Range-wide spatial mapping reveals convergent character displacement of bird song

A test of the mechanistic process behind the convergent agonistic character displacement hypothesis

In this era of rapid global change, understanding the mechanisms that enable or prevent species from co-occurring has assumed new urgency. The convergent agonistic character displacement (CACD) hypothesis posits that signal similarity enables the co-occurrence of ecological competitors by promoting aggressive interactions that reduce interspecific territory overlap and hence, exploitative competition. In northwestern Switzerland, ca. 10% of Phylloscopus sibilatrix produce songs containing syllables that are typical of their co-occurring sister species, Phylloscopus bonelli ("mixed singers"). To examine whether the consequences of P. sibilatrix mixed singing are consistent with CACD, we combined a playback experiment and an analysis of interspecific territory overlap. Although P. bonelli reacted more aggressively to playback of mixed P. sibilatrix song than to playback of typical P. sibilatrix song, interspecific territory overlap was not reduced for mixed singers. Thus, the CACD hypothesis was not supported, which stresses the importance of distinguishing between interspecific aggressive interactions and their presumed spatial consequences.

Mechanisms of reduced interspecific interference between territorial species

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.

Avian Diversity: Speciation, Macroevolution, and Ecological Function

The origin, distribution, and function of biological diversity are fundamental themes of ecology and evolutionary biology. Research on birds has played a major role in the history and development of these ideas, yet progress was for many decades limited by a focus on patterns of current diversity, often restricted to particular clades or regions. Deeper insight is now emerging from a recent wave of integrative studies combining comprehensive phylogenetic, environmental, and functional trait data at unprecedented scales. We review these empirical advances and describe how they are reshaping our understanding of global patterns of bird diversity and the processes by which it arises, with implications for avian biogeography and functional ecology. Further expansion and integration of data sets may help to resolve longstanding debates about the evolutionary origins of biodiversity and offer a framework for understanding and predicting the response of ecosystems to environmental change.

Evolution of communication signals and information during species radiation

Communicating species identity is a key component of many animal signals. However, whether selection for species recognition systematically increases signal diversity during clade radiation remains debated. Here we show that in woodpecker drumming, a rhythmic signal used during mating and territorial defense, the amount of species identity information encoded remained stable during woodpeckers' radiation. Acoustic analyses and evolutionary reconstructions show interchange among six main drumming types despite strong phylogenetic contingencies, suggesting evolutionary tinkering of drumming structure within a constrained acoustic space. Playback experiments and quantification of species discriminability demonstrate sufficient signal differentiation to support species recognition in local communities. Finally, we only find character displacement in the rare cases where sympatric species are also closely related. Overall, our results illustrate how historical contingencies and ecological interactions can promote conservatism in signals during a clade radiation without impairing the effectiveness of information transfer relevant to inter-specific discrimination.

Tape lures swell bycatch on a Mediterranean island harbouring illegal bird trapping

Mediterranean islands provide shelter and sustenance for millions of migrating birds each year. Humans have historically exploited bird migration through hunting. In Cyprus, trapping birds during their migratory peak is considered a tradition, but has long been against the law. Illegal bird trapping is lucrative, however, with trappers using tape lures that broadcast birdsong to increase capture rates. It results in the slaughter of millions of birds each year. Yet, scientific studies quantifying capture rates of target and nontarget species using methods employed by trappers are lacking. Here, we show using playback experiments that tape lures lead to an order of magnitude greater capture rates of target species, but also significantly increase bycatch, which may include species of conservation concern. Conservation efforts focusing on minimizing illegal bird killing should also consider tape lures and their contribution to the overall impact of trapping on avian populations.

Competition and hybridization drive interspecific territoriality in birds

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.

Range-wide spatial mapping reveals convergent character displacement of bird song

A long-held view in evolutionary biology is that character displacement generates divergent phenotypes in closely related coexisting species to avoid the costs of hybridization or ecological competition, whereas an alternative possibility is that signals of dominance or aggression may instead converge to facilitate coexistence among ecological competitors. Although this counterintuitive process-termed convergent agonistic character displacement-is supported by recent theoretical and empirical studies, the extent to which it drives spatial patterns of trait evolution at continental scales remains unclear. By modelling the variation in song structure of two ecologically similar species of Hypocnemis antbird across western Amazonia, we show that their territorial signals converge such that trait similarity peaks in the sympatric zone, where intense interspecific territoriality between these taxa has previously been demonstrated. We also use remote sensing data to show that signal convergence is not explained by environmental gradients and is thus unlikely to evolve by sensory drive (i.e. acoustic adaptation to the sound transmission properties of habitats). Our results suggest that agonistic character displacement driven by interspecific competition can generate spatial patterns opposite to those predicted by classic character displacement theory, and highlight the potential role of social selection in shaping geographical variation in signal phenotypes of ecological competitors.

Time spent together and time spent apart affect song, feather colour and range overlap in tinkerbirds

Most studies on the processes driving evolutionary diversification highlight the importance of genetic drift in geographical isolation and natural selection across ecological gradients. Direct interactions among related species have received much less attention, but they can lead to character displacement, with recent research identifying patterns of displacement attributed to either ecological or reproductive processes. Together, these processes could explain complex, trait-specific patterns of diversification. Few studies, however, have examined the possible effects of these processes together or compared the divergence in multiple traits between interacting species among contact zones. Here, we show how traits of two Pogoniulus tinkerbird species vary among regions across sub-Saharan Africa. However, in addition to variation between regions consistent with divergence in refugial isolation, both song and morphology diverge between the species where they coexist. In West Africa, where the species are more similar in plumage, there is possible competitive or reproductive exclusion. In Central and East Africa, patterns of variation are consistent with agonistic character displacement. Molecular analyses support the hypothesis that differences in the age of interaction among regions can explain why species have evolved phenotypic differences and coexist in some regions but not others. Our findings suggest that competitive interactions between species and the time spent interacting, in addition to the time spent in refugial isolation, play important roles in explaining patterns of species diversification.

Predicting evolutionary responses to interspecific interference in the wild

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.