only to register changes in the amount of light falling upon them. However, these simple structures, and the limited information they provided, had a profound influence on the evolution of animals. From a modern perspective these first eyes were game changers, the equivalent of today’s disruptive technologies. It is no exaggeration to suggest that the emergence of eyes changed forever the type, quality and quantity of information that underlies the interactions of animals with their environments, which, of course, includes their interactions with other animals. Ever since the simplest eyes evolved, the gaining of information about the world, its interpretation, and the uses to which it is put, has become increasingly complex and subtle.
The evolution of vision
Prior to the evolution of eyes, the ways that animals were able to gain information about their worlds was both slow and intimate. It depended primarily upon the transmission of chemicals through air or water. This information either took some time to arrive or it concerned objects that were very close to or, more likely, in direct contact with an animal’s body. The key advantage that even simple eyes brought was that information about events remote from the animals were received immediately. This advantage was so dramatic that the first simple light-sensitive structures, which could detect just the presence and intensity of light, rapidly evolved into something far more sophisticated.
This was the birth of ‘spatial vision’. This is the ability to determine not just that light is present and changing in intensity, but also the direction from which light is coming. The refinements of eyes and vision over the past 500 million years have been concerned primarily with elaborating the accuracy of spatial vision. This elaboration has been concerned with extending the degree of detail that can be extracted about light sources at various distances, extending the range of light levels over which this can be achieved, and increasing the volume of space about an animal from which information can be obtained at any instant.
The high utility of such information is indicated by the rapidity with which eyes evolved. It took probably less than 2 million years for eyes that simply registered the presence of light to evolve into ‘camera eyes’. These are eyes that show all of the main features of the eyes that we recognise in species of the present day, including in ourselves. It has been argued that the evolution of sophisticated eyes, showing all of the key features of modern eyes, could have involved only about 400,000 generations of change (Figure 3.1).
FIGURE 3.1 The evolution of well-focused camera eyes, starting from a light-sensitive patch on the surface of an animal. The diagram shows a theoretical model based on conservative assumptions about selection pressure and the amount of variation in natural populations. This model, proposed by Nilsson and Pelger from the University of Lund, suggests that an eye could have evolved very fast, in fewer than 400,000 generations. The starting point is a flat piece of light-sensitive skin (shown in blue) with a transparent protective layer over it, and below the receptor cells a layer of pigmented cells (shown in black). These absorb light not caught by the receptors and help provide integrity of the whole structure. The emerging chamber is filled with a clear fluid and eventually by a lens, while the original protective layer becomes curved and eventually takes on an optical role as the cornea. This evolutionary pathway is not just theoretical, it is informed by the fossil record. (Redrawn from the original scheme proposed by Nilsson and Pelger in 1994.)
The newly evolved ability of spatial vision provided information not only about objects that were close by, but also about those that were far away from the animal. By so doing it established vision as the primary source of information used to guide behaviour. This primary reliance upon vision is found today in nearly all animal taxa including, of course, birds.
Camera eyes (their basic design and functional divisions are described below) had been around for over 350 million years before the first birds appeared on the planet, about 150 million years ago. This means that the first birds, and their dinosaur ancestors, were highly likely to have had elaborate visual capacities. These capacities had been honed through the process of natural selection in response to the challenges of extracting information from the many different environments that had occurred on the earth over a number of geological eras. As sophisticated as these first bird eyes might have been, their evolution has continued. Changes in vision have occurred in response to the new environmental challenges that emerged as bird lineages diversified to exploit the wide range of habitats in which birds now exist. Across today’s 11,000 bird species, eyes exhibit both major and subtle differences in design and function.
It is difficult to know the paths and time courses over which eyes have evolved. However, Dan Nilsson of Lund University has argued that the evolution of eyes has not been linearly progressive. It seems that eyes, like many other structures, have evolved in fits and starts. This means that relatively long periods of stasis in eye structure were punctuated by periods of rapid change, triggered by the emergence of new tasks as the environment changed. These changes will have included the availability of new food sources and feeding opportunities, and the appearance of new species that provided new threats and opportunities.
Just what these tasks were is very difficult to determine. However, the overall tasks that seem to have driven the evolution of eyes in birds are concerned with the control of the bill or feet towards specific targets, especially in foraging, and in the detection of predators. These two main sorts of tasks typically make substantial, but often conflicting, demands upon eye design and most aspects of bird vision. It seems likely that other tasks, especially the control of locomotion (flight, swimming, or walking) are achieved within constraints imposed by these two key tasks of foraging and predator detection. Surprisingly, it seems that the control of locomotion may not be a prime driver of vision in birds. These arguments will be expanded upon later, but the important point to note here is that vision, alongside other aspects of an animal’s biology, is driven by its utility. In the case of vision (and other senses) that utility lies in gaining information for the control of behaviour. Vision of a certain kind is not just something that a bird happens to have, it must fulfil important functions.
The importance of vision in birds
The primary reliance of birds on vision is easily asserted. It can be based just upon casual observations of birds completing their everyday behaviours. However, this assertion is also well supported by evidence that in most species of birds relatively large portions of their brains are devoted to the analysis of information from vision. Also, the so-called ‘intelligent’ behaviours of birds seem to be based primarily upon visual information. Thus, extracting information from vision and using it to guide sophisticated behaviours is the essential function of the brains of most birds.
Only in a handful of extant bird species is vision not the primary sense. Even in such birds vision was at one time highly likely to have been the prime source of information. However, vision can become secondary through a process of regressive evolution. At the same time other senses, particularly olfaction, hearing, and touch sensitivity, can come to take on some of the primary functions usually carried out by vision. These include food detection, predator detection, and guidance of locomotion – all of which are underpinned by vision in most birds.
The prime examples of ‘non-visual’ birds are the five species of flightless kiwi, which probably lost their reliance on vision as they evolved in the absence of mammalian predators on the islands of New Zealand. The downgrading, but not complete loss, of vision in these birds is discussed in Chapter 9.
In other bird species, including some of the petrels (Procellariidae), shorebirds (Scolopacidae), and owls (Strigidae), olfaction, touch, and hearing respectively play a key role or one that is complementary to vision, especially when it comes to finding and ingesting food items. However, in these birds vision is still the primary guide for locomotion.
