Acoustic similarity of flight calls corresponds with the composition and structure of mixed-species flocks of migrating birds: evidence from a three-dimensional microphone array.

Animals that migrate in mixed-species groups may communicate with both conspecific and heterospecific individuals, providing a low-cost mechanism for navigation whenever individuals share similar migratory routes or destinations. Many migratory birds produce calls while flying, but the function of these calls, and the forces contributing to their evolution, are poorly known. We studied flight calls in mixed-species groups of wood warblers (Parulidae), a biodiverse group of migratory songbirds. We used a spatial approach to examine whether acoustic similarity of flight calls varies with group composition, recording flight calls of mixed-species flocks with a wireless microphone array and triangulating the positions of birds in three dimensions. We found that the acoustic similarity of flight calls was correlated with spatial proximity: birds with similar calls fly closer together during migration. We also found relationships between acoustic similarity, flock size and mixed-species flock diversity: birds with similar calls fly in smaller flocks and in flocks with lower species diversity. Our results support the idea that migrating birds use flight calls to maintain contact with acoustically similar individuals in mixed-species flocks, with communication transcending species boundaries. These results suggest that acoustically similar flight calls are used as cues of group assembly for migratory animals. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

The evolution of wood warbler flight calls: Species with similar migrations produce acoustically similar calls.

Diverse animal species engage in long-distance migrations. Many migrants travel in groups, and communication within these groups may be important to survival and successful migration. We examined migration and communication in 36 species of wood warblers (Parulidae), songbirds that breed in North America and migrate in mixed-species flocks to their wintering grounds. During migration, wood warblers produce short vocalizations called "flight calls." The function of flight calls and the patterns of acoustic similarity between species are poorly understood. We investigated whether acoustic similarity of flight calls of different species of warbler reflects the similarity in their migratory journeys or their phylogenetic relatedness. We found that phylogeny, similarity in breeding latitude, and overlap in the timing of migration predict acoustic flight call similarity across warbler species. Further, we found that phylogeny, similarity in migration distance, and overlap in wintering range predict acoustic flight call similarity in a subset of 12 species with highly similar calls, although this analysis has a small sample size. We conclude that migratory similarity may be an important force driving the evolution of acoustically similar calls in wood warblers, in addition to phylogenetic relatedness. Acoustic convergence in these species may facilitate communication between individuals with similar migrations.

Genomic insights into natural selection in the common loon (Gavia immer): evidence for aquatic adaptation.

BACKGROUND: The common loon (Gavia immer) is one of five species that comprise the avian order Gaviiformes. Loons are specialized divers, reaching depths up to 60 m while staying submerged for intervals up to three minutes. In this study we used comparative genomics to investigate the genetic basis of the common loon adaptations to its ecological niche. We used Illumina short read DNA sequence data from a female bird to produce a draft assembly of the common loon (Gavia immer) genome. RESULTS: We identified 14,169 common loon genes, which based on well-resolved avian genomes, represent approximately 80.7% of common loon genes. Evolutionary analyses between common loon and Adelie penguin (Pygoscelis adeliae), red-throated loon (Gavia stellata), chicken (Gallus gallus), northern fulmar (Fulmarus glacialis), and rock pigeon (Columba livia) show 164 positively selected genes in common and red-throated loons. These genes were enriched for a number of protein classes, including those involved in muscle tissue development, immunoglobulin function, hemoglobin iron binding, G-protein coupled receptors, and ATP metabolism. CONCLUSIONS: Signatures of positive selection in these areas suggest the genus Gavia may have adapted for underwater diving by modulating their oxidative and metabolic pathways. While more research is required, these adaptations likely result in (1) compensations in oxygen respiration and energetic metabolism, (2) low-light visual acuity, and (3) elevated solute exchange. This work represents the first effort to understand the genomic adaptations of the common loon as well as other Gavia and may have implications for subsequent studies that target particular genes for loon population genetic, ecological or conservation studies.