This also applies to the senses of birds, especially vision.
It is relatively easy to understand the structural bases for the different types of performance of birds’ wings or bills. For example, there may be obvious differences in the relative lengths and flexibility of bones, or in the number, hardness, and relative lengths of feathers. Bills also differ in their length, shape, and flexibility. It is not so immediately obvious when looking at an eye how variations in vision can arise. How is it possible for one bird to have quite different visual capacities from another?
Camera eyes
The camera type of eye is found in all vertebrates and in some invertebrates (octopuses, squids). A camera eye is also referred to as a ‘simple’ eye, and this label is not without good reason. Compared with the complexity of the multiple repeated structures which are found in the compound eyes of most invertebrates, camera eyes are structurally and conceptually simple. The important point, however, is that within this simplicity of basic design there is great potential for variation in each of the key components. Both gross and subtle variations in these components can profoundly alter the vision of an animal, and hence change the information that different eyes can extract from the same scene.
The basic structure of a camera eye has just two key functional components, an image-producing system and an image-analysing system (Figure 3.2). Not only can these components show much variation, they can also vary in their characteristics independently of each other. The image in the eye of one species will be different to that of another, as will the ways that these images are analysed. Furthermore, with two eyes in an animal’s head, they can be placed in different positions with respect to each other in the skull. This alters the region about the head from which visual information can be retrieved at any one instant and can profoundly influence what an animal can detect in the world that surrounds it.
These two main functional components of camera eyes are conceptually simple. The optical system produces an image of the world outside the eye, and the analysis system extracts information from that image. These two functional components can be matched in a straightforward manner to the main anatomical parts of an eye (Figure 3.2). Indeed, they can be matched to the key components of any camera, from the camera in your phone to a sophisticated video, single-lens reflex, or plate camera.
The optical system of a camera eye consists of the lens and the cornea. The initial extraction of visual information is carried out by the retina onto which the optical system projects an image of the world. The retina is a very thin structure of immense complexity, made up of layers of specialised neural cells. These include the layer containing the photoreceptor cells, and it these which detect the pattern of light within the image. The neural cells of the retina are anatomically part of the brain to which each eye is connected via the optic nerve.
Although the retina shows immense complexity – indeed, it is composed of many millions of cells – there is only a small number of different cell types. This applies especially to the photoreceptors, whose types are discussed in more detail later in this chapter. The important point to note is that significant differences in vision arise from the ways in which the different photoreceptor types are packed together and arranged across the retinas of different species.
This variation in packing and arranging high numbers of receptor cells of just a few types is not unique to vision. It is what underpins variation in other senses too. This will be discussed in later chapters showing, for example, how variation in touch sensitivity, taste, and smell arise. Each of these senses is based on a relatively small number of receptor types, but marked differences in sensory capacity occur because of their relative numbers, and how the receptors are arranged, in different species. Ultimately, these variations are what underpin the sensory ecology of different species.
Sources of variation in camera eyes
These functional components of an eye can be matched to the two main functional parts found in all human-made imaging systems. From large astronomical telescopes to the small cameras built into mobile phones, these systems all have one part that produces the image and another that analyses it, and it is clear that the properties of these two components differ greatly.
Even within the cameras of mobile phones properties can be varied to give images that differ markedly in the information they provide. These differences in information capacities result primarily from three fundamental attributes: the degree of detail that can be detected, the extent of the world that is available for analysis, and the range of light levels over which the camera will operate. These will be familiar and important to keen photographers, but even manufacturers of mobile phones draw attention to these features in their marketing materials.
Comparing a mobile phone camera with an astronomical telescope is straightforward. Both are doing essentially the same thing in the same way, but the levels of information they provide are phenomenally different. However, neither one can do the other’s job. The essential point is that the same consideration applies to eyes. They have evolved in different species to provide information for the conduct of different tasks and in different environments. Differences in their eyes are the result of relatively fine-tuning of both image production and image analysis, similar to the fine-tuning of components that underlies differences in phone cameras.
That people are willing to invest time and money in choosing between phone cameras indicates how differences in the information extracted by cameras are functionally significant. That eyes can differ markedly in all of these attributes suggests that if we were able to choose between different types of eyes, rather than having those we are born with, we might spend a lot of time in coming to a decision.
The optical systems of camera eyes
The optical system is composed of two main elements (Figure 3.2). The cornea is the relatively simple curved surface at the front of the eye. In eyes that operate primarily in air, the cornea is essentially a boundary between air and the fluid-filled chamber of the eye. The radius of curvature of the cornea is the key to its image-forming properties. A more highly curved surface produces a smaller image than a shallowly curved surface. The lens is suspended in the fluids that fill the chambers of the eye. It is also relatively simple. Like a magnifying glass, it has two convex surfaces, but these can vary in how curved they are. Unlike a magnifying glass, the interior of the lens is not uniform but is made up of a complex structure of transparent layers of different densities. The optical function of the lens is primarily concerned with making relatively fine adjustments that focus the image, already formed by the cornea, onto the retina.
FIGURE 3.2 Camera eyes can be divided into two main functional parts; the optical system and the image analysis system. The optical system in bird eyes have two components, the cornea and the lens. These produce a focused image that is projected onto the retina, which is where the first stage of image analysis begins, and from where information is sent via the optic nerve to the brain. The key thing is that although these two functional components are joined together in a single eye, they can to some degree evolve independently of each other. The lens and cornea can have many different optical properties depending on their shapes and sizes and can produce images with different properties (for example, size, brightness, and contrast). The retina can exhibit huge variation in the way receptors are arrayed across its surface. This means that eyes with different image-making properties can evolve and eyes with different image analysis systems can also evolve. Even eyes which are of the same size and overall shape can have very different properties. Analysis of different eyes has revealed a plethora of subtle differences in both image production and image analysis. This results in the eyes of different species