BIO 554/754
Ornithology

Vocal Communication


An updated version of these notes can be accessed from a new "Avian Biology' page
(http://people.eku.edu/ritchisong/avian_biology.html)
.

 

Birds produce a variety sounds to communicate with flock members, mates (or potential mates), neighbors, & family members. These sounds vary from short, simple call notes to surprisingly long, complex songs. Sometimes birds generate sounds by using substrates (like woodpeckers) or special feathers (like American Woodcock), but most sounds are produced by the avian vocal organ, the syrinx.
 


Source:
http://trc.ucdavis.edu/mjguinan/apc100/modules/
Respiratory/airways/syrinx/syrinx.html

The syrinx is located at the point where the trachea branches into the two primary bronchi. According to one model of syrinx function, sound is generated when:

With two separate passageways (and membranes), some birds are able to generate two different sounds at the same time:


Click on the sonagram to hear a 'self-duet' by a Clay-colored Robin
(Source: Doug Von Gausig's webpage at http://www.naturesongs.com/costa.html)

Characteristics of the sound (e.g., frequency) are influenced by vibrations of the internal tympaniform membrane (ITM). The 'characteristics' of  the ITM (e.g., degree to which the membrane protrudes into the air pathway), in turn, are influenced by pressure in the interclavicular air sac (or, as in the diagram above, clavicular air sac) (Gill 1995). Syringeal muscles also influence air flow and the characteristics of sound (Gill 1995):


Vocal organs of passerine birds. Syringes (vocal organs) of representative passerines are located
at the juncture of the trachea and bronchi. (a) Acanthisitta (New Zealand wren) showing a relatively
simple syrinx lacking complex musculature; (b) Campylorhamphus (woodcreeper) illustrating
a tracheophone syrinx; (c) Menura (lyrebird) showing an atypical oscine syrinx;
(d) Corvus (crow) showing a typical oscine syrinx. (Raikow and Bledsoe 2000). The avian syrinx
is categorized based on its location: 1. Tracheophone or Tracheal = in trachea (most New World suboscine
Furnari Passeriformes), 2. Haploophone or Bronchial = in bronchi (cuckoos, nightjars), and
3. Tracheobronchial = at junction of trachea and bronchi (oscine Passeriformes).

The quality of sound can be further influenced by tracheal length, by constricting the larynx, by muscles in the throat, or by the structure and/or movements of the bill (e.g., here are some complex 'Bird Songs in Slow Motion').

Although the above model has been generally accepted for several years, Goller and Larsen (1997a, 1997b, 1999) provide evidence that other structures (not the ITM) are the source of sound in both songbirds (oscines) and non-songbirds because birds can still vocalize when the medium (or internal) tympaniform membrane is experimentally kept from vibrating.
 
 

In pigeons (& other non-songbirds), there is a lateral (or external) tympaniform membrane (LTM) that spans two of the tracheal rings (T1 & T2) superior to the medial tympaniform membranes. It is these membranes that constrict the trachea, vibrate, and produce the sounds (Goller and Larsen 1997b).

 
In songbirds, "phonation is initiated by rostrad movement and stretching of the syrinx. At the same time, the syrinx is closed through movement of two soft tissue masses, the medial (ML) and lateral (LL) labia, into the bronchial lumen. Sound production always is accompanied by vibratory motions of both labia, indicating that these vibrations may be the sound source. However, because of the low temporal resolution of the imaging system, the frequency and phase of labial vibrations could not be assessed in relation to that of the generated sound. Nevertheless, in contrast to the previous model, these observations show that both labia contribute to aperture control and strongly suggest that they play an important role as principal sound generators" (Goller and Larson 1997a). 

Central motor control of song:

Different circuits (or impulse pathways) in the brain control song production and song learning. Song production is controlled via a pathway beginning in the brain & travelling to the syrinx:


Source: http://soma.npa.uiuc.edu/courses/physl490b/models/birdsong_learning/bird_song.html

The 'learning pathway' connects the HVC to RA via areas X, DLM, and LMAN. This forms a recursive loop because the neurons in LMAN also project to area X. Disturbances to this pathway affect song development, but not the production of song in adult males.


Virtual bird brain matches nature's tunes -- When birds sing, they force air from their lungs through the syrinx. Scientists at Rockefeller University and the University of Buenos Aires recently developed a simple computer model that mimics this process to produce sound. By simulating changes in the tension of the vocal folds and in the air pressure from the lungs, the model reproduced the song of a canary. But the song only sounded right if the lungs and the vocal folds vibrated with particular phase differences. How does a bird's brain give these complex commands? Some clues come from studies which show that a brain region called the high vocal centre is active when a bird sings. This activity excites neurons in the RA nucleus. Some neurons in this structure excite motor neurons that control muscles in the vocal folds or in the lungs. Others damp down the activity of nearby neurons. Laje et al. (2002) made a simple computer model of the RA neurons and were surprised to find that it changed a simple, constant signal from the high vocal centre into a complex series of bursts with the hallmark phase differences of birdsong. And when they fed the output signal from the virtual brain into their computer model of a bird's syrinx, it again sang like a bird. Simply varying the volume of the signal from the high vocal centre produced different song patterns. The model bird can accurately echo the song of the Chingolo sparrow (Zonotrichia capensis). The fake sparrow song sounds extremely similar to the real one. The big surprise is that the intricacies of birdsong arise from such simple instructions. Laje et al. plan to add more brainpower to the virtual bird, allowing it to listen as well as sing. This might help reveal how birds perfect their songs as they learn from other birds.  -- Hazel Muir, New Scientist
Photo ©Mike Danzenbaker
http://www.avesphoto.com/website/CH/species/SPARFC-1.htm

Testosterone (and melatonin; see below) appear to play some role in song production. For example:


Seasonal changes in plasma testosterone concentrations in male Song Sparrows.
Columns represent mean ± SEM (error bars) plasma testosterone concentrations in male Song Sparrows
collected at each of the four sampling times (Smith et al. 1997).

Melatonin Shapes Brain Structure In Songbirds -- Springtime's lengthening days spark the growth of gonads and a rush of sex hormones that drive songbirds to melodic flights of fancy. That much has been known for some time. But for the first time, Bentley et al. (1999) have also identified melatonin as a critical ingredient that regulates singing and fine-tunes the effects of testosterone on the brain."There is a lot of interest in melatonin," said co-author Gregory Ball, "but there has never been any indication that it affects brain plasticity like this. The fact that it would have a direct effect on a brain area in birds and influence its volume has never been addressed in other species." About 20 years ago, it was discovered that the high vocal center, or HVC, increases in volume as days grow longer. Scientists realized that longer daylight hours in the spring lead to a higher level of testosterone and prompts males to sing more. Scientists were able to link the rise in testosterone to physical changes in the HVC. But, scientists also noticed that even if songbirds are castrated, thus blocking the influence of testosterone, seasonal changes still affected the volume of HVC. "The changes weren't as large," observed Bentley, "but it was obvious that something else was controlling the change in volume. If it wasn't testosterone, what could it be? Because we knew that many hormones are controlled by photoperiod, we decided to look at an obvious candidate, melatonin." To conduct the study, European Starlings without testosterone were exposed to a range of artificial daylight hours that induced reproductive states characterized by different seasons. By providing birds with melatonin, researchers found they could still have a direct effect on the HVC, reducing overall volume or otherwise attenuating its growth despite the amount of daylight. "For example," Bentley said, "late in the summer, when birds terminate their reproductive activity and their gonads regress and testosterone disappears, the HVC doesn't really appear to shrink until later in the year. We think the effect of melatonin (which is secreted at night) is kicking in as the days get shorter, causing the volume of HVC to decrease slowly. To our knowledge, this is the first direct evidence of a role for melatonin in functional plasticity within the central nervous system of vertebrates."  The next step is to find out how melatonin and testosterone interact to encourage efficiency in brain volumes during the breeding season. -- Source: John Hopkins University and Science Daily

www.mdc.missouri.gov


Simplified view of the avian song 
control sytem showing distribution 
of steroid receptors. HVc, High 
Vocal Center (Brenowitz and
Lent 2002).


Sexual differentiation of the avian brain

In songbirds, males and females may have distinctly different brain structures, specifically in those areas involved in the production of song. In many songbirds, males sing while females do not (or sing very little). The ability to sing is controlled by six different clusters of neurons (nuclei) in the avian brain (see diagrams below). Neurons connect each of these regions to one
another. In male songbirds, these nuclei can be several times larger than the corresponding cluster of neurons in females, and in some species (e.g., Zebra Finches), females may lack one of these regions (area X) entirely (Arnold 1980, Konishi and Akutagawa 1985).


Classification of vocalizations:


Nocturnal flight call of a Black-and-White Warbler
RealAudio | AIFF | WAV (at 1/6 speed: RealAudio | AIFF | WAV)


Sonograms of distress calls from six species. (a) Sooty-
capped Bush-Tanager, (b) Black-capped Flycatcher, 
(c) Green Violet-ear (pictured below), (d) Gray-breasted Wood-Wren, (e) Streak-breasted Treehunter, and 
(f) Yellowish Flycatcher. Each sonogram represents 
1 sec of distress calling. 

Distress Calls of Birds in a Neotropical Cloud Forest (Neudorf and Sealy 2002 -- Distress calls are loud, harsh calls given by some species of birds when they are captured by a predator or handled by humans. Newdorf and Sealy (2002) recorded the frequency of distress calls in 40 species of birds captured in mist nets during the dry season in a Costa Rica cloud forest. They tested the following hypotheses proposed to explain the function of distress calls: (1) calling for help from kin or reciprocal altruists; (2) warning kin; (3) eliciting mobbing behavior; (4) startling the predator; and (5) distracting the predator through attraction of additional predators. Our results did not support the calling-for-help, warning kin, or mobbing hypotheses. Indeed, genera that regularly occurred with kin or in flocks were not more likely to call than non-flocking genera. There was no relationship between calling frequency and struggling behavior as predicted by the predator startle hypothesis. Larger birds tended to call more than smaller birds, providing some support for both the predator distraction hypothesis and predator startle hypotheses. Calls of higher amplitude may be more effective in startling the predator. Distress calls of larger birds may also travel greater distances than those of smaller birds, supporting the predator manipulation hypothesis. The adaptive significance of distress calls remains unclear as past studies have generated conflicting results. While more playback experiments are necessary to determine if calls indeed attract other individuals or predators, these results suggest that distress calls do not function to attract helpers or mobbers but are more likely directed toward predators.


Domestic Chicken - aerial predator alarm call
 


Domestic Chicken - ground predator alarm call


Low frequency calls of cassowaries -- Although some birds can detect wavelengths in the infrasound range,there has been litle evidence that birds produce very low frequencies. Mack and Jones (2003) made 9 recordings of a captive Dwarf Cassowary (Casuarius benneti) and one recording of a wild Southern Cassowary (C. casuarius) in Papua New Guinea. Both species produced sounds near the floor of the human hearing range in their pulsed booming notes: down to 32 Hz for C. casuarius and 23 Hz in C. benneti
     Natural selection should favor the evolution of vocalizations that reach their targets with minimal degradation, and low frequencies propagate over long distances with minimal attenuation caused by vegetation. New Guinea forests ofen have a fairly thick understory of wet leafy vegetation that could quickly attenuate higher frequencies. Thus, the very low frequency calls of cassowaries probably ideal for communication among widely dispersed, solitary cassowaries in dense rainforest. How cassowaries produce such low vocalizations is currently unknown. 
    All three cassowary species have keratinous casques rising from the upper mandible over the top of the skull up to 17 cm in height. Hypotheses concerning the function of the casque include: (1) a secondary sexual character, (2) a weapon in dominance disputes, (3) a tool for scraping the leaf-litter, or (4) a crash helmet for birds as they bash through the undergrowth. The later three seem unlikely based on field observations. Future research should include the possibility that the casque might play some role in sound reception or acoustic communication.
Photo by D. DeMello, Wildlife Conservation Society


http://www.valleyanatomical.com/


The functions of bird song may vary among species. Some known & hypothesized functions include:


Song complexity and the avian immune system -- There are three hypotheses to explain how the evolution of parasite virulence could be linked to the evolution of secondary sexual traits, such as bird song. First, female preference for healthy males in heavily parasitized species may result in extravagant trait expression. Second, a reverse causal mechanism may act, if sexual selection affects the coevolutionary dynamics of host-parasite interactions by selecting for increased virulence. Third, the immuno-suppressive effects of ornamentation by testosterone or limited resources may lead to increased susceptibility to parasites in species with elaborate songs. Assuming a coevolutionary relationship between parasite virulence and host investment in immune defense, Garamszegi et al. (2003) used measures of immune function and song complexity to test these hypotheses in passerine birds. Under the first two hypotheses, they predicted avian song complexity to be positively related to immune defense among species, whereas this relationship was expected to be negative if immuno-suppression was at work. They found that adult T-cell mediated immune response and the relative size of the bursa of Fabricius were both positively correlated with song complexity, even when potentially confounding variables were held constant. These results are consistent with the hypotheses that predict a positive relationship between song complexity and immune function, thus indicating a role for parasites in sexual selection. 

Regression of short-term song complexity (number of unique syllables within songs/song length) on T-cell mediated immune response, after removing allometric effects by using residuals after controlling for body mass. Datapoints are phylogenetically independent linear contrasts (N = 38). The line and equation are from linear regression forced through the origin.


In some species, females also sing. This is particularly true in the tropics (see 'duetting' below). Singing by females may be important in:


When do female birds sing? Hypotheses from experimental studies (Langmore 1998).


(a) The song of a female Superb Fairy-wren. Females use songs to defend their territories against both
males and females. (b) The song of a female Alpine Accentor. Female Alpine Accentors sing to attract males, and
the complexity of their songs increases with age. This song was recorded from a two-year-old female (Langmore 1998).


Singing by female Northern Cardinals -- Yamaguchi (2001) found that female Northern Cardinals learn to sing three times faster than males - the most dramatic example of learning disparities between male & female animals found to date. She collected nestling cardinals & raised them in sound chambers with microphones and speakers that play back the songs of adult cardinals. It takes about a year for a cardinal to learn to sing, and young songbirds learn by imitating adults. During the early sensitive phase, young don’t sing, but listen to singing adults to memorize their songs. Then the practicing begins. “Their initial attempts are pretty miserable,” she says, “but they practice & practice until it matches the memory that was formed earlier during the sensitive phase.”

   Yamaguchi (1998) also analyzed songs and found that females sing with more overtones, creating a slightly nasal sound. Young males also go through a nasal, warbly phase as their testosterone kicks in, but it’s as though the females continue to sing with an adolescent male’s voice.


Source: http://www.flmnh.ufl.edu

   More important, Yamaguchi (2001) discovered that female cardinals memorize adult songs three times faster than males. While both sexes ultimately learned the same number of song types, the females’ sensitive phase was only a third as long as the males’. The different learning rates may reflect an evolutionary adaptation. Like other songbirds, juvenile cardinals disperse from their parents’ territory about 45 days after hatching to establish their own turf before their first breeding season. Away from their natal nest, the young cardinals are suddenly immersed in the new song dialects of other adult cardinals. It appears that females lose the ability to learn new dialects when they disperse, while males are able to learn them and “fit in” with their new neighbors.

  “ It might be that males retain the ability to learn songs longer than females so that they can have a better chance of establishing territory in a new area,” Yamaguchi says. “For males, song-matching and fitting into the crowd in a new place are really important, while they’re not for females.” It’s not clear why female cardinals have a shorter window of vocal learning, she says, but then again, “we don’t really know why females sing at all, or how they use their songs.” One hypothesis, she says, is that females sing as a species identification tool, a greeting card to male cardinals that says, “I’m an eligible mate; come court me.” Other researchers have proposed that female cardinals sing to shoo away brightly colored mates from the nest, warning the males not to attract attention to the vulnerable chicks. “I know that female cardinals also use songs in aggressive behavior,” Yamaguchi says. “I’ve seen females battling each other in the field, and they’re singing the whole time as they bang into each other.”



Early bird: the European Robin starts singing over 
an hour before some of its neighbors (Photo by S. Crawford)
Early birds have big eyes -- Birds with large eyes begin singing earlier in the morning than their small-eyed neighbors because they can see better in low light. So say British researchers explaining why, on a spring day in Welsh woodland, robins and redstarts pipe up a good hour and a half before chaffinches and blue tits. The keen-sighted early birds probably do get the worms. Big eyes may help the hunt for breakfast, speculates Robert Thomas from the University of Bristol, who led the study (Thomas et al. 2002). Birdsong is an acoustic advertisement. Birds use it to attract mates and defend territory. But singing also alerts predators. So a bird does not sing before it can see well enough to spot approaching danger, speculates Thomas. Yet a bird also does not want to waste time crooning when the sun is high enough to search for food. The earlier it can see to sing, the earlier it can see to start foraging. The link between the dawn chorus and eye size was first mooted in the 1960s, but this is the first time that it has been tested. Previous data was very confusing, says Graham Martin, who works on birds' senses at the University of Birmingham, UK. Thomas's team measured light levels and the times at which 57 different bird species began singing at various sites across Britain and Europe. When they measured the diameter of each bird's exposed eye surface, they realized that big-eyed birds sing earlier than those of the same body size but with smaller eyes. They also found that small birds sing earlier than large birds with the same-sized eyes as them. One explanation for this may be hunger. Small birds cannot store as much food overnight and they use up energy more quickly than large birds. "Only large birds can afford to take it easy," says Thomas Szekely, another member of the research team. "Small birds may sing earlier because they have to get up earlier to search for food." -- Natasha McDowell, Nature Science Update

Song Variation

Locally distinct versions of song (or song 'dialects') have been described in several species. Hypotheses to explain dialects include:


Song Repertoires

In many species of songbirds, including Northern Cardinals, Carolina Wrens, and many others, males possess repertoires of song types. These repertoires typically consist of perhaps 6 - 12 song types, but may range anywhere from 2 to an apparently unlimited number (as in, for example,Northern Mockingbirds). Interestingly, however, males in other species, such as Common Yellowthroats, have just one song. Repertoires of song types may have a variety of functions:


Song sharing (Beecher and Brenowitz 2005) -- Song sharing is common in songbirds and is found in a variety of social contexts, not only in territorial neighbors (the most commonly studied context), but also in lekking species and communal breeders. Song sharing in the neighbor context is best understood in the context of the Dear Enemy hypothesis. According to this hypothesis, long-term neighbors are preferred to newcomers because newcomers are inherently expansionist, whereas old neighbors generally respect territory boundaries once they have been mutually established. Neither preferring nor cooperating with familiar neighbors requires shared songs, but shared songs are a reliable signal (a ‘badge’) of familiarity or locality because they must be learned in the local neighborhood. Consistent with this hypothesis, Wilson and Vehrencamp (2001) found that neighboring Song Sparrows sharing fewer songs were more aggressive with one another than were neighbors sharing more songs.
     A corollary of the Dear Enemy hypothesis for territorial songbirds is that established neighbors should use their songs in place of time- and energy-costly physical interactions to minimize unnecessary territorial conflicts. Playback studies of Song Sparrows and Banded Wrens have supported this prediction. Even when neighbors do not share any song types (with respect to the investigators' criterion), they might still be able to song match using songs in their repertoires that they perceive as being most similar.

Song learning

Avian vocalizations can be inherited, learned, or invented. In some species, like Brown-headed Cowbirds, males develop normal songs even when raised in acoustic isolation. However, for most species of songbirds, at least some aspects of their singing behavior is learned. That learning, though, is guided by inherited (innate) mechanisms.
 

Stages of song development (birds with fixed repertoires):

 1 - Critical learning period

 2 - Silent period  3 - Subsong period  4 - Song crystallization

Stages (and duration) of song development in White-crowned Sparrows


Timeline for zebra finch song learning.   Young birds first memorize the song of an adult 'tutor' during a
critical period for 'sensory' learning of song. Later, during a 'sensorimotor' learning period, they begin to sing and
gradually match their initially immature vocalizations to the memorized song using auditory feedback. After this learning
process, adult song normally remains unchanging, or 'crystallized'. For zebra finches, the periods of
sensory and sensorimotor learning overlap. Many other species, such as the white-crowned sparrow, do not crystallize
song until close to one year of age, and have a much clearer separation between the sensory and sensorimotor
learning periods. Still other species, such as the canary, seem to reiterate the processes of sensory and sensorimotor
learning each season, perhaps under the regulation of seasonal variability in hormone levels (Brainard and Doupe 2000).


Singing in their sleep -  Researchers have shown that young birds sleeping at night may be reviewing the songs they've learned during the day (Dave and Margoliash 2000). Normally, the brain is desensitized to outside stimuli during sleep, partly because of changing concentrations of a norepinephrine. But the robustus archistratalis (RA),a region of the Zebra Finch brain involved in singing, exhibits an increase in neurological activity during sleep. It is generally believed that the activity of the sleeping brain helps to consolidate what was learned during the day, but how this occurs has never been directly shown. Dave and Margoliash (2000) recorded electrical impulses from single neurons in the RA of anesthetized, asleep and awake birds as they listened to recordings of their own songs played back on a computer. Without fail, birds that were asleep or anesthetized exhibited reduced regular oscillations but showed occasional bursts of strong activity in their RA neural impulse patterns. When the birds occasionally woke up during the night, the bursting patterns quickly disappeared and were replaced by the steady oscillating pattern seen during the day. "This is surprising because the same neurons that show no response during the day have these strong responses to the bird's own song when they are asleep. It's possible that songs learned during the day affect the bursting patterns of the RA at night, serving to solidify the newly learned songs in the bird's mind," says Margoliash. 

What is learned?

 1 - Syllables?

 2 - Duration?  3 - Rhythm?  4 - Frequency?
Male Birds' Ability To Learn Song Affects Female Mating Response  -- Researchers have found that how well a male learns his song affects the female's mating response – the first evidence that female birds use song-learning ability as an indicator of male quality. Nowicki et al. (2002) tested the mating response of female song sparrows to songs of captive-raised males. They found that females preferred those songs that came closest to wild-type songs they heard when young and presumably learned as models. According to Nowicki, he and his colleagues in the field have long theorized that female songbirds pay attention to male song as an indicator of fitness. "We've developed experimental evidence that there is a link between early stress, male brain development and song-learning," he said. "But until now, experimental and field observations showing that females were interested in song only contrasted the presence or absence of song, or relatively gross features of song, like the size of the repertoire. This is the first study to explicitly demonstrate that females care about song-learning quality," he said. To test the effects of fine differences in song quality on female response, the researchers trained captive-reared male song sparrows to sing by exposing them to the recorded songs of wild birds. To induce variation in stress among the birds, some were placed on a restricted diet during development. Using spectrographic analysis, the researchers rated the captive-reared birds on two measures of song quality: (1) how much of the wild-bird song they copied versus how much they invented, a practice common among song sparrows. Those birds who did invent more song elements also tended not to copy well those elements they did copy, and (2) how close the males had come to actually matching just the wild-bird song elements they were attempting to copy. To determine the effects of song quality, the researchers exposed wild-caught adult females -- experienced in listening to male songs -- to the captive-reared males' songs. The scientists measured female response to the songs by the amount they performed characteristic and distinctive female mating presentation display -- which includes a shivering of the wings, the lifting of the tail and a characteristic call. As a control, the wild-caught females were exposed to what the scientists had judged as well-learned male songs, as well as the digitally recorded wild songs. The female birds responded equally to both. However, when exposed to the captive-reared males' songs, females responded more strongly to male songs that had been better learned by both of the scientists' measures. "The females showed a strong preference for songs that had been copied well, as opposed to songs that had been copied poorly," said Nowicki. "And by our measures, the males got points taken off for originality. That seems to make sense because we would argue that males that deviate from original song haven't learned the song as well."  In addition to insight into bird song, said Nowicki, such studies can give basic insight into the evolution of animal signals in general. "We know sexual selection is a very powerful evolutionary force that has led to phenomena such as the evolution of extravagant displays and the evolution of size differences between sexes. I believe that this work demonstrates that sexual selection might not be acting directly on the obvious trait that is expressed, but on the mechanisms that underlie the expression of that trait. In the case of bird song, a male's song reflects the birds' developmental history, and song expression is only the trait that the female can gain access to for information about -- in this case -- brain mechanisms."  Also, said Nowicki, the discovery that females assess song quality emphasizes the importance of studying the neurobiology of song expression and placing it in an evolutionary context.  While the current studies show clearly that females prefer well-learned songs, among the next research steps, said Nowicki, will be to determine how females learn to judge song quality. "There is only very thin evidence that females learn song, so it's a major scientific question whether females are learning something about the population that they're living in, and using that as a way of assessing males," he said. Such female studies also will reveal whether the female's ability to distinguish good songs from bad reflects the birds' fitness and influences evolution, said Nowicki.   (Source: www.sciencedaily.com/releases/2002/09/020911072517.htm)


Source: http://142.179.94.212/Gallery_9/songbird.html


Why have songbirds evolved the ability to learn their songs?


Wide repertoire wins mates -- Female Great Reed Warblers choose males who sing the widest repertoire of songs because it shows they were well brought up. Songs are learned at a vulnerable stage of fledgling development, so Nowicki et al. (2000) predicted that adult males with the most tunes were well fed as youngsters -- turning them into eligible bachelors. Although biologists know that male birdsong attracts females, the exact message it conveys is a mystery. "We just don't know how the male bird song can mean anything to the female," Nowicki explains. He now claims to have the first experimental data that can answer this question. "A male's songs may be an honest indicator of how well he developed in the face of nutritional or other stresses experienced early in life," he suggests. Hungry chicks, in other words, have more on their mind than choir practice. And undernourished fledglings generally make poorer fathers when they grow up. To test the idea, Nowicki et al. (2000) recorded year-old Great Reed Warblers (Acrocephalus arundinaceus) in Sweden; they also weighed the birds and measured the lengths of particular feathers -- the longest of which are usually found on the strongest, fittest birds. Warblers with the most warbles also had the longest feathers -- supporting Nowicki's idea. The researchers also found evidence of a link between wide song repertoire and body mass. So, what do female warblers think when they encounter males with limited songs? That they are being chatted up by real bird-brains, Nowicki says. Limited song learning indicates poorer brain development in general. "By assessing the output of song learning, the female may gain accurate information about critical cognitive abilities such as spatial navigation and memory," he says. These are important for skills that every good warbler father needs: the ability to defend territory, for example, avoid predators and find food. This argument provides "a convincing solution to the problem," says Nigel Mann of the bird and mammal sound communication group at St. Andrews University, UK. "Other studies have shown that ability in tits to remember where food stores are located correlates with the size of certain brain nuclei. An interesting further development of the idea, " Mann continues, "would be to look more directly for a connection between such aspects of foraging, spatial memory and repertoire size." -- David Adam, Nature Science Update
Male Great Reed Warbler

Photo by Hervé Michel
(Source: perso.wanadoo.fr/michelhp/encyclopedie/sylvidae.html)


Vocal mimicry, in which birds copy and imitate sounds produced by other birds (or just other sounds in general), occurs in about 15-20% of passerines. Among the best examples of 'mimics' are birds in the family Mimidae (Northern Mockingbird, Brown Thrasher, and Gray Catbird). The possible functions of mimicry include:


Duetting:


Complex song duet of the Plain Wren -- Mann et al. (2003) studied the duet of the Caribbean-slope subspecies of the Plain Wren (Thryothorus modestus zeledoni) in Costa Rica. It is one of the most complex duets to have been described. The duet proper consists of rapid, highly coordinated alternation of “A-phrases” from the female and “B-phrases” from the male. While the female initiates this section with her A-phrase, this cyclical part of the duet is almost invariably preceded by an introductory “I-phrase” from the male, so that it is the male that initiates the performance. Each male has a repertoire of I- and B-phrases, and each female has a repertoire of A-phrases. These are specifically associated with each other to form a repertoire of duet types. Mann et al. (2003) hypothesize that the pattern of song organization in this species facilitates more coordinated and precise duetting. The presence of the three components means that a full duet requires the cooperation of both members of the pair. 
   Some investigators believe that duets benefit both singers, and play a role in territorial defense, pair bonding, or maintenance of contact. Other authors believe a conflict of interests may be involved, e.g., a female whose mate is attempting to attract a second female might benefit by labelling him as already being mated. While a purely descriptive study such as this one cannot resolve such issues, highly complex and tightly coordinated duets such as those of the Plain Wren seem more likely to stem from cooperation than conflict. 
Standard duet format of the Plain Wren. A typical duet, initiated by a male introductory (I-) phrase and followed by an antiphonal cycle of female (A-) and male (B-) phrases. Recorded during October 2001 at La Suerte, Costa Rica.


Photo by P.J.B. Slater


More lecture notes:

Territoriality and Coloniality


Literature Cited

Arnold, A. P. 1980. Sexual differences in the brain. American Scientist 68: 165–173.

Arnold, A. P.,  F. Nottebohm, and D.W. Pfaff. 1976. Hormone concentrating cells in vocal control and other brain regions of the Zebra Finch (Poephila guttata). J. Comp. Neurol. 165: 487–512.

Becker, P.H. 1982. The coding of species-specific characteristics in bird sounds. Pp. 213-252 in Acoustic Communication in Birds, vol. 1 (D.E. Kroodsma and E.H. Miller, eds.). Academic Press, New York, NY.

Beecher, M. D. and Eliot A. Brenowitz. 2005. Functional aspects of song learning in songbirds. Trends in Ecology and Evolution 20:143-149.

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Useful links:
 

A Reason to Sing

A Song for Every Occasion

Birdsong Unplugged

Chickadee Karaoke

Duets and Drawls

Evolution of Brain Structure for Vocal Learning

Functions of Embryonic Vocalizations in Shorebirds

Hummingbird Studies Raise Questions About Birdsong Evolution

Links to lots of birdsong sites

Sensory Systems and Communication

Signalling Theory and Animal Communication

Singing in the Brain

Song Learning in the White-crowned Sparrow

Song Learning in the Zebra Finch

Stages of Song Development

The Life of Birds: Bird Songs

Those Amazing Birds: Color Me Beautiful

Vocal Copying

Vocal Development

Vocal Dialects

Vocal Functions

Vocal gymnastics and the bird brain


Back to BIO 554/754 Syllabus