Bird Sense (2 page)

Our predecessors’ starting point for understanding the senses was the sense organs themselves – the structures responsible for collecting sensory information. The eyes and ears were obvious, but others, such as that responsible for the magnetic sense of birds, are still something of a mystery.

Early biologists recognised that the relative size of a particular sense organ was a good guide to its sensitivity and importance. Once the anatomists of the seventeenth century discovered the connections between sense organs and the brain, and later realised that sensory information was processed in different regions of the brain, it became apparent that the size of different brain regions might also reflect sensory ability. Scanning technology, together with good old-fashioned anatomy, now allows us to create
3
-D images and measure with great accuracy the size of different regions of both human and bird brains. This has revealed, as Richard Owen predicted, that the visual regions (or centres, as they are known) in the kiwi’s brain are almost non-existent, yet its olfactory centres are even larger than he thought.
4

Once electricity had been discovered in the eighteenth century, physiologists like Luigi Galvani quickly realised that they could measure the amount of ‘animal electricity’ or nervous activity in the connections between sense organs and the brain. As the field of electrophysiology developed it became clear that this provided yet another key to understanding the sensory abilities of animals. Most recently, neurobiologists have used different types of scanners to measure activity in different regions of the brain itself to inform them of sensory abilities.

The sensory system controls behaviour: it encourages us to eat, to fight, to have sex, to care for our offspring, and so on. Without it we couldn’t function. Without any one of our senses life would be so much poorer and much more difficult. We strive to feed our senses: we love music, we love art, we take risks, we fall in love, we savour the scent of freshly cut meadows, we relish tasty food and we crave our lover’s touch. Our behaviour is controlled by our senses and, as a result, it is behaviour that provides one of the easiest ways of deducing the senses that animals use in their daily lives.

The study of senses – and bird senses in particular – has had a chequered history. Despite the abundance of descriptive information accumulated over the past few centuries, the sensory biology of birds has never been a hot topic. I avoided sensory biology as a zoology undergraduate in the
1970
s, partly because it was taught by physiologists rather than behaviourists, and partly because the links between the nervous system and behaviour were known only for what I considered rather unexciting animals like sea slugs, rather than for birds.

Part of my motivation for writing this book, then, is to make up for lost time. I have also been encouraged by a change in attitude, not so much among physiologists but among my animal behaviour colleagues, who during recent decades have effectively rediscovered the sensory systems of birds and other animals. While I was writing this book I contacted several retired sensory biologists and was surprised to discover that they all had a similar tale:
when I was doing this research no one was interested, or they didn’t believe what we had found
. One researcher told me how his entire career had been devoted to the sensory biology of birds and, apart from once being asked to write a chapter for an encyclopaedia of bird biology, had received relatively little recognition. On retirement he had burned all his papers, and then – to his simultaneous dismay and delight – I started asking him about his research.

Others told me how they had once planned to write a textbook on the sensory biology of birds but failed to find a publisher sufficiently interested. I cannot imagine what it must be like devoting your life to an area of research that few others find interesting. However, different areas of biology flourish at different times and I am optimistic that the sensory biology of birds is about to have its day.

So what’s changed? From my own perspective, the field of animal behaviour has changed dramatically. I describe myself as a behavioural ecologist first and an ornithologist second: a behavioural ecologist who studies birds. Behavioural ecology is a branch of animal behaviour that emerged in the
1970
s, with a tight focus on the adaptive significance of behaviour. The behavioural ecologist’s approach was to ask how a particular behaviour increases the chances of an individual passing on its genes to the next generation. For example, why does the buffalo weaver – an African, starling-sized bird – copulate for thirty minutes at a time when most other birds copulate for just a couple of seconds? Why does the male cock-of-the-rock display in groups of other males and play no part in rearing his offspring?

Behavioural ecology has been extraordinarily successful in making sense of behaviours that to previous generations had been a mystery. But behavioural ecology has also been a trap, for like all disciplines its boundaries have restricted researchers’ horizons. As the subject matured during the
1990
s many behavioural ecologists began to realise that, on its own, identifying the adaptive significance of behaviour wasn’t enough. Back in the
1940
s, when the study of animal behaviour was in its infancy, one of its founders, Niko Tinbergen (later a Nobel Laureate), pointed out that behaviour could be studied in four different ways: by considering its (i) adaptive significance; (ii) causes; (iii) development – how the behaviour develops as the animal grows up; and (iv) evolutionary history. By the
1990
s behavioural ecologists, whose entire focus during the previous twenty years had been on the adaptive significance of behaviour, began to realise that they needed to know more about the other aspects of behaviour and, in particular, the causes of behaviour.
5

Let’s see why. The zebra finch is a popular study species for behavioural ecologists, especially for studies of mate choice. Female zebra finches have an orange beak and males a red beak, a sex difference that suggests that the male’s brighter beak colour evolved because females prefer a redder beak. Some, but not all, behavioural tests suggest that this is true and researchers assume that because
we
can rank the beaks of male zebra finches from orangey-red to blood-red, female zebra finches can do the same. They have never tested this assumption in terms of what zebra finches can actually see, yet it is widely assumed that beak colour is an important component of female choice.
6

Another trait that female birds are thought to use in their selection of a mate is the symmetry of plumage markings, such as the pale spots on the throat and chest of male European starlings. Careful tests in which female starlings were ‘asked’ to discriminate between different levels of plumage symmetry (using images rather than live birds) revealed that, while they could identify males that were highly asymmetric in their spotting, their ability to discriminate smaller differences was not very good. In fact, to a female starling most males look much the same in this respect, demonstrating that they were unlikely to use plumage symmetry as a way of choosing a male.
7

Behavioural ecologists have also assumed that the degree of sexual dimorphism in birds – that is, how different males and females are in their appearance – might be linked to whether they are monogamous or polygamous. To test this they scored species according to the brightness of the male and female plumage – based on
human
vision. We know now that this is naive, for the avian visual system is not like ours because birds can see ultraviolet (UV) light. Scoring the same birds under UV light revealed that a large number of species – including the blue tit and several parrots – previously thought to lack sexual dimorphism, actually differed quite a lot when viewed – as females would see them – with UV vision.
8

As these examples illustrate, of all avian senses, vision – and colour vision in particular – is the area where the most spectacular recent discoveries have been made, mainly because this is where researchers have focused most effort.
9
Researchers now realise that to understand the behaviour of birds it is essential to understand the kind of worlds they live in. We are just beginning to appreciate, for example, that many birds other than kiwis have a sophisticated sense of smell; that many have a magnetic sense that guides them on migration, and, most intriguingly of all, that like us birds have an emotional life.

What we know about the senses of birds has been acquired gradually over centuries. Knowledge accumulates by building on what others have previously found, and, as Isaac Newton said, by standing on the shoulders of giants. Because researchers feed on each other’s ideas and discoveries, and since they both collaborate and compete with each other, the more individuals there are working on a particular topic, the more rapidly progress is made. Progress is accelerated, of course, by intellectual giants: think Darwin for biology, Einstein for physics and Newton for mathematics. But scientists are human, too, and susceptible to human foibles, and progress isn’t always rapid or straightforward. It is all too easy to become fixated on one idea – as we’ll see. Research is full of blind alleys and scientists constantly have to judge whether to persist in what they believe to be correct, or to give up and try a different line of enquiry.

Science is sometimes described as a search for the truth. This sounds rather pretentious, but ‘the truth’ here has a straightforward meaning: it is simply what, on the basis of the available scientific evidence, we currently believe. When scientists retest someone else’s idea and find that evidence to be consistent with the original notion, then the idea remains. If, however, other researchers fail to replicate the original results, or if they find a better explanation for the facts, scientists can change their idea about what the truth is. Changing your mind in the light of new ideas or better evidence constitutes scientific progress. A better term, then, is ‘truth for now’ – on the basis of the
current
evidence this is what we believe to be true.

The evolution of the eye is a good example of how our knowledge has progressed. Throughout much of the seventeenth, eighteenth and nineteenth centuries, it was believed that God in his infinite wisdom had created all life forms and had given them eyes to see with: owls have especially large eyes because they need to see in the dark. This way of thinking about the perfect fit between an animal’s attributes and its lifestyle was called ‘natural theology’. But there were some things that simply didn’t look like God’s wisdom: why males produced so many sperm, for example, when only one was needed for fertilisation. Would a wise God be so profligate? Charles Darwin’s idea of natural selection, presented in the
Origin of Species
in
1859
, provided a much better explanation for all aspects of the natural world than the wisdom of God, and as the evidence accumulated scientists abandoned natural theology in favour of natural selection.

Scientific studies usually begin with observations and descriptions of what something
is
. Once again, the eye provides a good example. Starting in ancient Greece, the early anatomists removed the eyes of sheep and chickens and cut them open to see how they were constructed, and made detailed descriptions of what they saw – and sometimes what they imagined they saw. Once the descriptive phase is complete, scientists start to ask other kinds of questions, such as ‘How does it work?’ and ‘What is its function?’ Often, while one kind of biologist may be an expert in anatomy and can provide a detailed description, it usually requires a different range of skills to understand how something like the eye actually works. As our knowledge increases and researchers become more and more specialised in their knowledge, they usually have to collaborate with others whose skills complement their own. Understanding how the eye works today, for example, requires expertise in several different fields, including anatomy, neurobiology, molecular biology, physics and mathematics. It is this interdisciplinary approach – the interactions between researchers with different kinds of expertise – that ultimately makes science both exciting and successful.

Ideas have a particularly important place in science. Having an idea about why something is the way it is is crucial since it provides the framework for asking questions – and asking the
right
questions. For example, why do the eyes of owls face forward, whereas those of ducks are directed sideways? One idea for the owl’s forward-facing eyes is that, like us, owls rely on binocular vision for depth perception. But there are other ideas, too, some of which, as we shall see, are even better supported by the evidence.