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Oceanic islands share distinctive characteristics thought to underlie a set of parallel evolutionary trends across islands and taxonomic groups – including life‐history traits, morphology and visual signals. To which extent acoustic signals also change in parallel on islands is less clear. Some important processes associated with insularity, such as founder effects and reduced sexual selection, could lead to a decrease in vocal performance and song complexity on islands. In a field‐based study, we recorded 11 insular species and their closest mainland relatives. Out of the 11 species pairs, 6 live in the tropics (São Tomé/Mount Cameroon) and 5 in the temperate region (Madeira/southern France). For each species, we measured two proxies of vocal performance (song duration and syllable rate) and one proxy of song complexity (syllable diversity). This study did not recover a clear relationship between the island environment and song traits. If as expected, syllable rate was lower in island species than in their mainland counterparts, the two other proxies showed no clear island–mainland pattern of divergence. Several factors may explain the absence of reduction for song duration and syllable diversity. Among those, relaxation of interspecific competition on islands may have led to an increase in syllable diversity, or correlations between song variables may have constrained song evolution. More studies on island species are needed to draw a better picture of divergence patterns and go beyond the confounding ecological factors that could explain peculiar song characteristics in islands. [ABSTRACT FROM AUTHOR]

The character release hypothesis—which predicts that a decline in interspecific competition leads to the expansion of trait expression—remains to be tested for communication signals. Taking advantage of the fact that oceanic islands host fewer species than the mainland, we tested whether island birds show an increase in frequency bandwidth of acoustic signals compared with mainland birds. Given the higher animal diversity and more saturated acoustic space in the tropics, we expected acoustic character release, if any, to be stronger in the tropics than in the temperate zone. We field recorded 22 bird species (11 pairs consisting of an endemic island species and its closest mainland relative) breeding at similar latitudes and in similar habitats: six tropical pairs (São Tomé Island/Mount Cameroon) and five temperate pairs (Madeira Island/southern France). For each species, we measured the degree of acoustic interference experienced when vocalizing and the spectral characteristics of its song (minimum and maximum frequencies, bandwidth). As expected, island species spent more time vocalizing alone, and any overlap in vocalizations involved fewer species. The vocalizations of island species spanned broader frequency bandwidths than their mainland counterparts in the tropics (true for all six pairs), but this pattern was less evident in the temperate region (2/5 pairs with no marked differences and 1/5 with opposite pattern). Overall, the character release of communication signals only occurred where the differential in number of species was large (tropics). We discuss latitude differences and the potential factors driving the observed differences. [ABSTRACT FROM AUTHOR]

Aim: On islands, species richness is reduced and interspecific competition relaxed in relation to the mainland, allowing species to use broader ecological niches. These factors are known to affect diet and morphology, but can also affect communication and acoustic signalling in particular. However, no study has ever compared insular and continental soundscapes to determine to which extent islands present reduced acoustic richness (number of co‐vocalizing species) and fewer constraints for vocalizing species. Location: São Tomé Island, Mount Cameroon, Madeira Island, Southern France. Taxon: Birds. Methods: We compared two pairs of insular and continental soundscapes: one in a temperate zone, the other in the tropics. We recorded sounds produced in similar types of primary forests and measured acoustic richness and ambient noise profiles. We then assessed acoustic niche organization by computing, for each community, species turnover, temporal and frequency overlaps, and acoustic avoidance. Results: We found fewer species co‐vocalizing on islands compared to mainland and in temperate compared to tropical region. Ambient noise was louder in the tropics and occupied a wider frequency range, especially on the mainland, thereby revealing a reduction in available acoustic space for tropical mainland birds. In this more crowded and noisy soundscape, species presented a higher acoustic turnover, overlapped less in time and in frequency with each other and acoustically avoided each other more when compared to the three other communities. Main conclusions: Soundscapes differed and imposed fewer constraints on vocalizing species along the species diversity gradient from tropical mainland to temperate island. Acoustic niche partitioning increased with species richness and was associated with increased levels of acoustic interference. Results set a scene for an effect of relaxed competition on song evolution on islands, especially in the tropics. [ABSTRACT FROM AUTHOR]

Insular ecosystems share analogous ecological conditions, leading to patterns of convergent evolution that are collectively termed as the 'island syndrome'. In birds, part of this syndrome is a tendency for a duller plumage, possibly as a result of relaxed sexual selection. Despite this global pattern, some insular species display a more colourful plumage than their mainland relatives, but why this occurs has remained unexplained. Here, we examine the hypothesis that these cases of increased plumage coloration on islands could arise through a relaxation of predation pressure. We used comparative analyses to investigate whether average insular richness of raptors of suitable mass influences the plumage colourfulness and brightness across 110 pairs of insular endemic species and their closest mainland relatives. As predicted, we find a likely negative relationship between insular coloration and insular predation while controlling for mainland predation and coloration, suggesting that species were more likely to become more colourful as the number of insular predators decreased. By contrast, plumage brightness was not influenced by predation pressure. Relaxation from predation, together with drift, might thus be a key mechanism of species phenotypic responses to insularity.