Low-amplitude Song Research Articles

Are signals of aggressive intent less honest in urban habitats?

AbstractHow anthropogenic change affects animal social behavior, including communication is an important question. Urban noise often drives shifts in acoustic properties of signals but the consequences of noise for the honesty of signals—that is, how well they predict signaler behavior—is unclear. Here we examine whether honesty of aggressive signaling is compromised in male urban song sparrows (Melospiza melodia). Song sparrows have two honest close-range signals: the low amplitude soft songs (an acoustic signal) and wing waves (a visual signal), but whether the honesty of these signals is affected by urbanization has not been examined. If soft songs are less effective in urban noise, we predict that they should predict attacks less reliably in urban habitats compared to rural habitats. We confirmed earlier findings that urban birds were more aggressive than rural birds and found that acoustic noise was higher in urban habitats. Urban birds still sang more soft songs than rural birds. High rates of soft songs and low rates of loud songs predicted attacks in both habitats. Thus, while urbanization has a significant effect on aggressive behaviors, it might have a limited effect on the overall honesty of aggressive signals in song sparrows. We also found evidence for a multimodal shift: urban birds tended to give proportionally more wing waves than soft songs than rural birds, although whether that shift is due to noise-dependent plasticity is unclear. These findings encourage further experimental study of the specific variables that are responsible for behavioral change due to urbanization.Soft song, the low amplitude songs given in close range interactions, is an honest threat signal in urban song sparrows. Given its low amplitude, soft songs may be a less effective signal in noisy urban habitats. However, we found that soft song remained an honest signal predicting attack in urban habitats. We also found that birds may use more visual signals (rapid fluttering of wings) in urban habitats to avoid masking from acoustic noise.

Song and aggressive signaling in Bachman's Sparrow

ABSTRACT Male Bachman's Sparrows (Peucaea aestivalis) have large vocal repertoires containing many song types and multiple categories of songs and calls. We examined the territorial defense function of the vocal repertoire of male Bachman's Sparrows by quantifying singing behaviors and aggressiveness in response to a simulated territorial intrusion. We compared vocal and other behaviors between the intrusion and post-intrusion periods, compared more aggressive males (attackers) with less aggressive males (non-attackers), and tested for signals that predict attack. During intrusion, subjects switched among their song types at higher rates, however, song switching did not differ between more and less aggressive males. More aggressive males sang more low-amplitude “whisper songs.” In line with previous studies of aggressive signaling in songbirds, Bachman's Sparrows appear to use low-amplitude song to threaten rivals. The eavesdropping avoidance hypothesis predicts that low-amplitude songs should have acoust...

Eavesdropping avoidance and sound propagation: the acoustic structure of soft song

Many species use low-amplitude (soft) song during close range interactions with conspecifics, such as in aggressive encounters or courtship displays. It has been suggested that soft song is adapted to limit eavesdropping by conspecifics or predators through reduced signal transmission range. If so, other structural features of soft song, besides amplitude, should be adapted to reduce transmission. The soft songs of white-throated thrushes, Turdus assimilis, have properties expected to lower transmission, such as higher frequency and broader bandwidth relative to broadcast songs. We compared the transmission properties of broadcast and soft syllables using a sound propagation field experiment. When played at the same amplitude as broadcast song, soft song showed greater excess attenuation and blur ratios and lower signal-to-noise ratios. Lowering the frequency of soft syllables recovered similar transmission properties as those of broadcast syllables. Further analysis of spectral traits showed that excess attenuation values were mainly affected by minimum frequency and peak frequency, while blur ratio variation was predominantly affected by bandwidth, and signal-to-noise ratios were predicted by bandwidth and peak frequency. Additionally, soft song had more frequency overlap with background noise than broadcast song. These results support the eavesdropping hypothesis: aspects of the acoustic structure of soft signals in addition to low amplitude seem to have been shaped by selection to reduce transmission and avoid eavesdroppers.

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.

Stridulated soft song by singing insects

The study of low-amplitude or ‘soft’ songs and calls has largely been limited to organisms that produce multiple call types that fall neatly into a bimodal distribution with respect to amplitude. The soft vocalizations of many of these animals, including birds and mammals, have proven to be extremely difficult to collect data on due to difficulty in hearing and recording such songs in the wild, the lack of production of these sounds in captivity, and the difficulty in standardizing measurements of the amplitude produced by free-moving animals. Here I suggest we consistently expand the working definition of soft song to allow for the inclusion of insects and other organisms whose calls do not easily fit into a ‘high-amplitude’ versus ‘low-amplitude’ signal paradigm. For instance, some species of moths produce extremely quiet ultrasonic courtship songs without also producing a high-amplitude song, and field crickets sing courtship songs that contain both relatively loud and quiet elements within the same song. Soft-singing moths and crickets may not only prove more practical to work with, but may also provide answers to heretofore untestable hypotheses about the function and evolution of soft song. • Low-amplitude signals receive little study, especially those produced nonvocally. • Common definitions of soft song only apply to species with loud and quiet songs. • Many moths produce only low-amplitude songs. • Many field crickets produce courtship songs with high- and low-amplitude elements. • The definition of soft song should expand to include these and other organisms.

Reliability of the Brownish-flanked Bush Warbler’s soft song in male-male conflict

Soft song is a low-amplitude song produced by many birds. Recent studies have confirmed that soft song is an aggressive signal. For example, the Brownish-flanked Bush Warblers Cettia fortipes use soft song in male-male conflicts, particularly prior to attacks. Although stable signaling systems require that signals be honest on average, models predict that cheating is an acceptable strategy for some individuals or in some contexts. This study aimed to test the reliability of soft song as an aggressive signal in the brownish-flanked bush warbler. We used mounted specimens accompanied by broadcast songs or soft songs to simulate a male attempting to invade an existing territory. We found the mounted specimen that coupled playback of soft songs suffered more and quicker attacks from the territory owner and that the relationship between soft song and subsequent attack in the territory owner was far from perfect. We observed territory owners that both over-signaled (i.e., produced soft song but did not attack) and under-signaled (i.e., attacked without producing soft song). Under-signaling territory owners were relatively more commonly than were over-signaling territory owners, particularly in simulated intrusion that coupled playback of soft song with a mount specimen. We discuss the cost of producing soft song and the potential benefit of the unreliable use of soft song and propose a new hypothesis for under-signaling with soft song; i.e., under-signaling territory owners might benefit from taking the initiative in fights.

Low-amplitude songs produced by male dark-eyed juncos (Junco hyemalis) differ when sung during intra- and inter-sexual interactions

The function and relative importance of low-amplitude (quiet) acoustic signals remains a poorly understood aspect of animal communication. Research in songbirds has predominantly focused on the role of low-amplitude songs as reliable predictors of attack and signals of aggressive intent. However, many species of songbirds produce low-amplitude song in the presence of both male and female conspecifics, suggesting a broader range of functions beyond aggressive behavior. Male dark-eyed juncos (Junco hyemalis) produce two distinct types of low-amplitude song: (1) soft long-range song (LRS), which is a lower amplitude version of junco LRS, and (2) short-range song (SRS), which is substantially more complex and structurally distinct from LRS. SRS is also sung at two distinct tempos (slow and fast) and has been observed in both male- and female-directed contexts. In this study, we presented free-living male juncos with a caged male or female conspecific accompanied by playback of LRS, a female precopulatory trill, or slow SRS and quantified the songs elicited by each social condition. Males produced slow SRS only in response to a female conspecific paired with a precopulatory trill or slow SRS, never to a male, even when males were paired with slow SRS. Together these observations indicate that slow SRS is a female-directed song. In contrast, males produced both LRS and soft LRS in the presence of conspecifics of both sexes, which suggests that LRS may serve a similar function with respect to male and female receivers regardless of amplitude. These results highlight the importance of investigating the production of low-amplitude songs in both intra- and inter-sexual contexts and provide further evidence that in juncos, slow SRS functions in courtship.

Soft song during aggressive interactions: Seasonal changes and endocrine correlates in song sparrows

It is well known that songbirds produce high amplitude songs (“broadcast songs”). Songbirds also produce low amplitude songs (“soft songs”) during courtship or territorial aggression in the breeding season. Soft songs are important social signals but have been studied far less than broadcast songs. To date, no studies have examined seasonal changes in soft song or its endocrine regulation. Here, in male song sparrows, we examined soft songs during a simulated territorial intrusion in the breeding season and non-breeding season. We also measured plasma testosterone and dehydroepiandrosterone (DHEA) levels in subjects immediately after the aggressive encounter. The total number of songs produced (broadcast+soft songs) did not vary between seasons. However, there was a dramatic increase in the percentage of soft song in the non-breeding season. Further, the percentage of soft song was negatively correlated with plasma testosterone levels in the non-breeding season. There were seasonal differences in the acoustic structure of two major elements of soft song, trills and buzzes. The minimum frequency of trills was lower in the non-breeding season, and the element repetition rate of buzzes was lower in the non-breeding season. To our knowledge, this is the first study to (1) examine soft songs outside of the breeding season and (2) to identify endocrine correlates of soft songs, which are important social signals in songbirds.

Deciphering Information Encoded in Birdsong: Male Songbirds with Fertile Mates Respond Most Strongly to Complex, Low-Amplitude Songs Used in Courtship

Research on the function of acoustic signals has focused on high-amplitude long-range songs (LRS) and largely ignored low-amplitude songs produced by many species during close-proximity, conspecific interactions. Low-amplitude songs can be structurally identical to LRS (soft LRS), or they can be widely divergent, sharing few spectral and temporal attributes with LRS (short-range song [SRS]). SRS is often more complex than LRS and is frequently sung by males during courtship. To assess function, we performed two playback experiments on males of a socially monogamous songbird. We compared responses of males whose mates were fertile or nonfertile with differences in song structure (SRS vs. LRS and soft LRS), amplitude (SRS and soft LRS vs. LRS), and tempo (slow SRS vs. fast SRS). Males responded more strongly to SRS than to LRS or soft LRS, indicating that song structure had a greater effect on response than song amplitude. SRS tempo did not detectably affect male response. Importantly, males responded more strongly to SRS when their mates were fertile, presumably because hearing SRS can indicate that a male's mate is being courted by an intruding male and a strong response can deter extrapair competitors. We conclude that low-amplitude songs can function in both inter- and intrasexual communication and should receive greater attention in future studies of mate choice and male-male competition.

Sing softly and carry a big stick: signals of aggressive intent in the song sparrow

Reliability of signalling has been the focus of much research in behavioural ecology, yet few studies have been able to show that putative aggressive signals actually predict subsequent escalation (e.g. attack). This is partly because until recently researchers have usually used stimuli (e.g. song playback) that did not permit subsequent escalation in the form of attack. We presented male song sparrows, Melospiza melodia, with a traditional simulated intrusion that involved only playback of their own song (no-mount condition), or a simulated intrusion that coupled playback of their own song with a taxidermic mount. We hypothesized that aggressive signals that are predictive of attack should increase in frequency when there is a visible intruder present (i.e. in the mount trials). Our results showed that only low-amplitude song (soft song) and wing waves (the latter effect only approached significance) increased in frequency when there was a mount present, consistent with previous research. We discuss these results in the context of how reliability of such a low-cost signal can be maintained, and propose new hypotheses for future research.

Song, Dance, and Smell Routine? Interpreting the Content and Function of Multimodal Signals in a Songbird, the Dark-Eyed Junco (Junco Hyemalis)

The study of animal communication has been dominated by a focus on signal types that are easily recognized and quantified by human observers. This approach has inevitably limited our ability to identify cryptic signals such as low-amplitude vocalizations and signals that transmit beyond the range of our sensory system, such as most olfactory signals. Only recently with the development of new technologies and less biased sampling techniques have we begun to unravel the importance and function of these non-traditional signal types. Here we report the results of two experiments focusing on poorly studied signals using a common songbird, the dark-eyed junco. We investigated the effect of low-amplitude song on male physiology and the occurrence of bill-wiping behavior during courtship and aggressive interactions. Preliminary results suggest that males do not alter their plasma testosterone or corticosterone levels in response to a song playback of high-amplitude or low-amplitude song, indicating that a stronger stimulus may be necessary to affect circulating hormones. Males that received intrusions of a live male or female conspecific performed significantly more bill-wiping in response to the female conspecific, suggesting that bill-wiping may be an important and overlooked courtship signal in this species.

Low-amplitude song predicts attack in a North American wood warbler

Many animal signals given during agonistic interactions are thought to convey levels of aggressive motivation of signallers, but few studies have experimentally tested whether such signals actually predict subsequent aggressive behaviour. In songbirds, the predictive content of most putatively aggressive vocal signals remains largely unresolved. Prior studies of agonistic vocal signalling in birds have generally assessed the reliability of vocal displays using indirect response measures such as approach to a playback speaker. However, this method does not allow aggression to be assessed directly, because subjects have no opportunity to engage in actual attacks. In this study, we assessed the aggressive content of vocal signals in black-throated blue warblers, Dendroica caerulescens , using a direct measure of aggression. We presented territorial males with a brief period of conspecific song playback, recorded their vocal responses, and then revealed a taxidermic mount, giving subjects an opportunity to attack. We found that one song feature, low-amplitude ‘soft’ song, was an extremely reliable predictor of attack; a logistic regression model in which the occurrence of soft song was the sole predictor correctly classified 93% of males as attackers or nonattackers. By contrast, four other vocal behaviours (total number of songs, rate of song type switching and numbers of two nonsong vocalizations) failed to predict whether birds would attack the mount. These data provide evidence that soft song in black-throated blue warblers conveys extremely reliable information about impending aggression. We suggest that the reliability of soft song observed in this study and recent studies in emberizid sparrows may be stabilized by vulnerability costs because the limited distance with which it can be transmitted forces signallers to place themselves at risk of retaliatory attack.