The Life of Birds

Introduction

It is easy to understand why so many of us are so fond of birds. They are lively; they are lovely; and they are everywhere. They have characters with which we can easily identify – cheeky and shy, gentle and vicious, faithful and faithless. Many enact the dramas of their lives in full view for all to see. They are part of our world yet, at a clap of our hands, they lift into the air and vanish into their own with a facility that we can only envy. And they are an ever-present link with the natural world that lies beyond our brick walls. It is hardly surprising that human beings have studied birds with a greater dedication and intensity than they have lavished on any other group of animal.

The first task of ornithology was to give names to birds. Every society, of course, has produced its own version, often in great detail. In the eighteenth century, the Swedish naturalist, Carl von Linné, proposed a uniform way of classifying all living things based on the names used by Greek and Roman naturalists. That, greatly refined and elaborated, remains the system used by scientists all over the world ever since. Today, two hundred and fifty years later, we have found names for some ten thousand different species of birds. Museums and other scientific institutions have accumulated cabinets full of bird specimens with dozens, sometimes hundreds of examples of each species, each one carefully prepared, meticulously measured with every tiny variation in coloration and size duly noted. The introduction of portable binoculars and, later, photography, allowed that high expertise to spread into the field. Now it is no longer necessary to shoot a bird to identify it. Now ornithologists have become so expert that they can identify a wild living bird from a snatch of song or the briefest glimpse of its plumage or silhouette. That is a skill which I greatly admire, but one, alas, that I do not possess.

But that is not what this book is about. My fascination with birds comes from watching how they behave. Ornithologists began to study this aspect of their subjects rather earlier than those working in many other branches of zoology. While big-game hunters were still shooting antelopes in the belief that establishing the maximum size of the horns of any species told us something important and were arguing, on the basis of skin patterns, how many species of giraffe exist, ornithologists were beginning to investigate the journeys birds make. To do this, they needed to identify individuals. One technique, which those working with other animals were somewhat slower to adopt, was to use tags or bands. Jean-Jacques Audubon, back in the early nineteenth century, is said to have tied coloured threads to the legs of the flycatchers that each summer visited his parents’ mill in Pennsylvania and so established that birds which nested there reappeared the following spring after their migration south and nested in exactly the same place. That in itself was an astonishing finding, but a mere hint of what would be discovered about the extraordinary abilities of birds.

Those discoveries, however, were a long time coming. Individual birds of the same sex and species tend to resemble one another more closely than do those of any other large animal. Scientists working with elephants quickly learn to distinguish individuals from the irregular shapes of their huge ears. Chimpanzees have faces that are as different from one another as those of human beings. Humpback whales have different white patterns on the flukes of their black tails. Lions, being of a quarrelsome disposition, tend to acquire characteristic scars on their muzzles – and even if they don’t, have whiskers that vary in their number and disposition. But it is virtually impossible to distinguish one fully fit male dunnock from another.

So watching one in our garden, we tend unthinkingly to make assumptions about its social arrangements. We assume that it is always the same one which collects a worm from the lawn and seldom question whether it is also always the same bird which devotedly supplies food for the chicks in the nest in the hedge. Until comparatively recently, it simply did not occur to anyone that there was any need to fit a ring on a bird that lived permanently in the garden to test such assumptions. But when an ornithologist caught dunnocks and did so, he discovered that the marital arrangements of these birds were of such a variety that had they been human beings they would have made headlines in the newspapers.

Once the use of leg rings extended beyond migration studies, other techniques soon came into use. Small devices were fitted on the backs of large birds which sent regular signals to a satellite high in the sky and relayed them down to a receiving station on earth. So it was discovered that a wandering albatross may travel 1,600 kilometres in order to gather a cropful of food and bring it back to its chick sitting by itself on a lonely Antarctic island. Penguins were persuaded to swallow tiny instruments that measure water pressure. These, carried in their stomachs, showed that king penguins regularly swim down to depths of 300 metres in the ocean in search of fish. Genetic fingerprinting was used to identify the exact parentage of young birds, and it was revealed that such was the complexity of the superb fairywren’s life that a male may not be the father of a single one of the chicks that he so devotedly feeds in his nest. The science of bird behaviour has now become a rich and fascinating subject.

And that is the subject of this book. Even though many of the investigative techniques are of such sophistication that they can only be used by full-time research scientists, there is still a great deal of research work that is done by dedicated amateurs. The ubiquity of birds and the devotion they inspire has produced a worldwide army of enthusiasts prepared to devote their spare time and endure the most uncomfortable of conditions in order to make observations and collect data. The literature recording all this work, both amateur and professional, is huge and widely scattered, much of it only found in specialised scientific journals. Without that great body of raw data and the distillations that have been made from it, barely a single page of what follows could have been written. My debt to such publications is huge. I have not, however, given individual references to all these sources in order not to clog the text. Nor, for a similar reason, have I used scientific names for the species I mention. Their precise identity, however, can be discovered by consulting the index in which their scientific name is listed beside the colloquial name I have used in the text.

It is my hope that the pages that follow will show something of the deep fascination to be found not only in naming birds but in discovering what they do and why they do it.

ONE

To Fly or Not to Fly

A bird of prey circles high in the sky above a limestone cliff that rises sheer and white above the green forests of Borneo. Earlier in the day, it had been roosting in the trees so motionless that few other animals could have noticed it. As the evening sun started to sink and lose its brilliance, the bird had flapped lazily into the air to start its patrol. Now it looks down towards the black mouth of a cave that gapes in the face of the cliff. Deep inside, a million bats hang from the ceiling, packed together so tightly that the rock above them is hidden. Unlike the hawk, they cannot see the sinking sun to take a cue from its impending disappearance. Nor can they have detected a drop in temperature for the conditions inside the cave are remarkably stable. Yet somehow the bats know that outside the day is coming to an end and that soon they will be able to fly out into the darkening forest to gather their nightly harvest of insects.

A few flutter uncertainly into the air and fly to and fro, navigating in the blackness by the echoes of their ultrasonic squeaks. Then, within a minute or so, they organise themselves into a column. Like a wavering black ribbon, it snakes across the ceiling only inches beneath the rock. It advances, winding round bumps and along crannies from one chamber into the next, until eventually it reaches the great hall that forms the cave’s entrance. Outside, the sun has vanished, but if you are sitting in the cave mouth there is still enough light for you to watch the black ribbon advancing diagonally across the ceiling until it reaches the highest point in the far corner. As it arrives, the bats break ranks and spill out into the open across the forest canopy.

The hawk watches. It does not seem to be in any hurry. There are so many bats in the cave that their exodus will last many minutes. It can choose its moment. Suddenly, it makes up its mind. It tips down, accelerates on rapidly beating wings and dives straight into the cloud of bats. Its feet come forward and it grabs a bat with its talons. Sometimes it rips its prey apart with its beak as it flies. On other occasions, it carries the crumpled bat back to its roost to feed. It may catch two or three bats before the last of them leaves the cave and darkness falls. The hawk has particularly large eyes but even so, after about half an hour, it can no longer see well enough to repeat its accurate high-speed pounce. However, it has gathered all it requires. Bats are certainly the most skilled and agile of all flying mammals but they are no match for the bat hawk. As long as there is light, the skies belong not to the mammals but to birds.

Perhaps that is to be expected. Birds have been flying for much longer than bats. The oldest bat fossils to be discovered date from around fifty million years ago and birds were flying at least a hundred million years earlier still, at the time of the dinosaurs. They were not, however, the first animals to colonise the air. Insects preceded them by some two hundred million years. Some of the first were giants with wings over 30 centimetres across. As the millennia passed, the descendants of these pioneer aviators evolved many different techniques for flying. Some had two pairs of wings, others just one. Some developed gyroscopic stability controls, others beat their wings faster than muscles could contract and managed to do so by hitching them to the vibrating shell of their thorax. But they did not grow very large. Insect bodies are constructed in such a way that they cannot operate above a certain size and no insect has ever exceeded those early giants. When birds appeared, the insects found themselves outflown and that superiority remains today.

If you are in doubt, watch a spotted flycatcher, that once common but now declining summer visitor to gardens all over Europe. It is a somewhat nondescript bird with no dramatic colour in its plumage, but it draws your eye immediately because of its actions. It usually sits, very upright, on a bare branch of a tree and every few seconds takes off in a swerving twisting flight before returning to its branch. Get nearer to it and you may hear a faint click in the middle of its excursions. That is the sound of its beak snapping shut on an insect. Dragonflies may zig-zag and dodge but they are lucky to escape if the bird gets anywhere near them. Flies and ichneumonid wasps are snatched from the air without any difficulty. The bird is so skilled that sometimes it returns to its perch with several small insects held deftly in its beak. Each victim is dealt with appropriately. Butterflies are stripped of their wings. Horse-flies are swallowed as they are, but bees and wasps, although they are of a similar size and general appearance, are recognised and made harmless by being rubbed vigorously against the perch so that their stings are discharged before the bird swallows them. The insects met their match in the air a long time ago.

When, then, did the birds first achieve this dominance of the skies? The answer was discovered during the last century in Bavaria not far from Munich. The countryside there is studded with quarries where men extract a lovely cream-coloured limestone that has been used for building since Roman times. It is so smooth, uniform and fine-grained that, in the nineteenth century, it was used for lithographic printing. In some places, it splits into thin slabs. Separating them, one after the other, from the top of a block is like opening the pages of a book. Most of these pages, it must be said, are blank, but every now and then, lifting one reveals the perfectly printed record of an animal – a bewhiskered shrimp, a fish with its fins and ribs immaculately preserved, a horseshoe crab lying at the end of a trail of its last footprints. Sometimes there is a faint brown stain around these remains showing the position of soft parts that decayed and dissolved soon after death. Even such insubstantial creatures as jellyfish have been found delicately delineated.

From these fossils it is not difficult to deduce the sort of conditions under which the limestone was deposited. It was once mud on the floor of a warm tropical lagoon. Land lay a few kilometres to the north and to the south a coral reef separated the lagoon from the open sea. Because there was little flow in or out and the rate of evaporation in the warm sunshine was high, the waters became so saline that no animal could make the lagoon its permanent home. But periodically an unusually high tide would sweep over the reef, carrying with it creatures from the sea beyond. Occasionally animals from the mainland flew across the lagoon and crashed into its tepid salty waters. So there are fossils of insects – mayflies, dragonflies, locusts, beetles, wasps – ample evidence of how advanced and sophisticated insects had become in the air even at this remote period. There are also exquisitely preserved skeletons of small flying reptiles – pterosaurs, with the faint outlines of their skinny wings plainly visible around their elongated straw-thin fingers.

Such wonderful, beautiful fossils have been collected and treasured for centuries. But in 1860, a quarryman working near the small village of Solnhofen split a block and made an unprecedented and astonishing discovery – a feather. It is quite small, only 15 centimetres in length and less than 6 centimetres wide, but it is preserved in the greatest detail. In a couple of places the filaments of the blade are separated and at the base of the quill there is a little tuft of isolated ones. The vane on one side of the quill is only half the width that it is on the other. This asymmetry has particular significance. The feathers on a modern bird’s wing are shaped in just this way and positioned on the wing with the narrow stronger edge at the front. Such a shape makes it clear that this ancient feather had an aerodynamic function. It looks scarcely different from a wing feather that one might pick up today on a country walk. Yet it must have fallen from the wing of a creature that flew across the lagoon one day a hundred and fifty million years ago.

What kind of creature was it? No living creatures grow feathers except birds. Indeed, feathers are taken to be the defining characteristic of birds, so the Solnhofen feather, by definition belonged to a bird. But what kind of bird? Science did not have to wait long for the answer. The very next year, in 1861, in a quarry only a few kilometres from that where the feather had been found, a nearly complete skeleton was discovered. It was the size of a chicken and it was surrounded by the detailed impressions of feathers.

But this was a very strange bird indeed. It had a long bony tail; each foreleg had three separate digits, each of which ended with a curved claw; and its skull, as was revealed by later finds, carried not a beak but bony jaws studded with teeth. It was clearly part-reptile and part-bird. The scientist who described it called it Archaeopteryx, a name based on two Greek words meaning ‘ancient wing’. Since that time several more specimens have been identified so that now we know a great deal about this extraordinary creature’s anatomy. Yet the debate still continues about exactly how it lived.

The claws on its wing fingers give some clues. A few birds still retain such things even today. Some swans, ducks, jacanas and several other birds have them, hidden out of sight beneath their plumage. The screamers, goose-like birds that live in South America, carry two very prominent and easily visible ones on each wing which are displayed during territorial disputes and used as weapons during fights between males. But perhaps the most likely clue as to how Archaeopteryx used its wing claws comes from an odd bird living in northern South America, called the hoatzin.

The hoatzin is a swamp-living leaf-eater with a rather clumsy lumbering flight. It makes an untidy platform of twigs as a nest on which it lays two or three eggs. The young, when they hatch, have two well-developed claws on each wing. As the nestlings grow, they become venturesome and clamber about in the mangrove trees using these wing claws to help them cling to the branches. If they become alarmed, perhaps by some predator, they will dive into the water beneath and then, after a short time, use their claws to clamber back to their nest. Once the birds become adult, they lose their claws. Maybe Archaeopteryx which retained them throughout its life used them in the same way as the hoatzin chicks do. Although no fossil logs or substantial branches have been found in the Solnhofen limestone, there are occasional leaves of conifers, cycads and maidenhair trees so undoubtedly forests were not far away. The rarity of the Archaeopteryx fossils suggests that these creatures only appeared above the lagoon very infrequently. They must have strayed across from the mainland and the forests that were their true home.

There are other indications that Archaeopteryx lived in trees. The big toe on each foot points backwards, as it does in most modern birds, so enabling the animal to grasp a perch. Its long tail also seems to belong to a bird that lived up in trees. It is beautifully preserved in several of the specimens and shows no sign of being bedraggled at the tips as it might well be if the creature spent much time on the ground. And it is so long that it would be a grave encumbrance if its owner lived on water.

But did it use its wings merely to glide from one branch to a lower one, or was it capable of beating them and so powering its flight? If it could flap, then it must have had muscles connecting its wings to the bones of its chest. There was no evidence in the first specimens of any bone that might have provided such an attachment, but this does not mean that such muscles did not exist. They could have been fixed to a piece of cartilage which would have rotted and left no trace to be fossilised. So the question had to remain open. But in 1992 new evidence appeared. Yet one more specimen, the seventh, was found in a quarry only a few kilometres from that which produced the first. It is smaller than the first and differs sufficiently in other details for it to be regarded as a different species. It has been named Archaeopteryx bavarica and it has something the other specimens lack, a large bony breastbone. This is more than adequate as an anchor for wing muscles. So the likelihood is that these pioneering aviators did not just glide but flapped their way through the forest – and occasionally across the lagoon into which some of them crashed. More recent work, using special x-rays, has revealed the structure of Archaeopteryx’s wing bones to be similar to that of modern birds, the structure most closely matching that of quails and pheasants, which are capable of short bursts of powerful flight.

Archaeopteryx could not have been the first backboned animal to have taken to the air. Its feathers have such a complex structure that they must be the product of a long evolutionary process that extended over many thousands of generations. But why should that process have started? Why should Archaeopteryx’s ancestors have found it advantageous to have feathers even of the simplest kind? The answer to that clearly depends on who those ancestors were.

One possibility is that they were dinosaurs. Indeed, the similarity between Archaeopteryx and a small dinosaur is so close that one specimen lay for decades in a museum classified as a dinosaur until a more careful inspection revealed the faint impression of feathers around the forelimbs and made it clear that it was, in fact, an Archaeopteryx. Such small dinosaurs were probably active predators that chased after their prey. To be as active as that, an animal’s body must be warm so that its chemistry works vigorously and produces a lot of energy. Fast-moving reptiles of today, such as lizards and snakes, achieve this condition by warming themselves in the sun. But some argue that small dinosaurs such as the velociraptors were able to generate their own heat internally, as mammals do. Such a process is very expensive in terms of energy and uses up a significant proportion of the calories taken in as food, but it brings considerable advantages. It would, for example, enable an animal to be active in the early mornings and gather food when its competitors were still cold and torpid. A warm insulating coat would then be invaluable. Reptiles of the time were covered in scales, as modern ones like the Australian shingleback lizard are today. If those scales increased in length and became fibrous then they might well provide such beneficial insulation.

Now imagine such a creature using its abundant energy to pursue its prey, say a large insect. It might well rise on its hindlegs, as the frilled lizard of Australia does when it wants to move at speed. That would leave its forelegs free. If they were covered with long fibrous scales – proto-feathers – then stretching them out might lift the animal into the air and enable it to snatch at its prey with its mouth. Alternatively, if it was running to escape a bigger animal, then such a manoeuvre might take it out of range and into safety. So this warm-blooded reptile would have taken its first step towards flight. Disbelievers in this theory maintain that such an animal would not, when running on its hindlegs and seeking to put on a turn of speed, suddenly stick out its forelegs since such an action would instantly increase its drag through the air and thus inevitably slow it down.

Disregarding this objection, the question must still be asked if the dinosaurs that might have been Archaeopteryx’s ancestors were, in fact, warm-blooded? Some maintain that they were. They seek evidence from several sources – from the ratio between their numbers and those of the vegetarian-grazing reptiles on which they preyed; from the microscopic structure of their bones; and from the size of their brain. But none of this evidence is accepted by everyone as conclusive. Other research suggests that they, like reptiles alive today, were not able to maintain their bodies at a constant temperature. Such creatures grow at different speeds at different times of the year. In consequence, their bones develop concentric rings somewhat similar to the annual rings in a tree trunk. Such rings have been found in the bones of fossil birds that succeeded Archaeopteryx in the ancient skies. This suggests that Archaeopteryx itself did not generate its own body heat either. Archaeopteryx fossils are so rare and precious that no one has yet sectioned one of the limb bones to confirm that this is so, but if it is, then the theory that feathered flight originated with creatures that ran along the ground is greatly weakened. Clearly, a cold-blooded animal would be very unlikely to develop an insulating coat of fibrous scales since that would prevent or at least hamper its owner from warming itself in the sun.

There is an alternative hypothesis. Perhaps the ancestral reptile, with or without the benefit of internally generated warmth, started to clamber four-leggedly up into the trees. There are several reasons why it might have done so. Maybe it was taking refuge from larger predatory reptiles on the ground; maybe finding a safe place for its eggs; maybe pursuing insects that lived in the branches. Once it was up in a tree it would need to move about. It is much quicker and less energy-consuming to jump from one tree to a lower branch in a neighbouring one than to descend, run across the ground and climb back up again.

Several animals today leap around in