Species such as white-crowned sparrows Zonotrichia leucophrys have a short period of a few months after hatching when they can learn songs called close-ended learners Figure 2. In contrast, open-ended learners like European starlings Sturnus vulgaris and canaries Serinus canaria can learn and add new songs to their repertoire throughout their lives, usually on a seasonal basis.
Which sex sings tends to depend on who competes to breed. In many species, only males sing; in others, both males and females sing equally in duets. As we already learned, variation in repertoire size among species is extensive, and the characteristic of having a repertoire with multiple song types likely evolved more than once in songbirds MacDougall-Shackleton So what favors large and small repertoire size?
In fact, in some species females prefer males with large repertoires, and males with larger repertoires have a higher reproductive success i. When maintaining a breeding territory, familiar neighbors are preferred over newcomers because newcomers, who do not yet have a territory, are more likely to expand their territories than established neighbors.
Territorial conflicts are energetically costly, and the song of a local population, or dialect, is thus thought to be favored as it can serve as an indication of regional origin, minimizing physically demanding defense measures. William Thorpe pioneered scientific research on song learning in the late s. He showed that chaffinches Fringilla coelebs , raised in a laboratory as nestlings without exposure to adult males of the same species, develop abnormal songs Thorpe However, when young of the same species were exposed to taped recordings of a wild chaffinch song tutor songs , they sang species-specific songs as adults.
This demonstrated that birds must learn songs early in life. This field of study was advanced further by Peter Marler and colleagues who showed that the song dialect is learned during a sensitive period, and birds have an innate predisposition to learn the songs of conspecifics Marler Similar to human speech development, song learning is a two-stage process Figure 2.
Birds first memorize a tutor song and form an auditory memory, or "template," in their brain sensory phase. They then translate the inner template into motor activity by practicing, comparing their own vocalization to the template, and refining the songs sensorimotor phase. With some exceptions e. This is because most young birds learn the species-specific songs during the first year of their lives. In the wild, birds grow up listening to the songs of a variety of different species. So why don't they learn the songs of multiple species? If given a choice, young birds preferentially learn conspecific over heterospecific songs, and if birds are raised in acoustic isolation, they sing abnormal songs yet still with species-specific elements.
Thus, together with the genetic predisposition for recognizing and learning species-specific songs, this minimizes the risk of learning wrong songs. When the sensory phase ends varies among species, but this timing depends partly on experience. Interestingly, if birds are raised in acoustic isolation, the sensory phase can be extended even into adulthood in some species. At the beginning of the sensorimotor phase, young birds first produce generic, variable, and quiet vocalizations called subsong, which is similar to human baby babbling Brenowitz et al.
They then produce louder, more structured songs called plastic songs, which are still variable but contain some elements of the tutor song. Songs finally crystallize to stable stereotyped songs that are similar to the songs they memorized. During the sensorimotor phase, birds need to hear their own vocalization in order to develop normal songs. If juveniles are deafened after the sensory phase but before the sensorimotor phase, they develop aberrant songs Konishi Birds of certain species produce more sounds during the sensorimotor phase than the sounds they will produce in adulthood.
This means that during song crystallization, those species select which sounds are incorporated into the crystallized song. As you might expect, this selection is not random. This can be advantageous because males that sing local dialects have a higher reproductive success than those that sing foreign dialects MacDougall-Shackleton et al. In either case, song selection during crystallization is based on functional significance to maximize reproductive output.
Neuroplasticity describes the lifelong ability of the brain to form new neural connections depending on season and experience. Testosterone administration during the sensory phase has little or no effect on song memorization. However, testosterone levels are high during song crystallization and in the spring, when songs are more stable. Testosterone administration prior to song crystallization triggers premature crystallization of simple songs, and both castration and blocking testosterone receptors can delay or prevent crystallization.
Thus, the timing and degree of elevation in testosterone are essential in proper development of birdsongs. Abbreviations for the brain nuclei: Area X Area X of the medial striatum , DLM medial portion of the dorsolateral nucleus of the thalamus , HVC used as a proper name , LMAN lateral subdivision of the magnocellular nucleus of the anterior nidopallium , and RA robust nucleus of the arcopallium. Birdsong is controlled by discrete brain regions that are interconnected. There are two such neural pathways: the motor pathway, necessary for song production, and the anterior forebrain pathway AFP , necessary for song learning and plasticity Figure 3.
RA then sends axons to two separate motor nuclei whose motor neurons innervate the vocal organ in birds, called the syrinx, and the respiratory muscles to produce song while coordinating breathing. Studies deactivating parts of the neural circuit for birdsong via lesions highlight functions of the AFP. Further studies have shown that lesions of Area X in juveniles prevent crystallization whereas lesions of LMAN in juveniles result in permanently crystallized, atypical songs. Lesions of LMAN, in fact, make their songs truly stable. It prevents adult birds from learning new songs and also shields already-learned song from deteriorating after deafening.
There are striking similarities between the development of birdsong and human speech. In both cases, dialects and languages are culturally transmitted during a sensitive period of learning. Both birdsong and human speech are controlled by discrete neural circuitry and auditory feedback is essential in normal learning.
These parallels have attracted scientists to use birdsong as a model for research in human speech pathology. In the past ten years, neuroscientists discovered that a mutation in one gene called FOXP2 causes a particular speech and language disorder White et al.
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Using songbirds as a model, researchers found that the FOXP2 gene is necessary to accurately imitate the sounds of a tutor song Haesler et al. These recent discoveries demonstrate that birdsong constitutes an excellent model for exploring the molecular basis and behavioral development of human speech.
I thank S. MacDougall-Shackleton, A. Diez, T. Farrell, Z. Hall, T. Luloff, S. Charles Darwin was one of many who struggled to attempt an answer, and the elaborate songs of male birds such as nightingales clearly inuenced his thinking as he developed the theory of sexual selection. Since then, biologists from many dierent disciplines, ranging from molecular biology to ecology, have found bird song to be a fascinating and productive area for research.
The scientic study of bird song has made important contributions to such areas as neurobiology, ethology and evolutionary biology. In doing so, it has generated a large and diverse literature, which can be frustrating to those attempting to enter or survey the eld. At the moment, the choice is largely between wrestling with the original literature or tackling advanced, multi-author volumes. Although our book is aimed particularly at students of biology, we hope that our colleagues in dierent branches of biology and psychology will nd it a useful introduction.
We have also tried to make it accessible to the growing numbers of ornithologists and naturalists who increasingly want to know more about the animals they watch and study. The literature on bird song has seen rapid growth in the sheer number of publications, at least more in the 12 years since our rst edition. The subject has also had its share of controversies. We have tried to do justice to the eld without being too partisan and without burdening the reader with too many citations. We hope that our many friends and colleagues will excuse our omissions, especially where the examples we have chosen might have been from their favourite bird, but were not.
In our eorts to achieve a balanced view and widen our expertise, each chapter has been read by one or more experts in each particular area, and we are extremely grateful to them for their considerable help. We have not always followed their advice, and we cannot guarantee that they will be pleased with the result, but we do know that the book has gained immensely from their eorts.
In addition, many of our immediate colleagues and respective research groups have answered questions and helped with discussion and clarication of numerous issues. Finally, we thank Karen Johnstone, who carefully redrew the gures for the rst edition, Nigel Mann for his delightful vignettes, and Martin Griths of Cambridge University Press for seeing the book into print for us. We are both ethologists, and so it is no coincidence that the book is structured around the four questions that Tinbergen prescribed.
We start with causation, continue with development, and then move on to function and nally to evolution. Therefore, although the book has many themes and variations, it is very much about the biology of bird song. We hope that it illustrates what the scientic study of bird song has contributed to biology in the past, and what exciting developments it may hold for the future. A comparison of this edition with the last one will illustrate just how rapid progress has been in the past decade: in several places it has even been necessary to introduce new sections to take account of this.
Chapter 1 is an introduction to some basic theory, terminology and methodology. In Chapter 2 we attempt to summarise the dramatic and exciting recent advances made by neurobiologists, perhaps the biggest growth area in the whole eld. This chapter centres upon the complex neural circuits concerned with song but also deals with sound production, hearing and perception.
Chapter 3 deals with the development of song in the individual. Most birds learn their songs during a sensitive period early in life. The intricate interplay between the genetic and environmental factors involved has made the study of bird song a classic example of x. Chapter 4 investigates the problems of sound transmission through the environment and illustrates how dierent habitats may have shaped the evolution of songs.
Song structure can also give important information regarding the location and distance of the singing bird. Chapter 5 considers the context in which song occurs: who does the singing and when? Is there really a dawn chorus, and why is this the best time to sing? Chapter 6 concentrates upon males and emphasises recognition of species, mates, ospring and territorial neighbours. Territorial defence appears to be an important function of bird song. Chapter 7 switches to the female and emphasises the role of song in sexual selection and female choice.
Female attraction appears to be another main function of male song.
In Chapter 8 we survey the extraordinary richness and variety of bird song and deal with complexities such as repertoires and duets. It searches for patterns and trends and oers some ideas concerning the evolution of such complexity. Finally, Chapter 9 ventures further along the evolutionary path and considers variation in both time and space. How do songs vary from place to place, do dialects exist, how do songs change as they are transmitted across generations? To some of these questions this book may provide the answers, but to answer others we will have to wait for another generation of biologists.
We hope that they will nd investigating the biology of bird songs to be as fascinating, challenging and rewarding as we have done. Although everyone may assume that they do know what a bird song is, how does it dier from the other sounds that birds make? There are calls, notes, syllables and phrases to consider and what are repertoires? Before we start using these words, it is just as well to dene them and become acquainted with a terminology which can be confusing.
Only then can we move on to consider the role of song in the lives of birds and to review the many studies that have attempted to shed some light upon it. Animal communication is a rapidly expanding eld, and at this early stage it is also useful to consider some of the recent theoretical background.
For example, what is communication and how do we know it has occurred? What is information and who benets from sending and receiving it? What are signals and why is sound transmission particularly eective? Finally, recording, analysing and experimenting with sounds is a highly technical eld, currently being revolutionised by the use of computers at various points.
We will also attempt to give the reader just a brief introduction to some of the more important techniques and any recent developments. But rst, let us set the current study of bird song in its historical context. But its detailed scientic study is a comparatively recent phenomenon. One reason for this is that making a permanent record of it, as we now do routinely on tapes or discs, was not easy until half a century or so ago. Song can be so rapid and complicated that only with such a permanent record that could be slowed down, repeated and analysed in various ways, is it possible to make a serious study of many aspects of it.
Some interesting work was done before that time. The Hon. Daines Barrington wrote a letter to the President of the Royal Society in recounting a variety of observations he had made. He established the existence of song learning, for example, because he heard the song of 2. Presumably this bird had been exposed to wren song at some stage and had picked it up.
At around the same time, in , the great English parson and naturalist Gilbert White described how birds previously known as willow wrens could be separated by their songs into three separate species. These we now call the willow warbler, the wood warbler and the chi-cha. Those with a good ear were also able to detect that birds had repertoires of songs and study the way these were strung together into sequences, as Craig did with eastern wood pewee song, or that song could vary from place to place, as found by Marler for the chanches singing in dierent glens in the Scottish highlands.
The depth of such studies was severely limited, not just by lack of the possibility of recording, except latterly on wax drums, but most importantly by the lack of analytical equipment. The real revolution came with the invention of the sound spectrograph, rst used to provide a visual representation of song by Thorpe in Such equipment was not cheap, and therefore its use was somewhat restricted, but it still led to a huge growth in studies of song.
Today, equivalent visualisations of song, together with many other forms of analysis, can be carried out using a variety of computer packages at a fraction of the cost. The detailed study of bird song is within the scope and budget of many laboratories and even amateurs: as a result the subject is advancing with great strides.
Thanks to these very powerful techniques, there are now few areas of animal behaviour research that have not been illuminated by studies of bird song. Because the song is a special structure used solely in communication, we call it a signal. But how do we know whether communication has occurred? It is generally held that if the signal modies the behaviour of the receiving animal then we can infer that communication has taken place Slater c.
For example, if we play 3. As the song appears to have modied his behaviour, we are entitled to conclude that communication has occurred. This is a somewhat restricted denition of communication, as it relies upon a behavioural response and thus excludes passive signal detection by the receiver.
For example, if we repeated the experiment on another great tit and obtained no response, it may be that the great tit had heard the song but decided for some reason not to respond. Such behaviour may often occur, so we must temper our denition with caution and also be sure to carry out a number of experiments before we draw any rm conclusions. Although it may be relatively easy to demonstrate that communication takes place, it is much more dicult to suggest why it has evolved. Early theories emphasised the benets that might accrue to both the sender and the receiver e.
Smith and saw communication as a sharing of information between individuals to their mutual advantage. Modern ethologists are much more inclined to view communication as the outcome of conict rather than cooperation between sender and receiver. Sometimes, cooperation rather than conict is involved, and they suggest that a system which benets both receiver and sender would give rise to the evolution of relatively quiet, inconspicuous signals.
For example, a great tit may give an alarm call to warn its edglings that a sparrowhawk is approaching. The call should be loud enough to reach the edglings but not loud enough to reach the hawk and give away the position of the caller. To be able to hear the call, the edglings should develop sensitive hearing, and so a coevolutionary process will lead to the production of calls that are cost-minimizing conspiratorial whispers.
The fascinating story of the evolution of alarm calls, and their possible detection by predators, will be discussed in detail later in the book. But where the interests of sender and receiver conict, as in a territorial dispute, there will be a dierent kind of coevolution. The male great tit this time sings as long and loud as he can manage, simply to force his message across.
As we will see in later chapters, loudness and repetition are a particular feature of the songs of males when defending 4. Although sales resistance may occur among rival males, there are even more compelling reasons why females should be wary of male signals. If a listening male makes a mistake, he may just waste energy in a display or a ght, but if a listening female chooses a male of the wrong species, or one of inferior quality, she may pay a severe penalty in reduced breeding success.
We will also see in later chapters that there is now considerable evidence that females have been selected for ne discrimination of both quantity and quality of male songs. So far, we have assumed that the signals transmitted give reliable information from sender to receiver. At this stage, we should mention that the word information is used in two dierent ways. Technically, information theory considers it to be a reduction in uncertainty about the senders future behaviour on the part of the receiver.
In other words, information is said to have passed from sender to receiver when the senders behaviour becomes more predictable to the receiver Halliday When information is transmitted between birds, it is generally about something quite precise, such as species, sex, identity, likely next action, and so on. There are some grounds for expecting that receivers will be selected to detect unreliable or false information.
Zahavi , has suggested that only signals which are honest indicators of size, strength or motivation should evolve. One reason for this is that many signals are costly to produce, and so it is dicult, for example, for a smaller, weaker animal to cheat or blu the receiver into accepting it as a larger, stronger rival or mate.
The view that, because of costs incurred by the sender, evolution has generally favoured honest advertising in communication has now become widely accepted. If the costs of long, detailed assessment are high in relation to the value of the extra information gained, then receivers might settle for cheaper, less reliable signals. If so, the receiver may be open to being blued, cheated and manipulated to the senders advantage. In one, the outcome is the evolution of honest signalling, but in the other we may nd unreliable signals have evolved as one animal attempts to manipulate the behaviour of the other.
The latter point out that reliability requires there to be a correlation between some aspect of the signal and some attribute of the signaler that the receiver benets from knowing about. Hence the receiver benets from assessing the signal rather than ignoring it. Deceit requires not only that the correlation between signal and attribute be broken, but that the signaler benets from that breakdown. The apparent coevolutionary arms race between senders and receivers involves many dierent aspects of communication, which we will be considering throughout this book.
It is important to emphasise that we will not just consider the signal song itself, but how it is transmitted through the environment, how it is perceived by receivers and, in particular, how males and females react to both natural and experimental signals. Sound is only one of several channels of communication that are open to birds, and the advantages and disadvantages of the dierent channels have been summarised in Table 1. In general, birds have rather a poorly developed olfactory system, and so this method is less important than the main channels of sound or vision.
This contrasts with mammals, where olfaction is a very important method of communication. Olfaction is rather less important to humans, as their tiny noses indicate, and, like birds, humans rely particularly upon sound and vision. There is no doubt that 6. What then are the particular advantages of sounds, especially when compared to visual signals?
Visual signals have several disadvantages, for example in darkness or poor light. But bad conditions for visual signalling can occur at any time in dense habitats such as forest or reeds and when animals move out of view behind objects. Try looking for a small bird as it moves through the canopy.
Now you see it now you dont! But if it calls or sings you can always hear it, long after it moves out of sight. Sound travels in all directions, it can penetrate through or round objects, and it travels over long distances. Sound is an ideal method for communicating over long distances, and although birds also call softly to each other, their songs are often loud and can carry for several kilometres. How natural selection may have acted to shape song structures for optimal transmission through dierent habitats is one of many topics we will discuss later in this book.
Other advantages stem mainly from the rapid and transient nature of sound communication. A song or call is only produced when needed, and large amounts of information can be transmitted rapidly and eciently through the sound channel. There might perhaps be one disadvantage if, as has sometimes been suggested, singing is very costly in terms of energetics. A song has to be made each time it is produced, and some birds sing thousands of songs per day. However, recent evidence suggests that this is not the case and that song is comparatively cheap to produce compared, for example, with the cost to a bird of hopping or ying around its cage e.
It does therefore seem that the many advantages of sound communication rather easily outweigh its costs. Birds, like humans, are intensely vocal creatures, and communication by sound has come to play a central role in their lives. The distinction is both traditional and arbitrary, but as these terms are still retained in the literature we must attempt some clarication. There is also a taxonomic reason for the distinction. One particular group, the oscines, were originally separated from the rest of the order Passeriformes, primarily on the number and complexity of their syringeal muscles.
As these birds 7. But, as we will see later, the dierences between oscines and sub-oscines may have more to do with how they learn their songs and the underlying brain structure rather than with the actual complexity of their vocalisations. As this book is largely devoted to the study of songs, it is only fair that we should attempt a denition. In general, songs tend to be long, complex, vocalisations produced by males in the breeding season. Song also appears to occur spontaneously and is often produced in long spells with a characteristic diurnal rhythm. But to these features there are innumerable exceptions.
Especially in the tropics, it is common for females to sing as well as males, and both sexes may do so throughout the year even though breeding only occurs during a restricted period e. Langmore Even in temperate regions, song may occur well before egg-laying and there is also often a bit of a resurgence in the autumn.
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In the European robin, for example, song may be heard in every month of the year and in the winter it is produced by both males and females singing on separate territories Lack However as far as complexity is concerned, it is not easy to generalise and, as we shall see in Chapter 8, species dier enormously in how varied their songs are. There are even songbirds that appear not to sing at all, but the simple cheeping of a male house sparrow on a rooftop may full the same function so that it is, in eect, a very simple song.
What then are calls? Calls tend to be shorter, simpler and produced by both sexes throughout the year. Unlike songs, calls are less spontaneous and usually occur in particular contexts which can be related to specic functions such as ight, threat, alarm and so on. As with the house sparrow example, there are obviously areas of overlap between simple song and complex calls, and plenty of exceptions to the criteria we have presented.
But in general, ornithologists and ethologists recognise these distinctions and continue to nd them useful. Why the oscines have evolved such complex songs, and a special brain pathway to learn them, is one of the central themes of this book. Having stated that one of the main characteristics of most songs is their complexity, we have a number of other categories and units to dene.
Most birds have more than one version of their species song, and some have many. For example, most male chanches have more than one version of their song and will repeat one several times in a bout of singing, and then 8. As explained in Section 1. Each version is called a song type, and the male chafnch is said to have a repertoire of song types see Fig. Moving our analysis to a more detailed level, we can also see that each chanch song consists of a number of distinct sections.
These are called phrases, and each phrase consists of a series of units which occur together in a particular pattern. Sometimes, the units in a phrase are all dierent, as in the end phrase shown in Fig. The units themselves are usually referred to as syllables. Syllables can be very simple or quite complex in their structure.
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When complex, they are constructed from several of the smallest building blocks of all, called elements or notes but the latter is usually avoided because of its musical connotations. One denition of an element is simply a continuous line on a sonagram, as illustrated in Fig. Songs, syllables and elements can also be dened by the time intervals which separate them, intersong intervals are the longest, and so on downwards.
Because of the great variety of form and structure in songs, individual workers often use and dene their own terms, which may be slightly dierent from those given above. These can only serve as a general guide for this book, for there can be linguistic as well as technical problems with 9. For example, in German there are two dierent words for song: gesang means the song of a particular species, whereas strophe means a particular delivery of a song.
This last word is now often adopted in English to refer to a single rendition. However, if the main objective is to determine what possible functions a sound has, then this is where to start. Currently, it has become fashionable in many branches of modern biology to construct a hypothesis, perhaps even a model, and then test selected predictions by experiment. Naturally, this book is full of such examples, as experiments have played a leading role in the scientic study of bird sounds.
But to formulate an appropriate hypothesis or model, a period of observation should rst be undertaken. This should preferably be a thorough eld study which relates the singing bird to its habitat, to its other behaviour and to its general life history. The experimenter may have rather less enthusiasm for this phase, regarding the necessary eld work as dicult, dull and somewhat oldfashioned. However, it is vitally important for several reasons.
For example, it will provide an accurate source of basic information from which a proper hypothesis can be constructed. It should reveal such essential information as when, where and to whom the bird sings. The rst clues as to the probable functions of song invariably come from simple, contextual observations in the eld. Does a male sing only in his territory? Does he countersing against rival males? Does he stop singing when he has paired with a female? These are very basic questions, and their answers will help to give initial clues to function and will allow appropriate hypotheses to be formulated.
Does the male sing only at dawn? Does he stop when his mate appears? Does he sing more in her fertile period? More precise questions such as these can also be answered by careful observations and may lead to the eventual design of suitable playback experiments to test more detailed functional hypotheses. Nor need the modern eld worker feel too old-fashioned. The traditional note-book can be replaced by an electronic one, and a number of software packages will allow a full, Apart from rather straightforward observation and later quantitative analysis, there are two more important techniques that should also be mentioned.
These are correlation studies and the comparative approach. Although these are not simply observations, they rely upon the observational rather than the experimental approach. For example, we may wish to test the prediction that males in a population that sing at a faster rate attract females earlier than their rivals who sing more slowly.
Bird Song: Biological Themes and Variations
To do this, we need to have good observational data on song rate and pairing date, and we can then test the relationship by appropriate correlation analysis. The comparative approach can also be used to test hypotheses, this time about how dierences between species may have arisen during evolution. For example, are observed dierences in song complexity related to the dierent mating systems seen in birds? Again, we need good, reliable observational data as well as a correlation analysis.
But comparative studies may involve examining many variables and many species from dierent taxonomic groups. There are problems not only in selecting variables and controlling for confounding ones, but also in selecting the appropriate level of taxonomic comparison species, genus, subfamily or family? Although the most obvious method might be to use species as independent data points, this would bias the results towards genera containing large numbers of closely related species. What we should really do is reconstruct a phylogenetic tree and only make comparisons between data points that appear to be independent in terms of evolutionary events.
A more up to date treatment, in what is a very fast moving eld, is provided by Felsenstein We shall describe some applications of the comparative method to song when discussing its evolution in Chapter 9. It should be clear by now that the observational stage is of great importance for many reasons. It provides a great deal of valuable insight, as well as initial information which can be used for testing hypotheses by context, correlation and the comparative approach. It also prevents the creation and testing of hypotheses which may well be ingenious but may also be inappropriate or even irrelevant to the biology of the bird in its natural environment.
There are several reasons for doing this, and the rst leads back to observation. We may need very accurate answers to the questions we are asking, such as how much does a particular individual sing. Although some songs can be counted or timed, it is best to do this from a permanent record which can be analysed and reanalysed at leisure in the laboratory.
As an alternative to the portable computer, a two-track tape recorder can provide parallel records of recorded song and associated behaviour patterns. There are also other reasons for recording songs. We may wish to investigate song structure and compare dierent males to see whether they share song types or have more song types than their neighbours or males in other populations. Finally, we may wish to conduct playback experiments, and for this purpose we will also need an adequate sample of songs from the birds in our population.
The recording equipment that we use should satisfy the following criteria. It should be of high enough quality to permit later analysis of recording in an acoustics laboratory and yet be light, portable and robust enough for use under eld conditions.
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Fortunately, as we will see in Chapter 2, birds and humans tend to share much the same frequency range and so most commercial equipment is adequate for recording birds. There is now a considerable variety of formats and recorders available, some remarkably small but of very high quality. In the past few decades, there has been a gradual move from open-reel to cassette and, more recently, from analogue to digital recorders. Hard disc recorders are now becoming increasingly popular, especially given the ease with which the data can be downloaded from them onto a computer.
Suitable equipment for recording birds is regularly reviewed in the journal Bioacoustics, as are the special microphone systems required. Here, the choice is between the long gun type, or a standard microphone mounted in a parabolic reector. Both of these are designed to meet the problems of recording a bird singing some distance away. The gun microphone is highly directional and tends to cut out noise from either side of the singing bird. Alternative, shorter, pistol microphones also now give pretty good directionality without being quite so cumbersome. The parabola is also directional, but works by collecting a wider sound spread round the singing bird and then reecting the sound waves onto The sound is thus amplied considerably.
The only problem with a parabola is its size. Low frequencies have long wave-lengths which are not reected by small objects, and so low, deep sounds, such as the hoots of owls, need a very large dish. Ingeniously, some recent makes can be rolled up for ease of travel. Indeed, this is now a very early step in any serious research into bird song.
Yet, as mentioned earlier, up until the s bird song research literally stopped at this point, and there was really no way to examine, measure or compare permanent records of sounds. The sonagraph or sound spectrograph, developed at that time by Bell Telephone, was basically a frequency spectrum analyser which broke sounds down into their constituent frequencies.
The main output from a sonagraph is a plot of sound frequency in kilohertz kHz against time in seconds. This has become the standard, conventional way to illustrate a song, and it is called a sonagram. As sonagrams will be used throughout the book, at this stage we will present a quick guide to their interpretation, using the sonagrams of dierent sedge warbler syllables shown in Fig. With a little practice, it becomes quite simple to read sonagrams and gain some impression of the original sound from the structure displayed. The main point to remember is that high-pitched sounds with a higher frequency appear higher on the y axis.
Perhaps the most common sound people associate with birds is a whistle. A short whistle of constant pitch will appear as a pure, unmodulated frequency trace on the sonagram a. A whistle which starts at a higher frequency and drops to a lower one is said to be frequency modulated and appears on the sonagram as a slope from left to right b.
If more rapid modulations appear, as in a slow c or fast d vibrato, they are also easily recognised. But not all bird sounds are pure tones like these. A completely dierent sound is the harsh noise produced when a wide frequency spectrum is used. A short burst of such white noise sounds like a click e , and if several occur close together a buzzing sound is produced f. Frequency modulations sometimes occur in more complex forms, and g , for example, sounds like a chirp. Another complication is when a sound has higher frequencies occurring as multiples of the rst or fundamental frequency.
Sonagrams of dierent syllable types produced by a male sedge warbler and described in the text from Catchpole These are called harmonics and in the example shown here h produce rather a gru, barking sound. With the sonagraph, it became possible to analyse, measure, classify and recognise the dierent sounds birds make, and this is also true with the numerous computer packages that have superseded it in the past decade or so.
It has become possible to discriminate between dierent species, populations, individuals, song types within individuals, and even dierent renditions of the same song type from an individual bird. Such visualisation has totally revolutionised the scientic study of bird sounds. Computer based sound analysis systems are more powerful and faster, and songs and their analyses can now be stored and led on disc as well as manipulated and even synthesised for experimental purposes.
With such methods it is sometimes possible to relegate the still subjective and labourintensive chore of analysing, recognising and classifying songs to the computer, and quantitative comparisons may even be made between them Tchernichovski et al. Deecke et al. However, computer methods themselves have a variety of dierent assumptions and are likely to vary in their applicability depending on the particular task in hand.
The human eye and brain are a sophisticated pattern recognition system, and for some tasks using it to scan sonagrams may still be the best means of splitting them into categories Janik albeit necessitating care in assessing inter-observer reliability Jones et al. This applies particularly to the study of bird song, where, as we shall see throughout this book, experiments have been used extensively in studies of causation, development, function and even evolution.
Although many dierent types of experiment have been used, there is one technique above all others which has been highly developed and rened by those who study bird songs the playback experiment. Playback, as the name suggests, is the technique of playing sounds to animals and observing their response. The sounds are usually recordings of natural signals, such as songs, but synthetic sounds can also be used. Playback of songs may occur in the eld most often within the territory of a male bird, but playback can also be used in laboratory experiments, for example to captive females. In the past the sounds usually originated from audiotape, but are now more usually stored in the memory of a computer.
Playback techniques with birds originated in the s, and the most important pioneer was J. Bruce Falls: his classic early experiments feature in later chapters of this book. Playback experiments have many advantages, not the least being that they are eective in both laboratory and eld conditions. Playback also isolates the sound stimulus from other confounding variables, such as the presence or behaviour of the bird itself, and gives the experimenter a great deal of control over the experimental variables present in the sound stimulus.
By holding all other variables constant, the feature under investigation can be varied and the responses measured in a variety of ways. With playback to territorial males, approach to the speaker is common and so measures such as latency of approach, nearest distance McGregor b reviewed the dierent measures that can be used in experiments on males. Females are elusive in the eld and so playback to captive females is the main technique used.
The number of sexual displays that females make is the main response measured, and Searcy b discussed this and other techniques used on females. There are now many dierent types of playback experiment. Although early ones used only one speaker, two-speaker designs can also be used to present subjects with a choice between stimuli or, for example, with the separate male and female components of a duet Rogers et al. More modern techniques attempt to treat birds as more than just passive receivers. Weary has reviewed experiments based upon operant conditioning.
He is the author of around scientific papers and several books, and has been studying acoustic communication, largely in birds, for thirty years. See details. See all 2 brand new listings. Buy It Now. Add to cart. Slater , Hardcover, Revised. Be the first to write a review About this product.
About this product Product Information Bird song is one of the most remarkable and impressive sounds in the natural world, and has inspired not only students of natural history, but also great writers, poets and composers. Extensively updated from the first edition, the main thrust of this book is to suggest that the two main functions of song are attracting a mate and defending territory. It shows how this evolutionary pressure has led to the amazing variety and complexity we see in the songs of different species throughout the world.
Writing primarily for students and researchers in animal behavior, the authors review over scientific papers and reveal how scientists are beginning to unravel and understand how and why birds communicate with the elaborate vocalizations we call song. Highly illustrated throughout and written in straightforward language, Bird Song also holds appeal for amateur ornithologists with some knowledge of biology.
Additional Product Features Dewey Edition. The study of bird song; 2. Production and perception; 3.
Related Bird Song: Biological Themes and Variations
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