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.

Trait Variation and Spatiotemporal Dynamics across Avian Secondary Contact Zones

A secondary contact zone (SCZ) is an area where incipient species or divergent populations may meet, mate, and hybridize. Due to the diverse patterns of interspecific hybridization, SCZs function as field labs for illuminating the on-going evolutionary processes of speciation and the establishment of reproductive isolation. Interspecific hybridization is widely present in avian populations, making them an ideal system for SCZ studies. This review exhaustively summarizes the variations in unique traits within avian SCZs (vocalization, plumage, beak, and migratory traits) and the various movement patterns of SCZs observed in previous publications. It also highlights several potential future research directions in the genomic era, such as the relationship between phenotypic and genomic differentiation in SCZs, the genomic basis of trait differentiation, SCZs shared by multiple species, and accurate predictive models for forecasting future movements under climate change and human disturbances. This review aims to provide a more comprehensive understanding of speciation processes and offers a theoretical foundation for species conservation.

Sympatry in a nightingale contact zone has no effect on host-specific blood parasite prevalence and lineage diversity

Parasites are a key driving force behind many ecological and evolutionary processes. Prevalence and diversity of parasites, as well as their effects on hosts, are not uniform across host species. As such, the potential parasite spillover between species can significantly influence outcomes of interspecific interactions. We screened two species of Luscinia nightingales for haemosporidian blood parasites (Plasmodium, Leucocytozoon and Haemoproteus) along an approximately 3000 km transect in Europe, incorporating areas of host distant allopatry, close allopatry and sympatry. We found significant differences in infection rates between the two host species, with common nightingales having much lower parasite prevalence than thrush nightingales (36.7% versus 83.8%). This disparity was mostly driven by Haemoproteus prevalence, which was significantly higher in thrush nightingales while common nightingales had a small, but significantly higher, Plasmodium prevalence. Furthermore, we found no effect of proximity to the contact zone on infection rate in either host species. Despite having lower infection prevalence, common nightingales were infected with a significantly higher diversity of parasite lineages than thrush nightingales, and lineage assemblages differed considerably between the two species, even in sympatry. This pattern was mostly driven by the large diversity of comparatively rare lineages, while the most abundant lineages were shared between the two host species. This suggests that, despite the close evolutionary relationships between the two nightingales, there are significant differences in parasite prevalence and diversity, regardless of the distance from the contact zone. This suggests that spillover of haemosporidian blood parasites is unlikely to contribute towards interspecific interactions in this system.

Rapid resource depletion on coral reefs disrupts competitor recognition processes among butterflyfish species

Avoiding costly fights can help conserve energy needed to survive rapid environmental change. Competitor recognition processes help resolve contests without escalating to attack, yet we have limited understanding of how they are affected by resource depletion and potential effects on species coexistence. Using a mass coral mortality event as a natural experiment and 3770 field observations of butterflyfish encounters, we test how rapid resource depletion could disrupt recognition processes in butterflyfishes. Following resource loss, heterospecifics approached each other more closely before initiating aggression, fewer contests were resolved by signalling, and the energy invested in attacks was greater. By contrast, behaviour towards conspecifics did not change. As predicted by theory, conspecifics approached one another more closely and were more consistent in attack intensity yet, contrary to expectations, resolution of contests via signalling was more common among heterospecifics. Phylogenetic relatedness or body size did not predict these outcomes. Our results suggest that competitor recognition processes for heterospecifics became less accurate after mass coral mortality, which we hypothesize is due to altered resource overlaps following dietary shifts. Our work implies that competitor recognition is common among heterospecifics, and disruption of this system could lead to suboptimal decision-making, exacerbating sublethal impacts of food scarcity.

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.

Geographic contact drives increased reproductive isolation in two cryptic Empidonax flycatchers

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 data sets 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 10,000 years. In contrast, substantial introgression continues to the present in a western contact zone where geographic overlap is much narrower and probably 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 coloration, 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.

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.

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.