The day after: effects of vocal interactions on territory defence in nightingales

1. Models on territory acquisition and tenure predict that territorial animals benefit by adjusting territorial defence behaviour to previous challenges they had experienced within the socially complex environment of communication networks. 2. Here, we addressed such issues of social cognition by investigating persisting effects of vocal contests on territory defence behaviour in nightingales Luscinia megarhynchos (Brehm). 3. Using interactive playback during nocturnal song of subjects, a rival was simulated to countersing either aggressively (by song overlapping) or moderately (by song alternating) from outside the subjects' territory. Thereby, the time-specific singing strategy provided an experimentally controlled source of information on the motivation of an unfamiliar rival. 4. Expecting that nightingales integrate information with time, the same rival was simulated to return as a moderately singing intruder on the following morning. 5. The results show that the vigour with which male nightingales responded to the simulated intrusion of an opponent during the day depended on the nature of the nocturnal vocal interaction experienced several hours before. 6. Males that had received the song overlapping playback the preceding night approached the simulated intruder more quickly and closer and sang more songs near the loudspeaker than did males that had received a song alternating playback. 7. This adjustment of territory defence strategies depending on information from prior signalling experience suggests that integrating information with time plays an important part in territory defence by affecting a male's decision making in a communication network.     

A sensorimotor area in the songbird brain is required for production of vocalizations in the song learning period of development

Sensory feedback is essential for acquiring and maintaining complex motor behaviors, including birdsong. In zebra finches, auditory feedback reaches the song control circuits primarily through the nucleus interfacialis nidopalii (Nif), which provides excitatory input to HVC (proper name)—a premotor region essential for the production of learned vocalizations. Despite being one of the major inputs to the song control pathway, the role of Nif in generating vocalizations is not well understood. To address this, we transiently inactivated Nif in late juvenile zebra finches. Upon Nif inactivation (in both hemispheres or on one side only), birds went from singing stereotyped zebra finch song to uttering highly variable and unstructured vocalizations resembling sub‐song, an early juvenile song form driven by a basal ganglia circuit. Simultaneously inactivating Nif and LMAN (lateral magnocellular nucleus of the anterior nidopallium), the output nucleus of a basal ganglia circuit, inhibited song production altogether. These results suggest that Nif is required for generating the premotor drive for song. Permanent Nif lesions, in contrast, have only transient effects on vocal production, with song recovering within a day. The sensorimotor nucleus Nif thus produces a premotor drive to the motor pathway that is acutely required for generating learned vocalizations, but once permanently removed, the song system can compensate for its absence. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1213–1225, 2016     

Song‐type sharing in a population of Song Sparrows in the eastern United States

Male Song Sparrows (Melospiza melodia) sometimes interact with neighboring territory owners by song‐type matching or repertoire matching. In some Song Sparrow populations, levels of song sharing are high and most neighbors can interact by matching, but levels of song sharing are much lower in other populations, limiting the degree to which males can match their neighbors. One explanation for variation in sharing levels is that the importance of song‐type and repertoire matching, and therefore the extent of song sharing, varies geographically in North America, being greater in western populations than eastern populations. However, to date, two studies of eastern populations have provided conflicting evidence concerning levels of song sharing by Song Sparrows. Thus, we measured sharing of whole songs and introductory phrases of songs between males with adjacent territories in another population of Song Sparrows in the eastern United States (North Carolina), near the eastern and southern limits of the species’ breeding range. Males (N = 17) in our study shared an average of only 8.7% of their song types with neighbors, and more than half of neighbor pairs shared no whole songs. Sharing of introductory phrases was more common (mean = 22.8%). The level of whole song sharing in our study is the second lowest yet reported for any Song Sparrow population, supporting the hypothesis that sharing is generally lower in eastern than in western populations.     

Partial muting leads to age‐dependent modification of motor patterns underlying crystallized zebra finch song

The crystallized structure of adult zebra finch (Taeniopygia guttata) song is modifiable if sensory feedback is altered during sound production. Such song plasticity has been studied by examining acoustic modifications to the motif; however, the underlying changes to the vocal motor patterns of these acoustic modifications have not been addressed. Adult birds in two age categories (young = 90–120 days or middle aged 150–250 days) that sang crystallized song were used in the experiment. Vocal motor patterns were monitored by recording respiratory air sac pressure before, during, and after song plasticity was induced by partial or complete reduction of phonation (i.e., “partial muting”). Birds were recorded until changes in air sac pressure patterns underlying the song structure were observed (up to 160 days). Young adult birds were more likely to insert shorter duration (<125 ms) expiratory pulses (EPs) into the motif than middle‐aged adults. These shorter duration EPs were produced with a unique pressure pattern relative to the intact song, and therefore appeared to be generated by novel motor gestures. Stuttering (atypical repetition of an EP) was observed when these novel EPs were inserted into the motif, regardless of age. The EP of the distance call, which is also a learned vocalization in zebra finches, showed a similar reduction in duration if EPs were also shortened in the song. The emergence of shorter duration EPs was not related to sound production, or nonspecific effects of the surgical procedure, which suggests an age‐dependent neural process for song plasticity. © 2004 Wiley Periodicals, Inc. J Neurobiol, 2004     

Environmental variability and acoustic signals: a multi-level approach in songbirds

Among songbirds, growing evidence suggests that acoustic adaptation of song traits occurs in response to habitat features. Despite extensive study, most research supporting acoustic adaptation has only considered acoustic traits averaged for species or populations, overlooking intraindividual variation of song traits, which may facilitate effective communication in heterogeneous and variable environments. Fewer studies have explicitly incorporated sexual selection, which, if strong, may favour variation across environments. Here, we evaluate the prevalence of acoustic adaptation among 44 species of songbirds by determining how environmental variability and sexual selection intensity are associated with song variability (intraindividual and intraspecific) and short-term song complexity. We show that variability in precipitation can explain short-term song complexity among taxonomically diverse songbirds, and that precipitation seasonality and the intensity of sexual selection are related to intraindividual song variation. Our results link song complexity to environmental variability, something previously found for mockingbirds (Family Mimidae). Perhaps more importantly, our results illustrate that individual variation in song traits may be shaped by both environmental variability and strength of sexual selection.     

Individual differences affect honest signalling in a songbird

Research in the past decade has established the existence of consistent individual differences or ‘personality’ in animals and their important role in many aspects of animal behaviour. At the same time, research on honest signalling of aggression has revealed that while some of the putative aggression signals are reliable, they are only imperfectly so. This study asks whether a significant portion of the variance in the aggression-signal regression may be explained by individual differences in signalling strategies. Using the well-studied aggressive signalling system of song sparrows (Melospiza melodia), we carried out repeated assays to measure both aggressive behaviours and aggressive signalling of territorial males. Through these assays, we found that aggressive behaviours and aggressive signalling were both highly repeatable, and moreover that aggressive behaviours in 2009–2010 predicted whether the birds would attack a taxidermic mount over a year later. Most significantly, we found that residual variation in signalling behaviours, after controlling for aggressive behaviour, was individually consistent, suggesting there may be a second personality trait determining the level of aggressive signalling. We term this potential personality trait ‘communicativeness’ and discuss these results in the context of honest signalling theories and recent findings reporting prevalence of ‘under-signalling’.     

Geographic variation and divergence of songs in the Olive Sparrow species complex

Phenotypic traits such as songs are important in species recognition. Variation in acoustic traits can form barriers to gene flow and promote speciation. Therefore, understanding song divergence is crucial in groups with controversial taxonomy such as Olive Sparrows (Arremonops rufivirgatus), a widespread Neotropical species of songbird with multiple allopatric populations. Taxonomic authorities disagree on the number of Olive Sparrow subspecies, placing them into either two or three main groups. These groups may represent separate species based on morphological traits, but trait divergence within the complex has not been examined. We studied geographic variation in the characteristics of the songs of Olive Sparrows at two geographical levels: among three proposed groups and among five allopatric populations. In a second analysis, we evaluated the strength of acoustic divergence within the complex by comparing acoustic distances among groups and allopatric populations of Olive Sparrows with the acoustic distance among three recognized species in the genus Arremonops. We analyzed 802 songs from 174 individuals across 81 locations and measured 12 variables to describe the fine structural characteristics of the songs of Olive Sparrows, Green-backed Sparrows (A. chloronotus), Black-striped Sparrows (A. conirostris), and Tocuyo Sparrows (A. tocuyensis). We found significant acoustic variation in the Olive Sparrow complex at both geographical levels. Our divergence analysis also revealed that vocal divergence within the complex is similar to or greater than that found between recognized species in the genus. Together, these results suggest that acoustic diversity within the Olive Sparrow complex probably originated by isolation in tandem with selective and/or non-selective factors.     

Songs and the Function of Song Elements in Four Duetting Bushcricket Species (Ensifera, Phaneropteridae, Barbitistes)

The structure of male songs and the timing of female replies with respect to the male songs are described for four species of the palaearctic bushcricket genus Barbitistes (B. constrictus, B. ocskayi, B. serricauda, B. yersini). In a male song, 3 to 16 syllables form a chirp followed by a trigger syllable after a longer interval. The trigger syllable releases a female reply with a latency of 30 to 50 ms in all four species. In B. serricauda songs, there is no clearly separated trigger syllable. Instead, the first syllable of a chirp functions as a trigger syllable. Some B. serricauda males may produce a short female-type syllable just at the moment, when a female would reply. The possible function of such a syllable is acoustical mimicry. When comparing at least two song parameters, each species occupies a specific combination of values. According to the overlap of parameters a close phylogenetic relationship between B. constrictus and B. serricauda and between B. ocskayi and B. yersini is assumed. This interpretation is compared with a hypothesis based on morphological investigations.     

Shared Song Detector Neurons in Drosophila Male and Female Brains Drive Sex-Specific Behaviors

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Different modes of acoustic communication in deep‐diving short‐finned pilot whales (Globicephala macrorhynchus)

Toothed whales use a pneumatic sound generator to produce echolocation and communication sounds. Increasing hydrostatic pressure at depth influences the amplitude and duration of calls but not of echolocation clicks. Here we test the hypothesis that information transfer at depth might be facilitated by click‐based communication signals. Wild short‐finned pilot whales (27) instrumented with multisensor DTAGs produced four main types of communication signals: low‐ and medium‐frequency calls (median fundamental frequency: 1.7 and 2.9 kHz), two‐component calls (median frequency of the low and high frequency components: 2 and 9 kHz), and rasps (burst‐pulses with median interclick interval of 21 ms). Rasps can be confused with foraging buzzes, but rasps are shorter and slower, and are not associated with fast changes in body acceleration nor reduced acoustic output of buzzes, characteristic of prey capture attempts. Contrary to calls, the energy flux density of rasps was not significantly affected by depth. This, and a different information content, may explain the observed increase in the relative occurrence of rasps with respect to calls at depth, and supports the hypothesis that click‐based communication signals may facilitate communication under high hydrostatic pressure. However, calls are produced at depth also, indicating that they may carry additional information relevant for deep‐diving animals, including potential communication among whales diving at the same time in this highly social deep‐diving species.     

Individual right whales call louder in increased environmental noise

The ability to modify vocalizations to compensate for environmental noise is critical for successful communication in a dynamic acoustic environment. Many marine species rely on sound for vital life functions including communication, navigation and feeding. The impacts of significant increases in ocean noise levels from human activities are a current area of concern for the conservation of marine mammals. Here, we document changes in calling behaviour by individual endangered North Atlantic right whales (Eubalaena glacialis) in increased background noise. Right whales, like several bird and primate species, respond to periods of increased noise by increasing the amplitude of their calls. This behaviour may help maintain the communication range with conspecifics during periods of increased noise. These call modifications have implications for conservation efforts for right whales, affecting both the way whales use sound to communicate and our ability to detect them with passive acoustic monitoring systems.     

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.