birds

Several recent studies indicate that the consistency of song or syllable repetitions signals male quality. However, the comparability and efficacy of different measurement methods is not known. Here, I compared two approaches to measuring the consistency of syllable repetitions within the trills of House Wren (Troglodytes aedon) songs. In the first approach, I calculated the coefficient of variation (CV) in standard time–frequency measurements within each trill. In the second approach, I used spectrogram cross-correlation (SPCC), which measures the maximum pixel-by-pixel similarity of two spectrograms. The two approaches gave correlated estimates of trill consistency, but SPCC was more strongly related to two putatively biologically relevant traits: specifically, SPCC differed more strongly between age classes, and more variation in SPCC could be attributed to individual differences. CV-based measures complemented SPCC measures by clarifying some of the specific acoustic features whose consistency changed with age. Although additional comparisons between measurement approaches would be useful to assess generality, it appears that researchers interested in song or trill consistency should consider using an SPCC, or a combined SPCC- and CV-based, approach.

In several avian and mammal social species, contact calls are hypothesized to function in maintaining group cohesion, coordinating activities between group members and negotiating fissions and fusions of multiple groups during foraging and roosting. They are therefore expected to exhibit acoustic features that make them suitable for communication of identity information. We investigated this in the monomorphic Peach-fronted Conure, a tropical parakeet living in open forest and scrubland habitats. The species is highly social outside the breeding season where it is assumed to live in a fission–fusion social system. In this study, contact calls from 10 visually isolated male and female Peach-fronted Conures were recorded outside the breeding season. Results from the multivariate analysis of variance and spectrographic cross-correlation with principal coordinates analysis procedure showed that contact calls varied highly between individuals and indicated that caller identity could be encoded in the duration, peak frequency, max frequency and 25%, 50% and 75% frequency energy quartiles of contact calls. The 50% frequency and 75% energy quartile was also significantly higher for females than for males. Furthermore, the results showed an inverse relationship between two morphometric measures and two spectral parameters, suggesting that variation in call parameters could also allow receivers to obtain information about the body size of the caller. Discriminant function analysis showed that contact calls could be correctly classified to individuals in 100% and to sexes in 91% of the cases, suggesting that there is sufficient variation in contact call parameters to enable vocal recognition of individuals and sex.

The Red-billed Leiothrix (Leiothrix lutea) is an invasive species which has recently been found to be locally abundant in eastern Liguria (Italy). The song production of this bird was recorded over the course of an entire year (2011) and evaluated using both aural information and applying innovative automatic processing metrics. Our findings revealed that the Red-billed Leiothrix has loud vocalizations that are heard for most of the year, including in July and August when other birds are silent. Its song production has been estimated to amount to 37% of the sounds uttered by the entire bird community. The next most vocal species are the European Robin (36%), the Common Blackbird (18%), the Subalpine Warbler (5%) and the Sardinian Warbler (1%). The Red-billed Leiothrix can be seen as a new acoustically dominant species and, consequently, a potential modifier of the soundscape patterns of the indigenous bird community. The modification of acoustic traits in the repertoire of both indigenous species and the Red-billed Leiothrix is to be expected according to a co-evolution hypothesis.

Animal displays may vary both within and among individuals and also within and among populations. This variation may contain important information used by animals for individual recognition. Suboscine birds are thought to develop song by fully innate mechanisms and are poorly studied relative to oscine birds, where song learning results in significant variation in song structure among individuals and the development of dialects. Recent research, however, demonstrates that suboscine song is often individually distinctive and in some cases shows signs of regional variation. We used spectrogram cross-correlation and canonical discriminant function analysis to examine individual and geographic variations in songs of Eastern Phoebes (Sayornis phoebe), suboscine birds with two song types. Both song types were individually distinctive and showed significantly higher cross-correlations within than among individuals. Discriminant function analysis correctly assigned 85.3% of “phee-bee” and 90.0% of “phee-b-be-bee” songs to the correct male, levels that are significantly higher than expected by chance. The individually distinctive characters of songs were also significantly repeatable among recording sessions. Eastern Phoebe song did not vary geographically between two populations separated by 640 km; permuted discriminant function analysis assigned 65% of “phee-bee” and 70% of “phee-b-be-bee” songs to the correct population, which did not differ significantly from chance expectations. Variation among males in song characteristics could be used by both males and females to discriminate among individuals. These detailed bioacoustic analyses support the idea that individual distinctive acoustic signals are widespread across suboscine birds.

In territorial songbirds, duets between mates represent one of the best-known examples of communal display, with the main function being the defence of joint resources. In this study, we found evidence of a coordinated choral display performed by neighbouring heterospecific songbirds. In such choruses, most of the participating species perform a deliberate temporal overlap of songs, thus suggesting agonistic behaviour. However, they then utilize a complex form of behaviour to avoid signal jamming, making the aggressive purpose unlikely. We define these displays as “coordinated interspecific choruses”. We recorded dusk choruses of songbirds living in a mixed turkey oak wood in central Italy, and then carried out a niche overlap analysis using null models that were intended for investigations of concurrent emissions of songs, finding that species tend to sing concurrently instead of using the refractory period of another species. Among the species singing concurrently, about half used the same frequency range, but instead of finding considerable spectral overlap between their vocalizations, the number of real spectral overlaps was lower than would be expected by chance. We propose a tentative explanation for this, where such choruses are the expression of the existence of a neighbourhood of different species that has evolved a communal signal that is similar to that used by mates in a pair, i.e. coordinated vocalizations. As coordination requires experience of each other's songs, we propose that evolution has selected individuals that are more skilled at learning heterospecific songs.

Offspring recognition has been confirmed in many bird species, and vocal signatures appear to be its major component. Up to now most studies dealt with colonial species, but recent findings indicate that similar recognition is also present in non-colonial birds. By integrating spectrographic analysis and playback experiments, we investigated for the first time parent–offspring vocal recognition in the Stone Curlew (Burhinus oedicnemus), a highly vocal, crepuscular-nocturnal species that usually rears a maximum of two precocial and nidifugous chicks. Even though the species is usually non-colonial and non-cooperative, in Taro River Regional Park, Parma, Italy, breeding territories are densely packed with distances between simultaneously active nests as close as 40 m, which creates the possibility that chicks might be exchanged. Our analysis identified two main vocalizations in the chick call repertoire: (1) the S-call, a brief strangled soft sound often uttered when parents and chicks were at close quarters; and (2) the C-call, a chirping sound with a complex structure, which seems to be a long-distance contact vocalization. Our acoustic analysis showed that, even though the structure was remarkably different between chicks, C-calls were an unreliable individual signature given their high intra-individual variability. This was confirmed by playback experiments. Indeed, when presented with simulated C-calling chicks, adults approached both their own and foreign chicks with equal responses. These results suggest that parent Stone Curlews are unable to recognize their offspring by voice. Furthermore, experiments are needed to test whether the chicks are able to recognize their parents' voice and, more generally, to understand the details of parent–offspring communication in this species.

Avian vocal duets are joint displays where paired birds produce temporally and structurally coordinated vocalisations. Duets show great variety in form that can reflect different functions, such as mate guarding, mutual recognition, pair bond maintenance or territory defence. By describing the structure of duets and singing behaviour, we can investigate whether these signals are based on cooperation or conflict and which functions they might have. Here we describe the singing behaviour and song repertoire of the Crimson-breasted Shrike in the Kalahari Desert, and assess four main hypotheses proposed for the function of duets. We found that Crimson-breasted Shrikes have a sex-specific repertoire, and the individuality of males, females and pairs is expressed acoustically. Differences in vocal strategies between the sexes indicate male mate guarding as one possible function. Temporal coordination is high and duetters follow strict codes, suggesting that duetting could be a cooperative endeavour in this species. The observed duetting behaviour is also consistent with the predictions for mutual recognition because each bird has its individual vocal characteristics that are consistent over time. Our results provide no support for the hypothesis that duets serve pair bond maintenance in this species as no partner-directed adjustment of temporal coordination took place.

Although it is highly recognisable, relatively little is known about the repertoire composition and singing behaviour of the Hermit Thrush (Catharus guttatus). To address this, we recorded spontaneously singing males in two eastern populations (Halifax, NS (n = 11) and Hancock County, ME (n = 7)) and analysed the recordings with respect to repertoire size, basic song type characteristics and song syntax. Males had song type repertoires of 7–12 song types, and no song type sharing between individuals was observed within or between populations. While frequency-related structural characteristics of the song types were the same between the populations, song type duration (especially that of the introductory note) differed significantly. The song types within each repertoire could be categorised into high and low song types based on introductory note frequency, and these song type categories also differed with respect to the amount and distribution of spectral energy. In both populations, males sang with immediate variety, never repeating the same song type consecutively, and preferentially used some song-type to song-type transitions more than others. In addition, we found correlational evidence of changes in singing behaviour (e.g., the relative use of low versus high song types) over the course of the breeding season. We discuss these findings in the context of other songbird research as well as their implications for future work examining how Hermit Thrush males utilise their songs in defending territories and attracting mates.

The spectacled warbler (Sylvia conspicillata) is a small passerine with a patchy distribution throughout the circum-Mediterranean region, including the North Atlantic archipelagos of Madeira, Canary Islands and Cape Verde. Here we characterize the species song structure on the island of Fuerteventura, quantifying repertoire size, inter- and intra-individual spectrographic variation, to determine whether acoustic variation occurred within an island population. Male song display was organized in song bouts of a variable number of song phrases, which in turn were made up of 4–69 syllables. We classified syllable types to derive a measure of repertoire size (number of different syllables) per song bout, and then used rarefaction methods to calculate the estimated repertoire size for our population of males. Three categories of song bout length were considered in analyses: short song bouts of 10 phrases, average bouts of 19 phrases and long bouts of ≥ 29 phrases. The observed and estimated repertoire size per male (between 43 and 126 syllables per male) increased with song bout duration, although the relationship was not significant for the estimated values. To test whether songs could be individually specific, we measured 11 spectrotemporal parameters of the song. A discriminant analysis using these variables performed poorly in classifying songs to the individuals that uttered them, but we found less variation in the individual than in the population for three out of the 11 variables. These individually specific variables, involving the first or the most common syllable of the song, the trill, were the duration of the first syllable of the phrase, the duration and the dominant frequency of the trill syllable. Our study emphasizes the complexity of spectacled warbler songs, in which males continuously add novel syllables over the entire song bout. This complexity appears to be determined by individual innovation capabilities rather than by the behaviour of copying neighbour repertoires, since songs of close birds were not more similar than songs from far-away territories.

To more easily and non-invasively monitor urban Eastern Screech-Owl populations, we developed a method of distinguishing individual owls using their calls. A set of seven variables derived from recordings of ‘bounce’ calls taken from 10 known (either free-ranging birds recorded at a single site on a single night or identifiable captive owls) owls was tested using a model-based clustering analysis (Mclust) as a method of discriminating individual owls. The cluster analysis correctly classified these calls with 98% accuracy. A second set of calls from nine owls was used to further test the method and correctly classified 84% of the calls using the same variables. Four owls were recorded repeatedly from 2008 to 2010 to determine the extent to which calls changed over time; the cluster analysis correctly assigned 89% of the calls to the correct owl regardless of the year the recordings were made. Based on these results, we are confident that the Mclust analysis can be used to reliably and safely estimate abundance and survival of Eastern Screech-Owls within the time frame of a few years and of population sizes < 15 owls.