Sue Anne Zollinger

On the relationship between, and measurement of, amplitude and frequency in birdsong

A growing number of studies ask whether and how bird songs vary between areas with low versus high levels of anthropogenic noise. Across numerous species, birds are seen to sing at higher frequencies in urban versus rural populations, presumably because of selection for higher-pitched songs in the face of low-frequency urban noise, or because birds can avoid masking directly by shifting to higher-frequency sounds (Fernandez-Juricic et al. 2005; Slabbekoorn & den Boer-Visser 2006; Nemeth & Brumm 2009; Gross et al. 2010; Potvin et al. 2010). In addition to changing song frequency, birds are also reported to respond to increased background noise by singing at higher amplitudes (Brumm & Zollinger 2011). Nightingales, Luscinia megarhynchos, for example, sing with a higher sound pressure level in areas with intense traffic noise as compared to quieter locations (Brumm 2004). While frequencyand amplitude-based responses to ambient noise are often considered independently, the twomight also vary in tandem

Producing Song: The Vocal Apparatus

Abstract: In order to achieve the goal of understanding the neurobiology of birdsong, it is necessary to understand the peripheral mechanisms by which song is produced. This paper reviews recent advances in the understanding of syringeal and respiratory motor control and how birds utilize these systems to create their species‐typical sounds. Songbirds have a relatively homogeneous duplex vocal organ in which sound is generated by oscillation of a pair of thickened labia on either side of the syrinx. Multiple pairs of syringeal muscles provide flexible, independent control of sound frequency and amplitude, and each side of the syrinx exhibits a degree of acoustic specialization. This is in contrast to many non‐songbirds, including vocal learners such as parrots, which have fewer syringeal muscles and use syringeal membranes to generate sound. In doves, at least, these membranes generate a harmonic signal in which the fundamental frequency is regulated by respiratory pressure in the air sac surrounding the syrinx and the overtones are filtered out by the vocal tract. The songs of adult songbirds are generally accompanied by precisely coordinated respiratory and syringeal motor patterns that, despite their relative stereotypy, are modulated in real time by somatosensory feedback. Comparative studies indicate songbirds have evolved species‐specific motor patterns that utilize the two sides of the syrinx in specific ways and enhance the particular acoustic effects characterizing the species song. A vocal mimic tutored with heterospecific song uses the same motor pattern as the tutor species when he accurately copies the song, suggesting that physical or physiological constraints on sound production have had a prominent role in the evolution of species‐specific motor patterns. An understanding of the relationship between the central processing and peripheral performance of song motor programs is essential for an understanding of the development, function, and evolution of these complex vocal signals.

Bird song and anthropogenic noise: vocal constraints may explain why birds sing higher-frequency songs in cities

When animals live in cities, they have to adjust their behaviour and life histories to novel environments. Noise pollution puts a severe constraint on vocal communication by interfering with the detection of acoustic signals. Recent studies show that city birds sing higher-frequency songs than their conspecifics in non-urban habitats. This has been interpreted as an adaptation to counteract masking by traffic noise. However, this notion is debated, for the observed frequency shifts seem to be less efficient at mitigating noise than singing louder, and it has been suggested that city birds might use particularly high-frequency song elements because they can be produced at higher amplitudes. Here, we present the first phonetogram for a songbird, which shows that frequency and amplitude are strongly positively correlated in the common blackbird (Turdus merula), a successful urban colonizer. Moreover, city blackbirds preferentially sang higher-frequency elements that can be produced at higher intensities and, at the same time, happen to be less masked in low-frequency traffic noise.

The Lombard effect

What is it? This year marks the 100 year anniversary of the discovery of the Lombard effect, an involuntary vocal response by speakers to the presence of background noise. In the century since its discovery, this phenomenon has surely achieved importance far beyond what its discoverer could have ever imagined. In the simplest terms, the Lombard effect is an increase in vocal amplitude in response to an increase in background noise. Although most people are probably not aware of it, we all know the Lombard effect — just think of the last time you tried to engage in a conversation in a noisy pub or at a boisterous party (Figure 1).

Universal mechanisms of sound production and control in birds and mammals

As animals vocalize, their vocal organ transforms motor commands into vocalizations for social communication. In birds, the physical mechanisms by which vocalizations are produced and controlled remain unresolved because of the extreme difficulty in obtaining in vivo measurements. Here, we introduce an ex vivo preparation of the avian vocal organ that allows simultaneous high-speed imaging, muscle stimulation and kinematic and acoustic analyses to reveal the mechanisms of vocal production in birds across a wide range of taxa. Remarkably, we show that all species tested employ the myoelastic-aerodynamic (MEAD) mechanism, the same mechanism used to produce human speech. Furthermore, we show substantial redundancy in the control of key vocal parameters ex vivo, suggesting that in vivo vocalizations may also not be specified by unique motor commands. We propose that such motor redundancy can aid vocal learning and is common to MEAD sound production across birds and mammals, including humans.

Avian vocal production in noise

Birds use acoustic signals to mediate a number of crucial social interactions such as territorial defence, mate attraction and predator avoidance. Thus, differences in signalling efficiency are likely to have major fitness consequences. Acoustic signal transmission is considerably constrained by noise, e.g. sounds in the environment that interfere with the detection, discrimination or recognition of a signal. In this chapter, we discuss noise sources encountered by birds, and the diverse ways birds use to make their signals heard in this noisy world. One concept of signal evolution suggests that bird vocalisations undergo microevolutionary adaptations over time that tailor their sounds to the specific noise profiles of their species-typical habitats. On the individual level, birds across many different taxa also possess the vocal plasticity to make short-term adjustments to their signals to reduce masking in response to changing environmental noise conditions. Such adjustments can take different forms in different species. However, the widespread problem of acoustic communication in noise has also led to the evolution of one shared solution in birds: the Lombard effect, i.e. a noise-dependent regulation of vocal amplitude. In addition, birds may also change the frequency, the duration, the timing, and/or the redundancy of their vocal signals in noise, although in many cases it is not yet clear whether these additional changes are achieved through ontogenetic plasticity or through short-term regulation. In recent years, there has been a flurry of new studies reporting correlations between increased levels of anthropogenic noise and a variety of changes in the vocal behaviour of birds. While many of these studies have focused on increases in song or call frequency in birds exposed to high levels of traffic noise, it is not yet known whether these differences in vocal pitch are actually adaptive. We encourage future research studies to take a more rigorous and integrative approach to the study of vocal signalling in noise. Finally, we note the need for more research on the impact of noise on the evolution and usage of multi-component signals that combine vocal and visual signals.

On the evolution of noise-dependent vocal plasticity in birds

Signal plasticity is considered an important step in the evolution of animal communication. In acoustic communication, signal transmission is often constrained by background noise. One adaptation to evade acoustic signal masking is the Lombard effect, in which an animal increases its vocal amplitude in response to an increase in background noise. This form of signal plasticity has been found in mammals, including humans, and some birds, but not frogs. However, the evolution of the Lombard effect is still unclear. Here we demonstrate for the first time the Lombard effect in a phylogentically basal bird species, the tinamou Eudromia elegans. By doing so, we take a step towards reconstructing the evolutionary history of noise-dependent vocal plasticity in birds. Similar to humans, the tinamous also raised their vocal pitch in noise, irrespective of any release from signal masking. The occurrence of the Lombard effect in a basal bird group suggests that this form of vocal plasticity was present in the common ancestor of all living birds and thus evolved at least as early as 119 Ma.

Metabolic and respiratory costs of increasing song amplitude in zebra finches

Bird song is a widely used model in the study of animal communication and sexual selection, and several song features have been shown to reflect the quality of the singer. Recent studies have demonstrated that song amplitude may be an honest signal of current condition in males and that females prefer high amplitude songs. In addition, birds raise the amplitude of their songs to communicate in noisy environments. Although it is generally assumed that louder song should be more costly to produce, there has been little empirical evidence to support this assumption. We tested the assumption by measuring oxygen consumption and respiratory patterns in adult male zebra finches (Taeniopygia guttata) singing at different amplitudes in different background noise conditions. As background noise levels increased, birds significantly increased the sound pressure level of their songs. We found that louder songs required significantly greater subsyringeal air sac pressure than quieter songs. Though increased pressure is probably achieved by increasing respiratory muscle activity, these increases did not correlate with measurable increases in oxygen consumption. In addition, we found that oxygen consumption increased in higher background noise, independent of singing behaviour. This observation supports previous research in mammals showing that high levels of environmental noise can induce physiological stress responses. While our study did not find that increasing vocal amplitude increased metabolic costs, further research is needed to determine whether there are other non-metabolic costs of singing louder or costs associated with chronic noise exposure.

Effect Sizes and the Integrative Understanding of Urban Bird Song (A Reply to Slabbekoorn et al.)

Effect Sizes and the Integrative Understanding of Urban Bird Song (A Reply to Slabbekoornet al.)Author(s): Erwin Nemeth, Sue Anne Zollinger, and Henrik BrummReviewed work(s):Source: The American Naturalist, Vol. 180, No. 1 (July 2012), pp. 146-152Published by: The University of Chicago Press for The American Society of NaturalistsStable URL: http://www.jstor.org/stable/10.1086/665994 .Accessed: 08/06/2012 07:18

Why birds sing loud songs and why they sometimes don't

In birdsong, and in most commonly studied acoustic communication systems, research has often focused on temporal and frequency-related signal parameters. However, although variations in amplitude are often overlooked and seldom measured, they are just as critical in communication. Recent studies have demonstrated that vocal amplitude plays an important role in both territorial behaviours and mate choice in birds. Several songbird species have been shown to produce low-amplitude songs, used primarily in aggressive encounters between males. On the other hand, loud songs may be an honest signal of current condition in males and recent studies have shown that females may prefer high-amplitude songs. Although it is generally assumed that louder song is more costly to produce, there is little empirical evidence to support this assumption. Here we review data on the metabolic costs of singing at different vocal amplitudes, and discuss recent studies from our laboratory showing that louder songs elicit stronger aggressive responses from territorial males. Together, these findings suggest that while the energetic costs of singing loudly are negligibly small, social aggression may be a key constraint that limits the upper amplitude of vocal signals. Finally we discuss future directions that can increase our understanding of the role that amplitude plays in acoustic communication in animals.