Bird song for beginners

David M. Logue

This chapter is an introduction to the scientific study of bird song. You may wonder, ‘why am I reading about bird songs for a psychology course?’ The answer is that, like many kinds of animal behaviour, bird song is studied by both biologists and psychologists. In the middle part of the last century, a group of biologists known as the ethologists studied the behaviour of wild animals. They used evolutionary theory to explain natural behaviour in a wide variety of animals. Meanwhile, comparative psychologists were studying sensation, learning, and other mechanisms of behaviour in laboratory settings. They focused on a few model species, like cats, pigeons, and rats, and their goals were to use animals to better understand human behaviour.

In the 1970’s, the two fields began moving toward one another. Comparative psychologists integrated more natural behaviour into their research and adopted the evolutionary framework, while evolutionary biologists became increasingly interested in the mechanisms of behaviour, including learning. Today, an animal behaviour scientist can work and publish in both disciplines. For example, I am trained as a biologist, but I have published in both biology and psychology journals and I am now teaching in a department of psychology.

Bird song research has a lot to offer psychology. It has provided crucial insights into such ‘psychological’ themes as learning, communication, sex differences, adaptation and constraint, and the neural and hormonal control of behaviour. This chapter is a comprehensive introduction to bird song. It doesn’t go very deep into any particular area of study, but attempts to explain the basics of how and why birds sing.

What is a bird song? All birds make sounds, but not all bird sounds are songs. Mechanical sounds are produced by body parts other than the vocal tract. For example, some manikins make clicking sounds by slapping their wings together (Prum, 1998), and hummingbirds have special tail feathers that make chirping sounds when they fly (Clark & Feo, 2008). Mechanical sounds are interesting, but they are not songs.

Short, simple vocal sounds (sounds produced by the vocal tract) are known as calls (Benedict & Krakauer, 2013). Most calls are innate, which means that they develop normally without learning how to produce them. Birds use calls in many ways. Young birds use begging calls to solicit food from their parents. Aggressive calls precede attacks. Alarm calls warn other birds of danger. Some species produce different kinds of alarm calls that correspond to different kinds of danger. For example, smooth-billed anis (Crotophaga ani) produce one kind of alarm call when they see a flying predator and a different kind when they see a terrestrial predator. If you play a recording of the flying predator call to anis in the field, they dive into the bushes. If you play the terrestrial alarm, they fly up into the trees (Grieves, Logue, & Quinn, 2014). But this chapter is not about bird calls, it’s about bird songs. Songs are longer, louder, and more complex (usually) than calls. They are only produced by ‘perching birds’ (the order Passeriformes), which include most little ‘tweety’ birds, but does not include things like ducks, penguins, ostriches and hawks. In many tropical and southern hemisphere species both sexes sing, but in most north-temperate species only males sing. This is not a perfect definition, but in most cases it’s pretty easy to tell a song from a call, so we’ll move on.

How do people study songs? To study something scientifically, we must be able to accurately measure it and compare it to other things. It’s hard for people to compare and measure sounds. Many bird songs are fast and complex, compounding the problem. Bird song scientists turn sounds into images to make it easier to study them. We use many kinds of images, but the most common is the sound spectrogram (Fig. 1). Spectrograms are graphs with time on the X-axis, frequency (pitch) on the Y-axis, and amplitude (loudness) represented by darkness. We use spectrograms to measure songs (e.g., How long are they? How many notes do they contain?), classify songs (e.g., What species made them? What song-type are they?), and modify songs for playback experiments (e.g., by speeding them up or by adding more notes).

Figure 1. A spectrogram of an Adelaide’s warbler’s (Setophaga adelaidae) song. Many distinctive characteristics of this song are evident in its spectrogram. For example, the rapidly repeated notes indicate that this song is a ‘trill’. Variation in the vertical position of the notes shows the changes in the frequency of the song (frequency modulation), which sounds like a shifting pitch. The introductory section is slower than the main body of the song, and the terminal note contains the highest frequency (pitch) in the song. Working directly with the software that produced this spectrogram, it would be possible to make detailed measurements of the song’s frequency and temporal characteristics.

Bird songs evolved because they affect the behaviour of the birds that hear them. We use playback experiments to learn how song variants (e.g., fast vs. slow songs; local vs. foreign songs) influence listening birds. In these experiments, one or more kinds of sounds are played to living animals whose responses are recorded. The ani alarm call study described above involved playback experiments. I’ll discuss a few more examples later in the chapter. Bird song biologists ask many different kinds of questions about bird song, but all of them address at least one of the following four causes of behaviour:

1) Development: How does it develop? Here we might ask questions about song learning, or whether a bird can get better at singing by practicing. Questions about the genetic basis of song are usually lumped in with development. 2) Mechanism: How does it work? For example, how do the nerves and muscles work to produce song? 3) Function: Why did it evolve / what it is good for? The question, “why do birds match the song- type that their neighbour sang” is a functional question. Most questions with “why” in them are functional. 4) Evolution: How did evolution shape the behaviour? Examples of evolutionary questions include, “how many times has female song evolved in the song birds?” or “how have song structures changed as birds have evolved to use different habitats?”

Nobel Prize winning ethologist Niko Tinbergen (Tinbergen, 2005) came up with these four causes, and they’ve proven to be a remarkably durable framework for thinking about all kinds of behaviour, not just bird song. The latter part of this chapter is organized around the four causes. First, though, I will explain what bird song is like.

Description: What is bird song like? In a nutshell, it’s quite diverse (Fig. 2). Each species sings a unique kind of song. Species that live in dense forests (especially tropical forests) tend to sing slow songs, because fast songs would get jumbled as they reverberate through the forest. Species that live in open habitats can produce remarkably fast and complex songs. Most songs are made up of syllables. The characteristic order of syllables in a song is the song’s syntax. In many species, syllables can be rearranged and recombined to make different song-types. Many birds sing more than one song-type. For example, a song sparrow (Melospiza melodia) sings 7-11 song-types (Beecher, Stoddard, Campbell, & Horning, 1996) and a brown thrasher (Toxostoma rufum) sings over 1,000 types (Boughey & Thompson, 1981). Most black-capped chickadees (Poecile atricapillus) sing only one type, but they can shift the pitch of the song up or down to generate variety (Christie, Mennill, & Ratcliffe, 2004). The word repertoire describes all of the songs that an individual bird sings (Fig. 3). Individual birds often share some or all of their song-types with other individuals.

A given song-type will vary among and within individuals. So this bird’s version of Song X may be a little different than that bird’s version, even though they’re both recognizable as Song X. There is also some variation within each individual. So the way a bird sing Song X in the morning, may be a little different than the way he sings it in the afternoon, and the way he sing it when he’s relaxed may be different than the way he sings it when he’s ready to fight.

Figure 2. Spectrograms showing structural diversity in songs from a sample of Panamanian songbirds. Top row (left to right): great-tailed grackle (Quiscalus mexicanus), variable seedeater (Sporophila corvina); middle row: black-headed saltator (Saltator atriceps), rufous-and-white wren (Thryophilus rufalbus), clay colored thrush (Turdus grayi); bottom row: long-billed gnatcatcher (Ramphocaenus melanurus), buff-breasted wren (Cantorchilus leucotis), black-bellied wren (Pheugopedeus fasciatoventris). Spectrograms are based on recordings by David E. Gammon.

Birds don’t sing their songs in random order (Kershenbaum et al., 2014), but we do not yet understand why or how they choose to sing them in the order that they do. One rather obvious form of non-random singing occurs when a birds sings the same song-type over and over before switching types. This is called repeat mode singing. In contrast, when a bird switches song-type after every song, he is said to sing in switch mode. A given species may specialize in repeat mode or switch mode, or it may switch between them.

Birds do most of their singing in the breeding season. Many species participate in a dawn chorus, which means that they sing a lot very early in the morning. During the dawn chorus and later in the day, neighbouring birds often sing back and forth with one another, a behaviour called countersinging (Hyman, 2003). Many neighbours may countersing with one another at the same time. Together with all of the birds that are listening to these interactions, they form a communication network (McGregor & Peake, 2000). While countersinging, a bird may overlap his neighbour’s song. It isn’t clear whether this behaviour has any function – it may just occur by chance (Naguib & Mennill, 2010; Searcy & Beecher, 2009). Song-type matching occurs when a birds sings the same song-type that his neighbour just sang. This behaviour seems to be important, but its function is hotly debated (Akcay, Tom, Campbell, & Beecher, 2013; Logue & Forstmeier, 2008).

Figure 3. Spectrograms showing song-types and song-type sharing in Adelaide’s warbler. The spectrograms on the top row represent three distinct song-types from male LgRLg’s repertoire. His entire repertoire comprises about 25 song-types. The spectrograms on the bottom row represent the same song-types, as sung by male RbRbO. The high similarity between the two males’ renditions indicates high-fidelity cultural transmission of song structure.

In most North American songbirds, only males sing. The story is much the same in Europe and northern Asia, but female song is common in tropical and southern latitudes. Because most songbirds live outside of the northern hemisphere, female song may be ‘the rule’ rather than the exception (Odom, Hall, Riebel, Omland, & Langmore, 2014; Riebel, Hall, & Langmore, 2005). The historical scientific focus on male song is a consequence of the geographical distribution of bird song scientists (most of us are from the northern hemisphere) and perhaps our gender as well (historically, most of us have been men, but there are now many prominent women in the field). When female birds sing, they may sing similar songs to the males of their species (Mennill & Vehrencamp, 2005) or they may sing unique, female-only song- types (Logue et al., 2007). Song-type repertoires, vocal learning, countersinging, and song-type matching have all been observed in female birds. In most species with female song, females sing less than males, but in at least one species, females sing more (Illes, 2015). In many species, pair mates sing together, producing duets. In others, social groups of three or more individuals (often families) sing choruses. Both of these forms of interactive singing can be remarkably well-coordinated, to the degree that they sound like the song of a single bird.

Development: How a bird acquires its song About half the species of bird in the world are passerines. The passerines can be further divided into suboscines and oscines. Suboscines are mostly tropical, but they include some temperate species like kingbirds and other fly-catchers. Suboscine songs tend to be simple, with little regional variation. The bulk of the evidence suggests that song is innate in most or all suboscines. In other words, the song develops normally without the bird having to learn it by hearing another bird. Evidence comes from experiments in which baby birds developed normal sounding songs without exposure to any adult song (e.g., Touchton, Seddon, & Tobias, 2014). Oscines are also known as songbirds. Most of the smallish birds you are familiar with are oscines. Birds like sparrows, robins, wrens, warblers, and even jays and crows are in this group. Oscines sing the most complex songs. Unlike suboscines and most other birds, they learn their songs from adults of their own species (Marler, 1970). Experiments in which young songbirds were tutored with recorded songs have shown that oscines have a sensitive period early in life, during which time they can learn new songs. They practice messy versions of these songs for a while. Eventually the songs become more-or-less crystallized, after which point they do not change much. Some oscines, like northern mockingbirds (Mimus polyglottus), are open-ended learners, meaning they can learn new songs throughout their lives (Brenowitz & Beecher, 2005).

Scientists use song learning in oscines as a model of vocal learning in humans. Models like this are important in science because you can’t do learning experiments or brain dissections on human babies, but you can on songbirds. Songbirds are a good model of vocal learning in humans because their learning processes are similar to ours. Both songbirds and humans have a sensitive period early in life when vocal learning is fastest. Both practice messy versions of our vocalizations until they become more-or-less crystallized.

A couple of elegant studies link the ways in which songbirds and humans learn to produce and sequentially arrange sound elements. In the first experiment, young white-crowned sparrows (Zonotrichia leucophrys) were tutored with pairs of adjacent phrases (Rose et al., 2004). Suppose that a complete song comprises the syllables A-B-C-D, in that order. Some juveniles were tutored with the two- syllable combinations A-B, B-C, and C-D. Remarkably, those birds learned to sing the complete song: A-B- C-D. All that the birds needed to learn were the syllables and the ‘links’ between the syllables. A memory of a whole song is not necessary to produce a full song. Some of the birds in the experiment were tutored with the reversed combinations B-A, C-B, and D-C. What do you think they sang?

In the second (Lipkind et al., 2013), young zebra finches (Taeniopygia guttata) were tutored with songs comprising a repeated three-syllable sequence (A-B-C-A-B-C…). Once they learned those songs, the tutor switched the syllable order (A-C-B-A-C-B…). The young birds did not learn the new songs gradually. Rather, each of the new transitions (A-C, C-B, and B-A) appeared in their songs suddenly, indicating that they learned the new syllable order one transition at a time. An analysis of babbling in human babies showed the same pattern: When babies first learn a syllable they just repeat it over and over. Over time, they learn to combine it with other syllables. The authors of that study hypothesize that in both birds and humans, the motor gesture that produces a syllable is generated by a chain of neurons in the brain. Transitions between syllables are only possible when the tail of one of these chains connects to the head of the next. The generation of the connections corresponds to the individual learning new syllable-type transitions.

Bird song scientists have also made fascinating discoveries about the role of sleep in vocal learning. Individual neurons in a specific part of the brain fire in specific patterns when a bird is singing (Dave & Margoliash, 2000). These same neurons fire in exactly the same patterns when the bird is sleeping. Perhaps birds dream about singing? Perhaps their silent nocturnes are part of their brain’s nightly housekeeping. The notion that vocal learning processes occur during sleep finds support from a remarkably detailed study of vocal development (Deregnaucourt, Mitra, Feher, Pytte, & Tchernichovski, 2005). The researchers literally recorded every song that young zebra finches sang. By comparing their songs to the model from which they were learning, the scientists could see how much progress the young birds were making. Each morning when the birds woke up, they actually sang worse than the day before. But their songs quickly improved during their morning singing, to the point that they sang better than they had the prior day. After this initial improvement, they showed no more improvement the rest of the day. The results of several control studies indicate that the birds’ brains undergo reorganization during sleep, generating both the initial drop, and the subsequent increase in imitation performance.

Vocal dialects are a consequence of vocal learning in both songbirds and humans (Jenkins, 1978). In humans, dialects reveal themselves as regional differences in grammar, vocabulary, and pronunciation. In birds, regional differences include the structure of syllables and their order. When females have been tested, they prefer males that sing the local dialect (Anderson, 2009; Baker, Spitlernabors, & Bradley, 1981). In both birds and humans, dialects form because young individuals learn vocalizations imperfectly from older individuals. Learning has the effect of homogenizing vocalizations within the region. Imperfect copying (either through error or invention) generates variation between regions. Copy errors and improvisation are the source of new song-types. Some new song-types go on to become popular and successful, whereas others quickly die out. Vocal dialects in songbirds are often cited as a form of culture in non-human animals.

We know that genes interact dynamically with the environment; turning on and off in specific cells, in response to specific stimuli. Genes thus play crucial roles in both the development of the singing apparatus (e.g., song centres in the brain) and the mechanisms of song expression in adults, so we should think of genetic causes as including both developmental causes and mechanistic causes. Traditionally, scientists treat genetic causes of behaviour as a subset of developmental causes, which is why I’m writing about them here.

Scientists are beginning to identify some of the genes that play important roles in song learning and production. It turns out that many of these genes play similar roles in the acquisition and production of human speech (Pfenning et al., 2014). One way to study the genetics of bird song is to look at gene expression (when a gene is active) in different tissues. To identify genes involved in vocal learning you can compare the gene expression (1) in parts of the brain that are involved in song learning versus other parts, (2) in species or sexes that learn song versus those that do not, and (2) when a bird is learning song versus when he is not. Differences in expression patterns of a particular gene would suggest that the gene may be involved in song learning. Once a potential vocal learning gene is identified, researchers might disrupt its activity in a specific part of the brain to see how it affects song learning or production. Further evidence for a gene’s effects on vocal learning could come from cross-species comparisons, which could reveal how genomes differ in song learners versus non-learners.

The best-studied song learning gene is called FOXP2 (Fisher & Scharff, 2009; Scharff & Adam, 2013; Scharff & Petri, 2011; Wada et al., 2006). FOXP2 encodes a transcription factor, which is a kind of molecular signal that can turn other genes on and off. Transcription factors can have very powerful effects on development and behaviour because of their ability to influence many other genes. Birds express FOXP2 in a key vocal learning area of the brain (the ‘basal ganglia’) during critical learning periods early in life. Birds whose expression of FOXP2 is reduced during development produce incomplete, inaccurate, and overly variable songs. Interestingly, humans with mutations in FOXP2 suffer equivalent symptoms: their words are incomplete, inaccurate and overly variable. There is a growing consensus that FOXP2 affects the plasticity of neural circuits. Expression of FOXP2 permit neurons to change, and these changes are the mechanism of learning. Scientists are now working to understand which genes ‘turn on’ FOXP2, and which genes are affected by it (remember, FOXP2 is a chemical messenger whose job is to turn other genes on and off).

Of course, given the recent surge in genomics and transcriptomics (the large-scale study of gene expression) the genetic basis of bird song is an extremely hot topic. As I was writing this chapter, a major paper came out in the journal Science showing that many of the genes underlying song learning in birds are also involved in vocal learning in humans (Pfenning et al., 2014). Another active research area in bird song development addresses the development of birds’ responses to song. For example, do females learn which male songs to prefer, and if so, how? A related research question that interests me a great deal is how birds learn the rules to interact vocally with other birds. For example, how do they learn how to duet?

Mechanism: How a bird produces and hears its song When scientists ask about mechanisms, they’re asking how something works. In this section, I focus on the four most important ‘parts’ of the bird for making a song: The syrinx (sound production organ), the ears (sound reception organs), the brain (control centre for song learning and production over short time scales), and the endocrine system (a control mechanism of song learning and production over longer time scales).

The syrinx Humans produce sound in our larynx (voice box). The sound produced by the larynx is modified by our vocal tract, which includes our mouth and nose. Similarly, birds produce sound in their syrinx, and the sound is modified in their vocal tract, which includes the trachea and the bill. A key difference between sound production in birds and humans concerns the location of the sound producing organ: the larynx is in the throat, but the syrinx is way down in the bird’s chest, just above the lungs. Another major difference is that the larynx has a single sound source, but the syrinx has two. This property allows birds to produce part of a song on the left part of the syrinx and another part on the right side. Some birds produce different sounds on the two sides of the syrinx at the same time or allow the two sides to interact, creating complex sounds (Nowicki & Capranica, 1986; Suthers, 1990).

The ears Hearing in birds is quite similar to hearing in humans, with one important exception. Most birds’ auditory sensitivity is similar to humans, and we can detect sounds over a similar frequency range. So if you can hear a bird song, a bird standing where you are can probably hear it too. Songbirds distinguish frequency differences almost as well as humans. Like humans, birds can use the subtle differences between the sounds that reach their two ears to locate the spatial origin of a sound. The only really big difference between bird and human hearing is temporal resolution: birds can resolve very rapid changes in sound that humans cannot. So, for example, a house wren song sounds like a crazy jumble to you and me. To another house wren, however, the song would reveal a degree of acoustic details that we could only hear if we slowed it down (this is one of the reasons bird song scientists make spectrograms rather than relying on their ears).

The brain Songbirds, especially zebra finches and canaries (Serinus canaria), are models of vocal learning in the brain (this section draws heavily from the review by Jarvis, 2004). We can do things with these animals that we cannot do with humans (see below), so we know much more about the neuroscience of vocal learning in birds than in humans. Today, there are about 100 labs studying vocal learning in bird brains.

Basic brain anatomy is conserved across the amniotes, so human brains and songbird brains have the same basic parts: Hindbrain, midbrain, cerebellum, thalamus, and cerebrum. In humans, the midbrain is responsible for innate and non-conscious processing, and the cerebrum is the seat of complex learning, conscious processing and voluntary behaviour. Sensory information runs from receptor neurons (e.g., hair cells in the inner ear), to the spinal cord or hindbrain, then to the midbrain, thalamus and cerebrum. The control of movement goes in the opposite direction

One of the first interesting questions you can ask about song and the brain is ‘where in the brain does song learning occur’? The answer could be ‘all over,’ or it could be ‘in this specific place’. As it turns out, the latter answer is closer to the truth. Pioneering neuroethologist Fernando Nottebohm discovered the avian song centre by creating brain lesions (i.e., destroying small parts of the brain) in canaries. When he destroyed a region near the back of the cerebrum, adult canaries were no longer able to sing learned songs, but they could still make calls. This region is now called the HVC (neuroscientists love acronyms, so try and get comfortable labeling brain-parts with nonsensical strings of letters). By tracing the neuronal connections between brain regions, scientists discovered that the HVC connects to another cerebral region called the RA. Lesions in the RA also eliminate learned song.

We now know that three interconnected pathways are largely responsible for the perception, learning, memory and production of bird song. Nottebohm’s song control centres are part of the posterior vocal pathway (the PVP, which includes the HVC, RA, NIf, and Av). Outputs from the PVP travel to the midbrain, and then to the hindbrain areas that control breathing and the syrinx. If you record electrical activity in the PVP during singing, you first see activity in the NIf, then in the HVC, and then in RA. All of these events occur just millisecond before song production, clearly linking the brain activity to sound production. Unsurprisingly, there is a great deal of gene transcription in the PVP during singing. Putting all of the evidence together, it appears that the PVP generates learned vocalizations by controlling the motor neurons that produce sound and modulate the breath.

The aptly named anterior vocal pathway (the AVP, which includes Area X, MAN, and Mo) is further forward in the brain than the PVP. The AVP connects to the PVP, but it does not have strong connections to the hindbrain, suggesting that the AVP does not directly control muscle movements during singing. Lesions to the AVP produce a fascinating effect: subjects can still sing the vocalizations they already know, but they cannot learn new vocalizations. They are stuck at whatever developmental stage they were at before the lesion. So the AVP is not required for song production, but it is required for song learning and modification. Specifically, it appears that the MAN area adds variability to developing songs whereas Area X reduces variation.

The female’s PVC and the AVC are much smaller in species without female song, but similar in size to the male’s in species with female song. In species that sing seasonally, the vocal centres shrink during the non-breeding season and grow in the breeding season. This discovery in songbirds led directly to the discovery that adult human brains are capable of growing new neurons.

Finally, the auditory pathway leads from the bird’s inner ear to the hindbrain, and then to the cerebrum. The fact that sensory information travels from the ears up to the cerebrum (the centre of ‘higher’ learning and cognition) is evidence of the mechanistic basis of sensory learning.

Hormones Hormones are chemical messengers that are produced in some part of the body, and then travel to other parts of the body where they exert their effects. When hormones affect irreversible aspects of development, they are said to exert organizational effects. When they affect short term, reversible characteristics (especially behaviour), they are said to exert activational effects. What follows is a brief overview of the organizational and activational effects of the hormone testosterone (T) on bird song.

In songbirds, sex is determined chromosomally: males have two W sex chromosomes, whereas females have a W and a Z. This genetic difference causes the gonadal tissue in male embryos to differentiate into testes. Testes produce T, which affects the developing embryo’s brain. Specifically, the T is converted to estrogen in the brain, where it promotes the development of the song centres.

Songbirds exhibit annual cycles of behaviour with a strong peak in singing activity in each spring (I’m focusing on north temperate species, because we know much more about these species than we do about tropical or southern species). Changes in day length precipitate changes in the brain, which in turn precipitate testicular growth (i.e., songbird testes literally shrink in the fall and grow in the spring). The seasonally enlarged testes produce high levels of testosterone, which stimulates the enlargement of song centres and the resumption of singing. In short, T activates singing behaviour in north temperate birds. Experimental castration strongly reduces song output, and testosterone therapy reverses the effect. Even females of species without female song can be induced to sing with testosterone implants (Nottebohm, 1980).

Function: Why birds sing (the way that they do) Scientists look at function in two ways. Ideally, we would like to know what kind of selection pressure in the past favored the evolution of some trait. According to the theory of natural selection (Darwin, 1859), IF in the past, (1) some males had larger repertoires than others, (2) repertoire size was heritable (fathers with large repertoires tended to have sons with large repertoires and vice versa), and (3) males with large repertoires consistently reproduced more than males with small repertoires because females were more attracted to large repertoires than small repertoires, THAN, all else being equal, (4) large repertoires would evolve in the population (5) BECAUSE they are attractive to females. In this scenario, the adaptive function of large repertoires is mate attraction. Because this approach to studying function requires us to study organisms over evolutionary time scales, it can only be accomplished by longitudinal studies (but the time scales involved are often too long to study this way) or by the comparative approach, in which one studies many related species and makes statistically guided inferences about their evolutionary history.

Most of the time when scientists study the function of singing behaviours, they are studying the current function. That is to say, they use observations and experiments to ask ‘what good does this type of behaviour do for a bird today?’ By ‘good’ we usually mean something that is likely to give the animal a fitness advantage over other alternative behaviours that might exist (e.g., singing a smaller repertoire). Sometimes the current function is the same as the adaptive function, but that is not always the case. For example, we use our ears for holding up eye glasses, but that is clearly not the reason that they evolved. Similarly, a penguin uses its wings to swim, but we can be sure that wings did not evolve for swimming (the evolution of stiff, paddle-like wings, however, is an adaptation for swimming).

With that distinction in mind, the two primary current functions of song in songbirds are territory defense and mate attraction. Many studies support this conclusion, but I will focus on a couple of particularly compelling reports. Notably, both of these use the powerful ‘ablation’ paradigm, in which the consequences of removing the character of interest (song) reveal the normal function of the property. Krebs (1977) famously removed great tits from their territories and replaced them with speakers that played either great tit songs or a control sound. The control territories were consistently re-colonized faster than the experimental territories, demonstrating that song deters territorial invasion. McDonald (1989) surgically muted male seaside sparrows (Ammodramus maritimus) at two points in the breeding season. Males that were muted in the early part of the breeding season took longer to acquire territories and mates, than did control (sham-operated) males. Males that were muted later in the breeding season lost their mates. All of the effects were reversed when muted birds recovered their voices. That study provides compelling evidence that song is necessary for mate attraction and retention.

So if scientists are confident that territory defense and mate attraction are the two main functions of bird song, have they stopped doing research on song function? Of course not, but rather than focusing on the question, “why do birds sing,” we ask “why do they sing the way that they do?” For example, several scientists are trying to understand why many species of bird sing so fast. One line of research has found some support for the hypothesis that songs evolved to be fast because fast songs are more attractive to females (e.g., Ballentine, 2004). Another interesting property of song is the occurrence of song-type matching. There are two competing hypotheses that attempt to explain the function of song- type matching. First, it was proposed that matching signals aggressive intent (Searcy & Beecher, 2009). An alternative hypothesis states that matching makes it easier for females to accurately compare songs (Logue & Forstmeier, 2008). According to this hypothesis, superior singers match inferior singers to accentuate their own superiority. These are just two of many examples of ongoing research into the function of bird song. Evolution: What is the evolutionary history of song? We tend to think of evolution as occurring over very long time scales. While it is certainly true that evolutionary change can be remarkably slow, it is also true that populations are constantly evolving – every generation is different than the previous generation. Cultural traits, like learned bird song, can evolve particularly rapidly. A few studies look at the evolution of bird song over short time scales. For example, Derryberry (2007, 2009) compared recordings of white-crowned sparrows from 1970 to recordings taken in 2005. Both males and females responded more strongly to the recent recordings, and changes in song structure appear to have improved song transmission as the local vegetation changed.

Song doesn’t leave fossils, so to study song over long evolutionary time scales we must use the comparative method. In other words, we must study multiple species, assess their evolutionary relationships, and employ quantitative analyses that allow us to discern patterns of evolutionary change. Podos (2001) applied the comparative method to study the evolution of song structure in Darwin’s finches. These birds have evolved different bill shapes in response to the varied feeding opportunities on the Galapagos Islands. Podos found that bill size constrains the kinds of songs that the finches could sing. Specifically, species with large bills (e.g., those that specialized on large seeds) could not change pitch as rapidly as those with small bills.

Another example of a comparative study asked whether the relative size of the HVC co-evolved with repertoire size. In other words, when HVCs evolve to become larger, does the song repertoire tend to increase (and vice versa when they evolve smaller HVCs)? The answer, based on data from 41 species, is ‘yes’ (Devoogd, Krebs, Healy, & Purvis, 1993).

Conclusion Bird song research has made, and continues to make, major contributions to psychology. Without it, we would know much less about the development of learned vocalization, the hormonal and neurological control of vocal behaviour, and the biological basis of culture. I hope this chapter gave you a better appreciation for the value of birdsong research to psychology. And I hope that the next time you hear a bird sing, you sit silently for a moment and appreciate it for the wonder of nature that it is.

Acknowledgements The author thanks A. Matisz and D. Rendall for helpful comments on earlier drafts of this chapter. Thanks also to D.E. Gammon for permission to use his Panamanian songbird recordings.

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