Sympatry drives colour and song evolution in wood-warblers (Parulidae)

The Evolution of Multiple Color Mechanisms Is Correlated with Diversification in Sunbirds (Nectariniidae)

How and why certain groups become speciose is a key question in evolutionary biology. Novel traits that enable diversification by opening new ecological niches are likely important mechanisms. However, ornamental traits can also promote diversification by opening up novel sensory niches and thereby creating novel inter-specific interactions. More specifically, ornamental colors may enable more precise and/or easier species recognition and may act as key innovations by increasing the number of species-specific patterns and promoting diversification. While the influence of coloration on diversification is well-studied, the influence of the mechanisms that produce those colors (e.g., pigmentary, nanostructural) is less so, even though the ontogeny and evolution of these mechanisms differ. We estimated a new phylogenetic tree for 121 sunbird species and combined color data of 106 species with a range of phylogenetic tools to test the hypothesis that the evolution of novel color mechanisms increases diversification in sunbirds, one of the most colorful bird clades. Results suggest that: (1) the evolution of novel color mechanisms expands the visual sensory niche, increasing the number of achievable colors, (2) structural coloration diverges more readily across the body than pigment-based coloration, enabling an increase in color complexity, (3) novel color mechanisms might minimize trade-offs between natural and sexual selection such that color can function both as camouflage and conspicuous signal, and (4) despite structural colors being more colorful and mobile, only melanin-based coloration is positively correlated with net diversification. Together, these findings explain why color distances increase with an increasing number of sympatric species, even though packing of color space predicts otherwise.

Migratory singers dynamically overlap the signal space of a breeding warbler community

Migratory species inhabit many communities along their migratory routes. Across taxa, these species repeatedly move into and out of communities, interacting with each other and locally breeding species and competing for resources and niche space. However, their influence is rarely considered in analyses of ecological processes within the communities they temporarily occupy. Here, we explore the impact of migratory species on a breeding community using the framework of acoustic signal space, a limited resource in which sounds of species within communities co-exist. Migrating New World warblers (Parulidae, hereafter referred to as migrant species) often sing during refueling stops in areas and at times during which locally breeding warbler species (hereafter breeding species) are singing to establish territories and attract mates. We used eBird data to determine the co-occurrence of 19 migrant and 11 breeding warbler species across spring migration in SW Michigan, generated a signal space from song recordings of these species, and examined patterns of signaling overlap experienced by breeding species as migrants moved through the community. Migrant species were present for two-thirds of the breeding season of local species, including periods when breeding species established territories and attracted mates. Signaling niche overlap experienced by individual breeding species was idiosyncratic and varied over time, yet niche overlap between migrant and breeding species occurred more commonly than between breeding species or between migrant species. Nevertheless, the proportion of niche overlap between migrant and breeding warblers was similar to overlap among breeding species. Our findings showed that singing by migrant species overlapped the signals of many breeding species, suggesting that migrants could have unexplored impacts on communication in breeding species, potentially affecting song detection and song evolution. Our study contributes to a growing body of research documenting the impacts of migratory species on communities and ecosystems.

The extent of rapid colour change in male agamid lizards is unrelated to overall sexual dichromatism

Dynamic colour change is widespread in ectothermic animals, but has primarily been studied in the context of background matching. For most species, we lack quantitative data on the extent of colour change across different contexts. It is also unclear whether and how colour change varies across body regions, and how overall sexual dichromatism relates to the extent of individual colour change. In this study, we obtained reflectance measures in response to different stimuli for males and females of six species of agamid lizards (Agamidae, sister family to Chameleonidae) comprising three closely related species pairs. We computed the colour volume in a lizard-vision colour space occupied by males and females of each species and estimated overall sexual dichromatism based on the area of non-overlapping male and female colour volumes. As expected, males had larger colour volumes than females, but the extent of colour change in males differed between species and between body regions. Notably, species that were most sexually dichromatic were not necessarily those in which males showed the greatest individual colour change. Our results indicate that the extent of colour change is independent of the degree of sexual dichromatism and demonstrate that colour change on different body regions can vary substantially even between pairs of closely related species.

Pressure for rapid and accurate mate recognition promotes avian-perceived plumage sexual dichromatism in true thrushes (genus: Turdus)

Ecological conditions limiting the time to find a compatible mate or increasing the difficulty in doing so likely promote the evolution of traits used for species and mate recognition. In addition to interspecific character displacement signalling species identity, intraspecific traits that signal an individual's sex and breeding status reduce the challenge of identifying a compatible conspecific mate and should be more common in migratory rather than sedentary species, species with shorter breeding seasons and species breeding under high sympatry with many closely related heterospecifics. Here, we tested this recognition hypothesis for promoting plumage sexual dichromatism in the true thrushes (Turdus spp.), a large and diverse genus of passerine birds. We used receptor-noise limited models of avian vision to quantify avian-perceived chromatic and achromatic visual contrasts between male and female plumage patches and tested the influence of breeding season length, spatial distribution and sympatry with other Turdus species on plumage dichromatism. As predicted, we found that (1) true thrush species with migratory behaviour have greater plumage sexual dichromatism than non-migratory species, (2) species with longer breeding seasons have less plumage sexual dichromatism, and (3) greater numbers of Turdus thrush species breeding in sympatry is associated with more plumage sexual dichromatism. These results suggest that social recognition systems, including species and mate recognition, play a prominent role in the evolution of plumage sexual dichromatism in true thrushes.

The effect of spatial and temporal scale on camouflage in North African rodents

Background matching, a common form of camouflage, is a widespread anti-predator adaptation that hinders detection or recognition by increasing the resemblance of prey to its environment. However, the natural environment is complex and both spatially and temporally variable, which constrains effective background matching as an anti-predator strategy. Here, using remote sensing data (publicly available satellite imagery), we investigated how variation of habitat parameters predicts background matching in 16 Sahara–Sahel rodent species across spatial and temporal scales. All fur colour parameters (hue, saturation and brightness) strongly matched the respective habitats of the different species. Background matching in terms of hue was best at the microscale, whereas results for saturation and brightness showed more variation across spatial scales among species. Camouflage across the temporal scale (from 1 to 3 years before capture) was variable among species for all colour parameters. These complex interactions suggest that, in desert rodents, colour parameters are differentially sensitive to the respective scale of the habitat, plausibly reflecting the behaviour and life history of the species and the ecological properties determining their activity patterns. Consequently, the division between habitat (camouflage) generalists and specialists might become blurred in temporally changing and spatially variable environments.

Experimental test of selection against hybridization as a driver of avian signal divergence

Signal divergence may be pivotal in the generation and maintenance of new biodiversity by allowing closely related species to avoid some costs of co-occurrence. In birds, closely related, sympatric species are more divergent in their colour patterns than those that live apart, but the selective pressures driving this pattern remain unclear. Traditionally, signal divergence among sympatric species is thought to result from selection against hybridization, but broad evidence is lacking. Here, we conducted field experiments on naïve birds using spectrometer-matched, painted 3D-printed models to test whether selection against hybridization drives colour pattern divergence in the genus Poecile. To address selection for male colour pattern divergence without the influence of learning or the evolution of female discrimination in sympatry, we simulated secondary contact between Poecile species, and conducted mate choice experiments on naïve, allopatric females. We found that female black-capped chickadees (Poecile atricapillus) are equally likely to perform copulation solicitation displays to sympatric and allopatric heterospecific congeners when they are paired with conspecifics, but exhibit a strong preference for less divergent males when presented with paired heterospecific congeners. These results suggest that increased colour pattern divergence among sympatric species can reduce the likelihood of mixed mating in some contexts, and therefore should be favoured by selection against hybridization.

Sympatry drives colour and song evolution in wood-warblers (Parulidae)

Closely related species often exhibit similarities in appearance and behaviour, yet when related species exist in sympatry, signals may diverge to enhance species recognition. Prior comparative studies provided mixed support for this hypothesis, but the relationship between sympatry and signal divergence is likely nonlinear. Constraints on signal diversity may limit signal divergence, especially when large numbers of species are sympatric. We tested the effect of sympatric overlap on plumage colour and song divergence in wood-warblers (Parulidae), a speciose group with diverse visual and vocal signals. We also tested how number of sympatric species influences signal divergence. Allopatric species pairs had overall greater plumage and song divergence compared to sympatric species pairs. However, among sympatric species pairs, plumage divergence positively related to the degree of sympatric overlap in males and females, while male song bandwidth and syllable rate divergence negatively related to sympatric overlap. In addition, as the number of species in sympatry increased, average signal divergence among sympatric species decreased, which is likely due to constraints on warbler perceptual space and signal diversity. Our findings reveal that sympatry influences signal evolution in warblers, though not always as predicted, and that number of sympatric species can limit sympatry's influence on signal evolution.