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loadings obtained by principal component analysis on the responses of Common Nightingale and Thrush Nightingale males during playback experiments

loadings obtained by principal component analysis on the responses of Common Nightingale and Thrush Nightingale males during playback experiments

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Knowledge of the mechanisms facilitating the coexistence of closely related competing species is crucial for understanding biodiversity patterns. The concept of convergent agonistic character displacement (ACD) suggests that interspecific interference competition may lead to convergence in territorial signals between species, which helps to establi...

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... physical territorial response was monitored during the play- back itself (3 min) and during the subsequent 3 min of the post- playback phase (i.e. 6 min in total). The responses were described by a set of behavioral categories (see Table 1) ranging from low to high level of aggressiveness as follows: jumping on the ground (count), flyovers (count), and direct physical contact (% of time attacking); no flight attacks were observed in the present experiments. In addi- tion, we evaluated the strongest physical response shown by the bird during the experiment (as defined above, ranging from 1 = inter- est in the dummy without close approach, to 5 = physical attack) as an ordinal variable. ...
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... song parameters (Table 1) were extracted from the recordings covering 6 min of both the playback and post-playback phases, based on variables already used in previous studies on Luscinia species (e.g. Kunc et al. 2005;Turčoková et al. 2011 ...
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... first used Principal Component Analysis (PCA) to sum- marize multiple song and behavioral variables measured during the experiments into independent factors suitable for further analyses. PCA was based on a correlation matrix of all 13 variables describ- ing both physical and vocal responses (Table 1). It was run using the "principal" function in the "psych" package v. 1.5.8 (Revelle 2017) with a varimax rotation to maximize the variances of the squared normalized factor loadings across variables for each factor, thus facilitating interpretation of the resulting principal components. ...
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... to the linear mixed-effects model, the physical territo- rial response of the tested males (reflected by PC1) significantly dif- fered among playback stimuli (Χ 2 = 34.4, P = 3.4 × 10 −8 , df = 2, 70), with a more aggressive response to conspecific stimulus than to both heterospecific ones, either pure or mixed (Figure 3a, Supplementary Table 1). We did not observe a significant difference between the mixed and pure heterospecific stimuli (Figure 3a). ...
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... the males exhibited a stronger reaction to the conspecific stimulus than to pure and mixed heterospecific stimuli (Figure 3b, Supplementary Table 1), demonstrated by more time spent singing, a higher song rate, longer song duration and accordingly shorter pauses in song bouts. In contrast to the physical response, however, the vocal response to the mixed heterospecific stimulus was significantly more intense than the response to the pure heterospecific stimulus (Figure 3b, Supplementary Table 1). The intensity of the response to the mixed heterospecific stimulus was thus intermediate between the responses to pure heterospecific and conspecific stimuli. ...

Citations

... Most documented cases of divergent ACD involve evolutionary shifts in agonistic signals and competitor recognition (Grether et al. 2009;Grether et al. 2013;Grether et al. 2017;Latour and Ganem 2017;Moran and Fuller 2018a;Moran and Fuller 2018b;Zambre et al. 2020). Conversely, species with a contested resource in common may converge in agonistic signals and competitor recognition to facilitate resource defense and partitioning, i.e., convergent ACD (Cody 1973;Grether et al. 2009;Reif et al. 2015;Pasch et al. 2017;Souriau et al. 2018;Kirschel et al. 2019). Interspecific mate competition arising from reproductive interference has also been shown to cause convergent ACD (Drury, Okamoto, et al. 2015;Grether et al. 2020). ...
Article
Interspecific territoriality has complex ecological and evolutionary consequences. Species that interact aggressively often exhibit spatial or temporal shifts in activity that reduce the frequency of costly encounters. We analyzed data collected over a 13-year period on 50 populations of rubyspot damselflies (Hetaerina spp.) to examine how rates of interspecific fighting covary with fine-scale habitat partitioning and to test for agonistic character displacement in microhabitat preferences. In most sympatric species, interspecific fights occur less frequently than expected based on the species’ relative densities. Incorporating measurements of spatial segregation and species discrimination into the calculation of expected frequencies accounted for most of the reduction in interspecific fighting (subtle differences in microhabitat preferences could account for the rest). In 23 of 25 sympatric population pairs, we found multivariate differences between species in territory microhabitat (perch height, stream width, current speed, and canopy cover). As predicted by the agonistic character displacement hypothesis, sympatric species that respond more aggressively to each other in direct encounters differ more in microhabitat use and have higher levels of spatial segregation. Previous work established that species with the lowest levels of interspecific fighting have diverged in territory signals and competitor recognition through agonistic character displacement. In the other species pairs, interspecific aggression appears to be maintained as an adaptive response to reproductive interference, but interspecific fighting is still costly. We now have robust evidence that evolved shifts in microhabitat preferences also reduce the frequency of interspecific fighting.
... Thrush Nightingale song is very complex and shows pronounced geographic variation (Sorjonen 1987;Marova et al. 2015b). Moreover, in the secondary contact zone with the closely related Common Nightingale (Luscinia megarhynchos) in central Europe, Thrush Nightingales often include parts of that species' song to their repertoires, such behaviour being reported as "mixed-singing" (Sorjonen 1986;Vokurková et al. 2013;Souriau et al. 2018). ...
... com). The song delivery rate in the playback recordings (mean ± sd) was within the range typical for the average spontaneous song of the species (Souriau et al. 2018): 8 ± 1.05 song/min, for a mean of 40 ± 1.05 songs. ...
... The experimental setting generally followed our previous experiments (Reif et al. 2015;Souriau et al. 2018). A remotely operated speaker (MIPRO MA-101) was placed on the ground in the open towards the bird's singing post, at the foot of a Thrush Nightingale taxidermic dummy perched on a stick. ...
Article
Among the broad diversity of songbird vocalisations, song can serve a wide range of different functions depending on the species and context. In many species, aggressive motivation has often been linked with the use of fast repeated series of elements typically referred to as trills. However, only a few studies explored the role of this specific component in species with a large repertoire and high song complexity. Here, we investigate the potential role of trills in the territorial vocal response of males Thrush Nightingale (Luscinia luscinia), a species with complex songs characterised by the frequent use and diversity of their trills. We performed playback experiments simulating territorial intrusion to test if trills signal aggressive motivation in this species. If so, we expected tested males to respond by changing their trill rate or frequency of trill use in songs, and/or using different trill types than before the stimulation. Contrary to our expectation, males did not modify their trill rate or differed in trill type use before, during or after playback. There was a tendency for decrease in trill duration and the number of elements in the trills during the playback stimulation, reflecting to some extent the overall use of shorter songs by males during that stage of the experiment. Altogether, our study does not support the role of trills as motivation signals for territory defence in Thrush Nightingales, but advocates for more research on their potential roles in signalling other information.
... Nightingales also sometimes glean foliage on trees and shrubs, and very occasionally catch flying insects (Cramp and Brooks 1992). Previous studies have provided experimental evidence for intense interference competition between the two species (Reif et al. 2015;Souriau et al. 2018). A comparison of species morphology between sympatric and allopatric 1 3 populations showed an increased divergence in bill size in sympatry compared to allopatry (Reifová et al. 2011a). ...
... It has been shown that bird song can be affected by vegetation structure ('acoustic adaptation hypothesis', Boncoraglio and Saino 2007). Nevertheless this explanation is unlikely in our system as no difference in vegetation density was observed between the species, and the species show song convergence rather than divergence in sympatry (Vokurková et al. 2013;Souriau et al. 2018). ...
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Article
Competition-driven feeding niche separation is assumed to be an important driver of the morphological divergence of co-occurring animal species. However, despite a strong theoretical background, empirical studies showing a direct link between competition, diet divergence and specific morphological adaptations are still scarce. Here we studied the early steps of competition-driven eco-morphological divergence in two closely related passerines: the common nightingale (Luscinia megarhynchos) and the thrush nightingale (Luscinia luscinia). Our aim was to test whether previously-observed divergence in bill morphology and habitat in sympatric populations of both species is associated with dietary niche divergence. We collected and analysed data on (1) diet, using both DNA metabarcoding and visual identification of prey items, (2) habitat use, and (3) bill morphology in sympatric populations of both nightingale species. We tested whether the species differ in diet composition and whether there are any associations among diet, bill morphology and habitat use. We found that the two nightingale species have partitioned their feeding niches, and showed that differences in diet may be partially associated with the divergence in bill length in sympatric populations. We also observed an association between bill length and habitat use, suggesting that competition-driven habitat segregation could be linked with dietary and bill size divergence. Our results suggest that interspecific competition is an important driver of species’ eco-morphological divergence after their secondary contact, and provide insight into the early steps of such divergence in two closely related passerine species. Such divergence may facilitate species coexistence and strengthen reproductive isolation between species, and thus help to complete the speciation process.
... Or does it instead stabilize coexistence by conferring the same benefits that territoriality does within species? Based on its history of neglect in both ecology and evolutionary biology, one might infer the former, but research on this topic has surged in recent years, and the hypothesis that interspecific territoriality is usually adaptive is gaining traction (9,(18)(19)(20)(21). ...
Article
Costly interactions between species that arise as a by-product of ancestral similarities in communication signals are expected to persist only under specific evolutionary circumstances. Territorial aggression between species, for instance, is widely assumed to persist only when extrinsic barriers prevent niche divergence or selection in sympatry is too weak to overcome gene flow from allopatry. However, recent theoretical and comparative studies have challenged this view. Here we present a large-scale, phylogenetic analysis of the distribution and determinants of interspecific territoriality. We find that interspecific territoriality is widespread in birds and strongly associated with hybridization and resource overlap during the breeding season. Contrary to the view that territoriality only persists between species that rarely breed in the same areas or where niche divergence is constrained by habitat structure, we find that interspecific territoriality is positively associated with breeding habitat overlap and unrelated to habitat structure. Furthermore, our results provide compelling evidence that ancestral similarities in territorial signals are maintained and reinforced by selection when interspecific territoriality is adaptive. The territorial signals linked to interspecific territoriality in birds depend on the evolutionary age of interacting species, plumage at shallow (within-family) timescales, and song at deeper (between-family) timescales. Evidently, territorial interactions between species have persisted and shaped phenotypic diversity on a macroevolutionary timescale.
... Traditionally, character displacement is studied by documenting a geographic pattern of trait variation consistent with character displacement (e.g., divergence or convergence in sympatry) and then attempting to rule out alternative explanations for the pattern, such as chance, genetic drift, hybridisation and species sorting, while also testing assumptions of the character displacement hypothesis (Schluter 2000;Pfennig & Pfennig 2012). This approach has been applied to several putative examples of both convergent and divergent ACD (Grether et al. 2009, and some new case studies are particularly compelling (e.g., darters, Moran & Fuller 2018a,b;antbirds, Tobias & Seddon 2009, Kirschel et al. 2019nightingales, Reif et al. 2015;Souriau et al. 2018;singing mice, Pasch et al. 2017;damselflies, Anderson & Grether 2010a,b;Drury & Grether 2014). However, while the traditional approach can provide strong evidence that character displacement has occurred in particular cases, it provides little information about its prevalence or predictability (Germain et al. 2018). ...
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Article
Many interspecifically territorial species interfere with each other reproductively, and in some cases, aggression towards heterospecifics may be an adaptive response to interspecific mate competition. This hypothesis was recently formalised in an agonistic character displacement (ACD) model which predicts that species should evolve to defend territories against heterospecific rivals above a threshold level of reproductive interference. To test this prediction, we parameterised the model with field estimates of reproductive interference for 32 sympatric damselfly populations and ran evolutionary simulations. Asymmetries in reproductive interference made the outcome inherently unpredictable in some cases, but 80% of the model’s stable outcomes matched levels of heterospecific aggression in the field, significantly exceeding chance expectations. In addition to bolstering the evidence for ACD, this paper introduces a new, predictive approach to testing character displacement theory that, if applied to other systems, could help in resolving long‐standing questions about the importance of character displacement processes in nature. This paper introduces a new, predictive approach to testing character displacement theory and uses it to test an agonistic character displacement (ACD) model with field data on 32 sympatric populations of rubyspot damselflies. The results provide some of the strongest evidence yet for ACD. Applying the same approach to other organisms could help resolve longstanding questions about the importance of character displacement processes in nature.
... In secondary contact zones, thrush nightingales sometimes even learn from their sister species, the common nightingale (Vokurková et al. 2013). Learning after their early life sensory phase as well as learning from their sister species can have direct consequences on their vocal territorial interactions during the breeding season (Naguib and Todt 1998;Reif et al. 2015;Souriau et al. 2018). As a result, the mean proportion of song types shared between breeding males can be as high as 47% in a stable population . ...
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Article
The songs of migratory passerine birds have a key role in mate attraction and territory defence during the breeding season. Many species also sing on their wintering grounds, but the function of this behaviour remains unclear. One possible explanation, proposed by the song improvement hypothesis, is that the birds take advantage of this period to develop their singing skills for the next breeding season. If so, non-breeding songs should reflect features of an early phase in song development, characterized by high vocal plasticity. In our study, we tested this prediction by comparing songs of thrush nightingales (Luscinia luscinia) recorded at two different breeding areas in Europe and one wintering area in Africa. While all songs from European localities had a typical structure characteristic of the study species, 89% of the songs recorded from Africa were highly variable, lacking such typical structure. We conducted further detailed analysis of breeding and winter songs that exhibited species-specific structure. First, we explored plasticity at the syllable level using a cross-correlation analysis, to obtain similarity scores as a measure of consistency. Second, we asked multiple human observers to quantify element variability. Our results showed significant differences in syllable consistency between breeding and wintering grounds, with more consistent delivery of syllables in the breeding areas. Likewise, element variability was substantially lower in the breeding populations. While both results fit the predictions of the song improvement hypothesis, more research is needed to elucidate the roles of singing on the wintering grounds. Significance statement Many migratory songbirds sing on their wintering grounds, outside the breeding period. While the role of singing during breeding has been broadly studied, our understanding of the function of winter singing remains limited. We analysed songs of the thrush nightingale, a migratory songbird with highly complex songs, comparing song structures recorded from breeding populations in Europe and an African wintering site. We demonstrate that males recorded at wintering locations sang songs with both significantly lower syllable consistency and higher element variability. Such characteristics are comparable to those observed during the sensorimotor phase of song development, previously described in other species. This pattern supports the song improvement hypothesis, suggesting that males singing on the wintering grounds may practice songs for the next breeding season. This study contributes to the understanding of the functions of songbird vocal behaviour out of the breeding context.
Article
Studies of niche segregation among avian species have mostly addressed food sites and foraging behavior, while partitioning relative to nesting sites differentiation has been little analyzed. The recent degradation and loss of suitable habitats have upraised concerns toward their effects on the distribution and evolution of ecological niches, particularly in threatened species. This study, conducted in Midelt from February to September, between 2015 and 2018, attempted to identify any potential differentiation in nesting habitats and feeding sites among wood pigeon, rock pigeon, turtle dove, collared dove, and barbary partridge as a game species mostly hunted in Morocco and Northwest Africa. Also, this study aimed to evaluate any possible sharing of habitats, principally with heavy loss of suitable habitats remarked recently. We analyzed breeding and feeding sites and diet composition with multivariate analysis to test the relevance of nest site and feeding site variables in the distribution of studied species. The results showed substantial segregation in nesting and feeding sites selected by studied species. Observed nesting sites and feeding features partitioning may reduce the potential competition between these species and enhance opportunities for their coexistence. While, the common use of feeding habitats and diet composition by turtle dove and rock pigeon is suggested to be controlled by the loss of available resources, which forces these species to use jointly the offered food resources. Finally, our data provide the primer and only comprehensive data on the partitioning of breeding and foraging resources among game species in their entire Northwest Africa. Information collected from this research provides valuable data for conservation measures of these highly appreciated games, mainly the vulnerable turtle dove. Moreover, these data provide a new window of large-scale comparative studies of the ecological niche of the game birds.
Article
Geographic contact between sister lineages often occurs near the final stages of speciation, but its role in speciation's completion remains debated. Reproductive isolation may be essentially complete prior to secondary contact. Alternatively, costly interactions between partially reproductively isolated species – such as maladaptive hybridization or competition for resources – may select for divergence, increasing reproductive isolation and driving speciation toward completion. Here, we use coalescent demographic modelling and whole‐genome datasets to show that a period of contact and elevated hybridization between sympatric eastern North American populations of two cryptic bird species preceded a major increase in reproductive isolation between these populations within the last ten thousand years. In contrast, substantial introgression continues to the present in a western contact zone where geographic overlap is much narrower and likely of more recent origin. In the sympatric eastern region where reproductive isolation has increased, it is not accompanied by character displacement in key morphometric traits, plumage colouration, or ecological traits. While the precise trait and underlying mechanism driving increased reproductive isolation remains unknown, we discuss several possibilities and outline avenues for future research. Overall, our results highlight how demographic models can reveal the geographic context in which reproductive isolation was completed, and demonstrate how contact can accelerate the final stages of speciation.
Article
Interspecific competition is assumed to play an important role in the ecological differentiation of species and speciation. However, empirical evidence for competition's role in speciation remains surprisingly scarce. Here we studied the role of interspecific competition in the ecological differentiation and speciation of two closely related songbird species, the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (L. luscinia). Both species are insectivorous and ecologically very similar. They hybridize in a secondary contact zone, which is a mosaic of sites where both species co‐occur (syntopy) and sites where only one species is present (allotopy). We analyzed fine‐scale habitat data for both species in both syntopic and allotopic sites and looked for associations between habitat use and bill morphology, which have been previously shown to be more divergent in sympatry than in allopatry. We found that the two nightingale species differ in habitat use in allotopic sites, where L. megarhynchos occurred in drier habitats and at slightly higher elevations, but not in syntopic sites. Birds from allotopic sites also showed higher interspecific divergence in relative bill size compared to birds from syntopic sites. Finally, we found an association between bill morphology and elevation. Our results are consistent with the view that interspecific competition in nightingales has resulted in partial habitat segregation in sympatry, and that the habitat‐specific food supply has in turn very likely led to bill size divergence. Such ecological divergence may enhance prezygotic as well as extrinsic postzygotic isolation and thus accelerate the completion of the speciation process. This article is protected by copyright. All rights reserved.