Tereza Petrusková

The Causes and Evolutionary Consequences of Mixed Singing in Two Hybridizing Songbird Species (Luscinia spp.)

Bird song plays an important role in the establishment and maintenance of prezygotic reproductive barriers. When two closely related species come into secondary contact, song convergence caused by acquisition of heterospecific songs into the birds’ repertoires is often observed. The proximate mechanisms responsible for such mixed singing, and its effect on the speciation process, are poorly understood. We used a combination of genetic and bioacoustic analyses to test whether mixed singing observed in the secondary contact zone of two passerine birds, the Thrush Nightingale (Luscinia luscinia) and the Common Nightingale (L. megarhynchos), is caused by introgressive hybridization. We analysed song recordings of both species from allopatric and sympatric populations together with genotype data from one mitochondrial and seven nuclear loci. Semi-automated comparisons of our recordings with an extensive catalogue of Common Nightingale song types confirmed that most of the analysed sympatric Thrush Nightingale males were ‘mixed singers’ that use heterospecific song types in their repertoires. None of these ‘mixed singers’ possessed any alleles introgressed from the Common Nightingale, suggesting that they were not backcross hybrids. We also analysed songs of five individuals with intermediate phenotype, which were identified as F1 hybrids between the Thrush Nightingale female and the Common Nightingale male by genetic analysis. Songs of three of these hybrids corresponded to the paternal species (Common Nightingale) but the remaining two sung a mixed song. Our results suggest that although hybridization might increase the tendency for learning songs from both parental species, interspecific cultural transmission is the major proximate mechanism explaining the occurrence of mixed singers among the sympatric Thrush Nightingales. We also provide evidence that mixed singing does not substantially increase the rate of interspecific hybridization and discuss the possible adaptive value of this phenomenon in nightingales.

Geographic Variation in Songs of the Tree Pipit (Anthus trivialis) at Two Spatial Scales

ABSTRACT. We studied geographic as well as individual variation of songs in five Czech populations of the Tree Pipit (Motacillidae: Anthus trivialis), a passerine bird with a medium-sized syllable repertoire and complex song. We focused on repertoire composition at the level of syllables (the smallest invariant song units) and of structures that are potentially important in recognition among individuals (i.e., bi-syllables, or regular repetitions of two different syllables) and among populations (introductory phrases). We tested the hypotheses that the repertoires reflect local dialects (i.e., different male origins) and that within-site similarities of male repertoires are distance dependent. The populations studied differed significantly in all analyzed characteristics. Male origin explained ∼18% of the variation in individual syllable repertoires. However, no clear within-site geographic structure in repertoire similarity was observed, either between closest neighbors or on larger scales. We observed a tendency for increased syllable sharing among males in a small and dense isolated population, which is consistent with recent studies that suggest the existence of strong effects of habitat fragmentation on geographic patterns of song variation. High individual variation was observed in bi-syllable repertoires, but whether this facilitates individual recognition is unknown and in need of further study.

Interspecific territoriality in two songbird species: potential role of song convergence in male aggressive interactions

In animals, interspecific interference competition is often associated with their aggressive behaviour. The intensity of interspecific aggression and the outcomes of interference competition between closely related species might be substantially modified by copying of vocal signals used in territory defence. Here we tested the hypothesis that song convergence observed in a secondary contact zone of two songbird species, the common nightingale. Luscinia megarhynchos, and the thrush nightingale, Luscinia luscinia, might be an adaptive response to interspecific interference competition. These species are morphologically and ecologically very similar. However, the thrush nightingale is slightly larger and several lines of evidence indicate its competitive dominance. In the secondary contact zone most thrush nightingales incorporate common nightingale song types in their repertoires. Using playback experiments, we evaluated the strength of nonvocal aggressive responses of both species to conspecific and heterospecific stimuli. The species did not differ in aggressive responses to a heterospecific stimulus, suggesting that competitive dominance is not associated with higher interspecific aggressiveness in nightingales. Interestingly, while the common nightingale reacted significantly more aggressively to the conspecific than the heterospecific stimulus, the thrush nightingale showed similarly strong responses to both stimuli. We suggest that this similar level of interspecific and conspecific aggression in the thrush nightingale results from mixed singing of this species in sympatry, as males may not distinguish conspecifics from heterospecifics by song alone. Our results are consistent with the concept of convergent agonistic character displacement. According to this theory, vocal convergence might be adaptive in species that overlap broadly in resource use, as it leads to better distinction of territory boundaries between the species and thus reduces the level of interspecific competition.

Blow fly Trypocalliphora braueri parasitism on Meadow Pipit and Bluethroat nestlings in Central Europe

The larvae of blow fly (Trypocalliphora braueri) are known to cause a subcutaneous myiasis in bird nestlings mainly in Nearctic species, while only a few records are available from the Palaearctic. Here, we report on infestations of this insect ectoparasite in two passerine species, Meadow Pipit (Anthus pratensis) and Bluethroat (Luscinia svecica), breeding in the Central European mountain range Krkonoše. The infestation of T. braueri parasite was relatively low in studied bird species (3.0% of infested nests in Meadow Pipit and 4.2% in Bluethroat) but varied strongly between years (0–33% of infested nests/year/species). The presence of the parasitic blow fly larvae was apparently controlled by temperature; they were found only during warm summers. The combination of parasite infection and heavy rainfalls in a critical period significantly reduced nestling survival.

Invasive American crayfish Pacifastacus leniusculus (Decapoda: Astacidae) in the Morava River (Slovakia)

The signal crayfish, Pacifastacus leniusculus (Dana, 1852), was recorded in the Slovak-Austrian stretch of the Morava (March) River in the Záhorie region in August 2006. This is the first confirmed record of a non-indigenous crayfish species of American origin in Slovakia. Most likely, signal crayfish were intentionally released to the river by Austrian fishermen, or invaded from adjacent water bodies in Austria. As this species has a substantial invasive capability and is a potential vector of the crayfish plague pathogen, it represents a threat to the native crayfish species and its spread should be monitored. Its invasion upstream to the territory of the Czech Republic is not unlikely in near future.

Structure and Complexity of Perched and Flight Songs of the Tree Pipit (Anthus trivialis)

We analysed 2166 songs of 90 males of the tree pipit (Anthus trivialis; Passeriformes: Motacillidae) from five Czech localities to evaluate variation in song characteristics, the song structure and syntax rules, and differences between songs performed in flight and from perches. We measured temporal and spectral song characteristics; structure analysis was based on the identification of individual syllable units. There was a high among-population variation in the repertoire, suggesting substantial differences in local dialects. Flight and perched songs significantly differed in most analysed quantitative characteristics. While both types of songs are apparently used in territorial defence, trill elements within flight songs may also indicate male quality to potential mates. Characteristic bi-syllable repeats within songs were often unique for individual males and may function in individual recognition. Features of tree pipit songs make it a good species for studying song variation and function in birds with complex repertoires.

Territorial meadow pipit males (Anthus pratensis; Passeriformes) become more aggressive in female presence

Although mate guarding as prevention of extra-pair copulation is common among birds, evidence for aggressive behaviour involving physical contact related to mate guarding in passerines is scarce and cases of the presence of one partner directly influencing the aggressiveness of the other are lacking. We investigated the intra-specific territorial behaviour of male meadow pipits (Anthus pratensis; Passeriformes: Motacillidae) at the beginning of the breeding season by placing a pipit model accompanied by an intra-specific song playback in the territory of socially paired males and compared the responses of males whose mates were physically present during trials with those whose females were out of sight. The level of aggression of males was significantly higher in the presence of the female; half of the males in this group physically attacked the model (the most intense and risky aggressive behaviour). Physical attacks did not occur among males whose female was absent during the trial; response to the playback by most of these males was only weak. This pattern may be related to the prevention of extra-pair copulation; if the risks involved in the conflict are outweighed by potential loss of paternity, such aggressive mate guarding may pay off. The apparently overlooked effect on the territorial behaviour of a partner’s passive physical presence during conflict should be further evaluated because it may be important for the design and interpretation of results of behavioural experiments.

A review of the distribution of Yellowhammer (Emberiza citrinella) dialects in Europe reveals the lack of a clear macrogeographic pattern

AbstractThe Yellowhammer (Emberiza citrinella) is a bird with a very long history of research in geographic song variation, dating back to the 1920s. Numerous features make the Yellowhammer a suitable model for studying songbird dialects: it is a common and easily recognizable species, has a simple song, keeps singing long into the season, and has dialects (defined by the final song phrase) that are relatively easy to identify. Numerous works have focused on Yellowhammer dialects and their geographic distribution in various parts of Europe, often revealing mosaic-like patterns even at relatively restricted spatial scales. However, it has been repeatedly suggested that Yellowhammer dialects can be divided into two groups showing a macrogeographic pattern of distribution (in some recent works, eastern and western groups of dialects have been mentioned). To evaluate this assumption, data scattered in various published sources need to be pooled. Comparing historical records is nevertheless challenging, as various nomenclatures for Yellowhammer dialects were used until the mid-1980s (when a detailed system coined by Poul Hansen in Denmark was adopted), and older studies often did not differentiate between dialects recognized at present. To facilitate further work on song variation of this species, we summarized published information on the distribution of Yellowhammer dialects in Europe, added data from recordings publicly available online and in selected sound collections, and unified the different dialect nomenclatures used in the past. We demonstrate that the continental-wide distribution patterns of Yellowhammer dialects do not support the existence of broad, geographically distinct dialect groups (eastern vs. western). Furthermore, some of the presently recognized distinct dialect types seem to be parts of a broader continuum. Based on our conclusions, we discuss potential future avenues for Yellowhammer dialect research.ZusammenfassungEin Überblick über die Dialektverteilung der Goldammer (Emberiza citrinella) in Europa zeigt das Fehlen eines klaren makrogeographischen Musters Die Geschichte der Erforschung der geographischen Strophenvariationen der Goldammer (Emberiza citrinella) ist sehr lang und beginnt in den 1920er Jahren. Zahlreiche Merkmale machen die Goldammer zu einem geeigneten Modell für die Untersuchung von Singvogeldialekten: Sie ist eine häufige und leicht erkennbare Art, hat eine einfache Strophe, die Gesangssaison ist lang und sie bildet Dialekte (durch den letzten Strophenteil bestimmt), die relativ leicht zu bestimmen sind. Zahlreiche Untersuchungen der Goldammerdialekte und deren geographische Verteilung in verschiedenen Teilen Europas ergaben oft mosaikartige Muster selbst in relativ kleinen Gebieten. Es ist jedoch wiederholt vorgeschlagen worden, die Goldammerdialekte in zwei Gruppen nach makrogeographischem Verteilungsmuster zu unterteilen (einige neuere Arbeiten erwähnen östliche und westliche Dialektgruppen). Um diesen Vorschlag zu überprüfen, war es erforderlich, die Ergebnisse der verschiedenen Quellen zusammenzustellen. Der Vergleich der historischen Daten ist dennoch eine Herausforderung, weil unterschiedliche Dialektbezeichnungen bis in die Mitte der 1980er Jahre verwendet wurden (bis das von Poul Hansen veröffentlichte ausführliche Bezeichnungssystem für Goldammerdialekte benutzt wurde) und weil oft in älteren Studien die Dialekte nicht nach der derzeitigen Systematik unterteilt wurden. Um die weiteren Untersuchungen der Gesangsvariabilität der Goldammer zu erleichtern, haben wir die veröffentlichten Ergebnisse über die Verteilung der Goldammerdialekte in Europa zusammengefasst, veröffentlichte Internetaufnahmen und ausgewählte Klangsammlungen ausgewertet sowie die verschiedenen Systeme der Dialektbezeichnung, die in der Vergangenheit verwendet wurden, vereinheitlicht. Wir zeigen, dass sich die kontinentalweite Verteilung der Goldammerdialekte nicht in breite geographische Dialektgruppen (Ost-West) einteilen lässt. Außerdem scheinen einige der kürzlich ermittelten verschiedenen Dialekttypen in einem größeren Gebiet vorzukommen. Auf der Grundlage unserer Ergebnisse diskutieren wir mögliche Wege der Goldammerdialektforschung für die Zukunft.

Repertoire-based individual acoustic monitoring of a migratory passerine bird with complex song as an efficient tool for tracking territorial dynamics and annual return rates

Summary In field ecological and behavioural studies, it is often necessary to identify specific individuals. In birds, colour rings are frequently used to mark individuals; however, rings are often difficult to observe, especially in small species and dense habitats. Acoustic-based monitoring detecting individuals by their characteristic vocalization is a potentially suitable alternative, but this approach is challenging in species with complex songs. On the example of the Tree Pipit (Anthus trivialis), a small migratory passerine often singing in flight or from perches obscured by foliage, we demonstrate that acoustic monitoring based on the syllable repertoire can be very efficient tool for individual recognition. During a 3-year study, we obtained over 500 recordings from males from one study population (a number of them returning after winter). Males banded with colour rings were repeatedly recorded throughout the seasons, and syllable repertoires were determined from spectrograms for each recording. The repertoire of each unambiguously identified male was distinct and stable within as well as between seasons; and males with similar syllable repertoires differed in syntax. Based on the congruence between identification based solely on spectrogram assessment, and that based on observation of colour rings, we inferred that reliable identification of singing males (including non-ringed ones) was possible in the studied population from assessing a repertoire and song syntax of <5-min recording (containing 20–30 songs). The acoustic-based data: (i) increased the overall estimated number of territorial males at the study locality (from 49 ringed to 61), and improved the estimates of the period of their presence; (ii) revealed dynamic within-season changes in territory occupancy that would otherwise be missed; and (iii) allowed identification of returning birds (including non-ringed ones and those actively avoiding approaching humans). Our results suggest that some commonly used methods may substantially underestimate return rates of migratory bird species. Individual acoustic monitoring should be applicable on various bird species with complex song and stable repertoires, and may be particularly useful for those living in dense habitat or sensitive to handling.

Can mixed singing facilitate coexistence of closely related nightingale species

Mixed singing in songbirds is usually considered to result from erroneous learning from other species, but we suggest it might be rather adaptive. Our results show that mixed singing of Thrush Nightingale males, which incorporate songs of the Common Nightingale in their repertoires in areas where both species co-occur, may help these 2 species to establish boundaries of their territories, and thus facilitate their coexistence before divergence in ecological niches evolves.