SONG REPERTOIRES AND REPERTOIRE SHARING IN A LOCAL GROUP OF BLACKBIRDS

ABSTRACT

The song repertoires of a local group of blackbird Turdus merula males were determined and quantified with respect to individuality and sharing between neighbours. We focused on the long-ranging, introductory whistle part of the song and its motifs of fixed sequences of sounds. These motifs, whether they are used to start the song or placed centrally in the song, constitute the largest units of song that are repeated in the same way every time they are sung. Blackbird motifs therefore are equivalents of song types in other species. The result shows that one needs to analyse more than 200 songs to estimate a male's repertoire, which averaged 44 different motifs. Relative to other species, this is a medium to large song repertoire. The size of the repertoire of start motifs (on average 32) varied only a little between the individuals, whereas that of central motifs (on average 12) varied rather much between the same individuals, suggesting that they form a potential cue for assessment of male quality. The males within the neighbourhood showed a high degree of start motif sharing, which, together with the relatively large repertoires, could be constraining neighbour recognition. However, in most males the possession of a few individually distinctive and frequently repeated motifs could counter such an effect.     

Blackbirds and Song Thrushes Constitute a Key Reservoir of Borrelia garinii, the Causative Agent of Borreliosis in Central Europe

Blackbirds (Turdus merula) and song thrushes (Turdus philomelos) were found to carry 95% of all spirochete-infected tick larvae among 40 bird species captured in Central Europe. More than 90% of the infections were typed as Borrelia garinii and Borrelia valaisiana. We conclude that thrushes are key players in the maintenance of these spirochete species in this region of Central Europe.     

A preliminary comparison of a songbird’s song repertoire size and other song measures between an urban and a rural site

Characteristics of birdsong, especially minimum frequency, have been shown to vary for some species between urban and rural populations and along urban–rural gradients. However, few urban–rural comparisons of song complexity—and none that we know of based on the number of distinct song types in repertoires—have occurred. Given the potential ability of song repertoire size to indicate bird condition, we primarily sought to determine if number of distinct song types displayed by Song Sparrows (Melospiza melodia) varied between an urban and a rural site. We determined song repertoire size of 24 individuals; 12 were at an urban (‘human‐dominated’) site and 12 were at a rural (‘agricultural’) site. Then, we compared song repertoire size, note rate, and peak frequency between these sites. Song repertoire size and note rate did not vary between our human‐dominated and agricultural sites. Peak frequency was greater at the agricultural site. Our finding that peak frequency was higher at the agricultural site compared to the human‐dominated site, contrary to many previous findings pertaining to frequency shifts in songbirds, warrants further investigation. Results of our pilot study suggest that song complexity may be less affected by anthropogenic factors in Song Sparrows than are frequency characteristics. Additional study, however, will be required to identify particular causal factors related to the trends that we report and to replicate, ideally via multiple urban–rural pairings, so that broader generalization is possible.     

Versatility and continuity in the songs of thrushes Turdus spp.

A relationship between continuity and versatility was proposed by Hartshorne as a general feature of song organization in birds and argued to be linked to the avoidance of habituation (the monotony threshold principle). Thrushes of the genus Turdus vary greatly in song repertoire size, from the Redwing with a single song phrase to the Song Thrush which normally possesses well over 100 (Table 1), so that this group is well suited to looking for such a relationship. Recordings of 14 Turdus species were used to do so. Species in which successive songs do differ were indeed found to sing with shorter intervals between songs, though no equivalent relationship was found at the level of individual sound elements. There was a suggestion that species might fall into two contrasting groups without intermediates, one with continuous and highly varied songs and the other with discrete and simple songs. It is suggested that the relationship between continuity and versatility is a real phemomenon but that it is unlikely to be explained in the way that Hartshorne proposed. More likely is the hypothesis that continuous and varied songs have evolved primarily as mate attractants through sexual selection, while the principal role of discrete and simple songs is in communication between males.     

Synanthropic birds influence the distribution of Borrelia species: analysis of Ixodes ricinus ticks feeding on passerine birds

Ixodes ricinus ticks collected from 835 birds and from vegetation in the Czech Republic were analyzed. Host-seeking ticks (n = 427) were infected predominantly by Borrelia afzelii (25%). Ticks (n = 1,012) from songbirds (Passeriformes) were infected commonly by Borrelia garinii (12.1%) and Borrelia valaisiana (13.4%). Juveniles of synanthropic birds, Eurasian blackbirds (Turdus merula) and song thrushes (Turdus philomelos), were major reservoir hosts of B. garinii.     

Sexually dimorphic phrase organization in the song of the indris (Indri indri)

Animal acoustic communication often takes the form of complex sequences, composed of multiple distinct acoustic units, which can vary in their degree of stereotypy. Studies of sequence variation may contribute to our understanding of the structural flexibility of primates' songs, which can provide essential ecological and behavioral information about variability at the individual, population, and specific level and provide insights into the mechanisms and drivers responsible for the evolutionary change of communicative traits. Several methods have been used for investigating different levels of structural information and sequence similarity in acoustic displays. We studied intra and interindividual variation in the song structuring of a singing primate, the indri (Indri indri), which inhabits the montane rain forests of Madagascar. Indri groups emit duets and choruses in which they combine long notes, short single units, and phrases consisting of a variable number of units (from two to six) with slightly descending frequency. Males' and females' contributions to the song differ in the temporal and frequency structure of song units and repertoire size. We calculated the similarity of phrase organization across different individual contributions using the Levenshtein distance, a logic distance that expressed the minimum cost to convert a sequence into another and can measure differences between two sequences of data. We then analyzed the degree of similarity within and between individuals and found that: (a) the phrase structure of songs varied between reproductive males and females: female structuring of the song showed a higher number of phrases if compared to males; (b) male contributions to the song were overall more similar to those of other males than were female contributions to the song of other females; (c) male contributions were more stereotyped than female contributions, which showed greater individual flexibility. The picture emerging from phrase combinatorics in the indris is in agreement with previous findings of rhythmic features and song repertoire size of the indris, which also suggested that female songs are potentially less stereotyped than those of males.     

Are there age-related differences in the song repertoire size of Eurasian blackbirds?

Most oscine bird species possess a repertoire of different song patterns, and repertoire size is thought to signal aspects of male quality. As age is assumed to be related to male quality in terms of experience and/or viability, repertoire size may be expected to reflect male age. Here, we investigated the relationship between repertoire size and age (yearlings or older) in Eurasian blackbirds, Turdus merula, a species with a large repertoire delivered in a highly variable manner. We found that older males tended to have larger repertoires than yearlings though the two age groups overlapped considerably. Thus, compared to other species with large repertoires, age-related differences in repertoire size seem rather small in male Eurasian blackbirds. We also compared repertoires of three individuals in two successive years (as yearlings and in the year following) and found a large element turnover. Our investigation revealed that this turnover was almost complete in the quiet terminating twitter part of the song. Such turnover may allow a young bird to adjust his repertoire to his neighbours?? repertoires, which could be useful for song matching interactions.     

Apostatic selection by song thrushes (Turdus philomelos) feeding on the snail Cepaea hortensis

There is no direct evidence that predators exert apostatic selection (the systematic overpredation of commoner forms) on live, naturally polymorphic prey. This study tested whether captive song thrushes ( Turdus philomelos ) select apostatically when presented with dimorphic populations of yellow five-banded and yellow unhanded morphs of the snail Cepaea hortensis. Four thrushes were used. Two were presented with a 9: 1 ratio of five-bandeds to unbandeds and two were presented with 1:9 ratios. Each thrush was given four trials in succession. In each trial 30 snails were presented and the trial was stopped when 15 had been eaten. There were no differences in shell size between morphs or between eaten and uneaten snails of each morph. Three thrushes selected apostatically and one thrush exerted virtually no selection. Overall, there was statistically significant apostatic selection.     

Seasonal and diel plasticity of song type use in individual ovenbirds (Seiurus aurocapilla)

Animal communication is an important aspect of ecology across taxa, and there is a growing area of research that examines how animals plastically adjust their signals to account for both abiotic and biotic factors. Song type use and the temporal plasticity of song have been described in many bird species, but as of yet, few studies have examined how song type use may change across both seasonal and diel timeframes, and no studies have considered individual-level variation in plasticity. Using a hierarchical framework, we examined temporal patterns of primary and flight song use in ovenbirds (Seiurus aurocapilla; N = 21 individuals). We recorded ovenbird songs (N = 99,259) with autonomous recorders over 24-hr periods once per week across a breeding season near Sault Ste. Marie, Canada. As predicted, the occurrence and frequency of both song types significantly decreased over the season and showed temporal separation over diel periods. Primary songs peaked at dawn and declined throughout the day, while flight songs peaked at dusk and night. Our results support that primary songs have multiple functions as they remained more frequent during dawn and morning across the breeding season, while flight songs likely serve an intersexual function as they decreased similarly for all diel periods as mating opportunities decreased. Individuals were consistent in how frequently they sang their primary songs, but not their flight songs, suggesting that flight songs are more plastically expressed. We highlight the importance of examining plasticity in animal communication at the individual level as we show that males significantly differed in both their song behaviours (random intercepts) and the seasonal plasticity (random slopes) in these behaviours. Integrating themes such as song type use, temporal plasticity, and individual variation will be important for examining the evolutionary mechanisms that shape animal communication systems.     

BIOACOUSTICS IN NEW ZEALAND AND JAPAN

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.     

Individual Distinctiveness in the Fine Structural Features and Repertoire Characteristics of the Songs of White‐eared Ground‐sparrows

Communication between social animals is often more effective when signals facilitate individual recognition. Two critical requirements for individual recognition are the occurrence of characteristics that are unique to each individual, and the consistency of these characteristics through time. In some animals, characteristics of acoustic signals are known to vary over time due to changes in a variety of factors, including physiological and environmental features. Such temporal variation requires careful evaluation when exploring the individual distinctiveness of animal signals. In this study, we evaluate individual distinctiveness in the songs of male white‐eared ground‐sparrows Melozone leucotis and the persistence of distinctive characteristics over time. We collected focal recordings from populations of banded ground‐sparrows during two consecutive breeding seasons, including multiple recording sessions within each breeding season. We evaluated individual distinctiveness in fine structural acoustic features of songs. We also extended our analysis to repertoire characteristics, focusing on whether the relative frequency of song type use may provide cues to individual identity. We found that each male white‐eared ground‐sparrow sang individually distinctive songs, although their fine structural features varied between recording sessions. We found the frequency with which males sang different song types was also individually distinctive, and this feature varied little between recording sessions. Receivers may be able to use these distinctive characteristics to differentiate individuals over extended time periods; this may be especially important for species that engage in long‐term social interactions, such as tropical birds that defend territories against rival conspecific animals throughout the year.     

A spatiotemporal analysis of acoustic interactions between great reed warblers (Acrocephalus arundinaceus) using microphone arrays and robot audition software HARK

Acoustic interactions are important for understanding intra‐ and interspecific communication in songbird communities from the viewpoint of soundscape ecology. It has been suggested that birds may divide up sound space to increase communication efficiency in such a manner that they tend to avoid overlap with other birds when they sing. We are interested in clarifying the dynamics underlying the process as an example of complex systems based on short‐term behavioral plasticity. However, it is very problematic to manually collect spatiotemporal patterns of acoustic events in natural habitats using data derived from a standard single‐channel recording of several species singing simultaneously. Our purpose here was to investigate fine‐scale spatiotemporal acoustic interactions of the great reed warbler. We surveyed spatial and temporal patterns of several vocalizing color‐banded great reed warblers (Acrocephalus arundinaceus) using an open‐source software for robot audition HARK (Honda Research Institute Japan Audition for Robots with Kyoto University) and three new 16‐channel, stand‐alone, and water‐resistant microphone arrays, named DACHO spread out in the bird's habitat. We first show that our system estimated the location of two color‐banded individuals’ song posts with mean error distance of 5.5 ± 4.5 m from the location of observed song posts. We then evaluated the temporal localization accuracy of the songs by comparing the duration of localized songs around the song posts with those annotated by human observers, with an accuracy score of average 0.89 for one bird that stayed at one song post. We further found significant temporal overlap avoidance and an asymmetric relationship between songs of the two singing individuals, using transfer entropy. We believe that our system and analytical approach contribute to a better understanding of fine‐scale acoustic interactions in time and space in bird communities.     

The modelling of avian visual perception predicts behavioural rejection responses to foreign egg colours

How do birds tell the colours of their own and foreign eggs apart? We demonstrate that perceptual modelling of avian visual discrimination can predict behavioural rejection responses to foreign eggs in the nest of wild birds. We use a photoreceptor noise-limited colour opponent model of visual perception to evaluate its accuracy as a predictor of behavioural rates of experimental egg discrimination in the song thrush Turdus philomelos. The visual modelling of experimental and natural eggshell colours suggests that photon capture from the ultraviolet and short wavelength-sensitive cones elicits egg rejection decisions in song thrushes, while inter-clutch variation of egg coloration provides sufficient contrasts for detecting conspecific parasitism in this species. Biologically realistic sensory models provide an important tool for relating variability of behavioural responses to perceived phenotypic variation.     

Sex Differences in the Song of <Emphasis Type="Italic">Indri indri</Emphasis>

In some primate species, males and females within a social group emit loud calls in a coordinated manner or chorus. Indri indri emits a very conspicuous loud call that elicits the loud calls of neighboring groups. Previous investigations have hypothesized that the main functions of the indri chorus are related to territorial announcement, intergroup avoidance, and group cohesion. We investigated sex differences in indri song. We recorded and analysed songs given by 10 different groups over 160 d. Overall singing duration did not vary between the sexes. However, males emitted significantly fewer but longer notes. Adult males and females of each group participated in the song with sex-specific repertoires. Females had a song repertoire of 8 note types; males shared all of their 6 notes with females. Apart from the initial roars, in all note types shared by both sexes, male notes were significantly longer than female ones, whereas variations in frequency parameters differed according to the note type. These findings suggest that indri song may provide cues to conspecifics, such as group size and sex composition, which could influence interactions between groups.     

Singing Activity Reveals Personality Traits in Great Tits

In animal communication, sexually selected signals have been shown to often signal individual attributes such as motivation or quality. Birdsong is among the best studied signalling systems, and song traits vary substantially among individuals. The question remains if variation in signalling also reflects more general and consistent individual characteristics. Such consistent individual differences in behaviour that are relatively stable over time and contexts are referred to as personality or behavioural syndromes. Here, we studied the relation between singing and explorative behaviour, a well‐studied personality trait, using great tits (Parus major) under standardized aviary conditions. The results show that singing activity measured as the number of songs sung in spring prior to breeding correlated with male but not with female explorative behaviour. In contrast, song repertoire was not related to explorative behaviour but varied over the day. The link between explorative and singing behaviour suggests that sexually selected signals are more than signals of quality but can also reflect other intrinsic behavioural characteristics such as personality traits.     

Thrushes now largely restricted to the built environment in eastern England

Abstract.  I studied the distribution and density of three thrush populations, blackbird Turdus merula , song thrush T. philomelos and mistle thrush T. viscivorus , in an urban and contiguous rural area, totalling 2636 ha, in eastern England. Populations of these species are declining and I found 826, 85 and 30 territories of blackbird, song thrush and mistle thrush, respectively. Significantly more territories of all species than expected were within the urban boundary. Significantly more territories than expected were found of all three species in the built environment (residential housing, factories, schools, etc. with their associated gardens and green-space), urban and rural areas combined. Farmland occupied 67% of the study area but held significantly fewer blackbird and mistle thrush territories than expected, associated mainly with the small amount of grass; song thrushes were not recorded on farmland. Rural woodland and scrub held more blackbird and song thrush territories than expected. Song thrush was significantly associated with scrub within the urban boundary. With all three species largely absent from farmland, residential habitats can be considered as habitat refuges. The consequences of this for conservation and planning are discussed.     

Stereotyped and non-stereotyped features of the temporal patterning of singing sessions in the ovenbird Seiurus auricapillus

Temporal patterning of recorded singing sessions of 26 different male ovenbirds Seiurus auricapillus (Fringillidae: Parulini) was analyzed computationally, in order to test whether differences among songs are potentially informative or merely reflect performance errors. Repeated songs within a singing session by a given male showed relatively little inter-individual variation in the duration of the song or in the number of units composing it, although these features varied substantially among individuals. On the other hand, within a session of singing by an individual male, the most variable and potentially informative aspect of temporal patterning was the relative placement of the peak amplitude within the song. These results support the hypothesis that diversity in the vocalization sessions of oscine passerines can be produced by other means than the use of a varied song repertoire, even in a species like the ovenbird that uses just one song type. Because variation among song was focused on a single feature, performance errors are an unlikely explanation, suggesting that the temporal patterning of singing sessions may play an informative role, such as the minimization of habituation on the part of receivers.