Oscar Johnson

Despite growing recognition that female bird song is widespread, large-scale comparative tests of the evolutionary drivers of female song remain rare. Most conceptual frameworks for studying birdsong have been developed and tested primarily using male traits, leaving open the question of whether ecological, morphological, and social pressures shape song evolution in females. Here, we apply phylogenetic comparative methods to test three classic hypotheses of signal evolution—acoustic adaptation, morphological adaptation, and species recognition—using female song data from 197 species of antbirds (Thamnophilidae), a socially monogamous Neotropical clade with widespread female vocalizations. We found that habitat structure and morphological traits constrain multiple aspects of female song. Species in unexposed (i.e., closed) habitats produced songs with fewer notes, slower pacing, and lower vocal performance, consistent with the acoustic adaptation hypothesis. Larger-bodied females with larger bills produced longer songs with lower peak frequency, narrower bandwidth, and lower vocal performance, supporting the morphological adaptation hypothesis. These results indicate that female song diversification is shaped by ecological and morphological constraints. We also found that syntopic species pairs—those that overlap in both habitat and elevation—exhibited reduced vocal similarity, supporting the species recognition hypothesis. However, divergence time also explained some variation in vocal similarity between syntopic pairs, indicating that both ecological interactions and evolutionary history contribute to female song divergence. Altogether, these results provide a rare comparative test of classic signal evolution hypotheses using female song. This study provides the first macroevolutionary test of multiple hypotheses of signal evolution in female birdsong. By showing that female song is shaped by both ecological and morphological constraints and diverges slightly more among syntopic species, our results emphasize the need to integrate female traits into evolutionary models of communication. Incorporating female perspectives will be essential to building a complete understanding of signal diversity and its evolution across species.

Avian migration has long captured human interest, but the causes of the evolution of migration remain unclear due to the limited study of the full spectrum of migratory strategies, including short-distance and intratropical movements. We examine the climatic drivers of migration across the roughly 1,300 species of suboscine birds, a group containing many intratropical migrants. Comparative analyses confirm that migratory behavior in temperate-breeding suboscines evolves in association with temperature seasonality. The evolution of migration in the tropics, however, has a more complex association with climatic variables, including precipitation and greenness seasonality. When we project the future climates suboscines will experience, we find that average temperature seasonality will decrease, favoring the loss of migration, but precipitation seasonality may increase, potentially favoring an increase in short-distance migration. The divergent impacts of climate on the evolution of different migratory strategies highlight the complexity of climate-movement associations and the challenges of projecting responses to climate change. 

Acoustic signal complexity varies widely in animals, from single notes to highly sophisticated vocal displays. In birds, vocal complexity can evolve as an honest signal of individual quality driven by sexual selection. However, this hypothesis is rarely explored in conjunction with alternative drivers, including competition for ecological resources (social selection) and intra-group communication, both of which may favour increased signal complexity. Using Bayesian phylogenetic models, we test whether these alternative mechanisms predict the complexity of innate songs in 1288 species of suboscine passerine birds, while accounting for ecological constraints on sound production, transmission and detection. We found that overall song complexity was reduced by sexual selection (estimated from mating systems) and declined with body size and vegetation density. Conversely, note count and song length increased in territorial species, particularly those using song to defend year-round territories during the non-breeding season. These findings challenge the common assumption that sexual selection is the main driver of increased signal complexity and highlight the role of social selection via territorial competition as a factor increasing the temporal complexity of songs. Our results suggest that signal complexity depends on social, cultural and ecological contexts, reflecting a combination of multiple inter-related drivers and constraints.

Aim: Assess how local variations in bird taxon composition across lowland Amazonia are associated with environmental factors and rivers.

Location: Lowland Amazonia below 500 m a.s.l., South America.

Time Period: Contemporary.

Major Taxa Studied: Birds.

Methods: We constructed maps illustrating changes in bird taxon composition across lowland Amazonia and calculated Jaccard dissimilarity between adjacent localities. We then applied geographically weighted regression (GWR) to evaluate how variation in environmental variables and riverine features (width, water discharge, meandering, floodplain extent) explains spatial turnovers in taxon composition. We used comparative phylogenetic analyses to test whether ecological traits predict cross-river taxon turnover.

Results: Geographic variation in bird taxon composition was mostly associated with the presence and physical characteristics of rivers, especially river width and discharge, which predicted composition dissimilarities along their lower courses. Away from rivers, variations in forest cover, habitat heterogeneity, and temperature seasonality were the strongest variables associated with composition turnovers. Phylogenetic analyses showed that taxa inhabiting terra firme forests were disproportionately likely to exhibit cross-river replacements, whereas dispersal-related morphological traits had limited explanatory power.

Main Conclusions: Local Amazonian bird taxon composition is primarily associated with variation in forest cover, habitat heterogeneity, and temperature seasonality, except across major rivers. The influence of river characteristics, particularly width and discharge, on bird taxon turnover highlights the role of rivers in structuring Amazonian biodiversity at local and global scales. Because these variables operate at different scales, taxon composition turnover patterns reflect a mosaic of ecological and geomorphological processes. These findings highlight the need for conservation strategies that account for bird habitat specialisation, river dynamics, and ongoing landscape changes.

Acoustic signal complexity varies widely in animals from single notes to highly sophisticated vocal displays. In birds, vocal complexity can evolve
as an honest signal of individual quality driven by sexual selection.
However, this hypothesis is rarely explored in conjunction with alternative drivers, including competition for ecological resources (social selection) and intra-group communication, both of which may favour increased signal complexity. Using Bayesian phylogenetic models, we test whether these alternative mechanisms predict the complexity of innate songs in 1,288 species of suboscine passerine birds, while accounting for ecological constraints on sound production, transmission and detection. We found that overall song complexity was reduced by sexual selection (estimated from mating systems), and declined with body size and vegetation density. Conversely, note count and song length increased in territorial species, particularly those using song to defend year-round territories during the non-breeding season. These findings challenge the common assumption that sexual selection is the main driver of increased signal complexity, and highlight the role of social selection via territorial competition as a factor increasing the temporal complexity of songs. Our results suggest that signal complexity depends on social, cultural and ecological contexts, reflecting a combination of multiple inter-related drivers and constraints.

This is the 25th supplement since publication of the seventh edition of the Check-list of North American Birds (American Ornithologists’ Union [AOU] 1998). It summarizes decisions made between April 25, 2024, and April 30, 2025, by the American Ornithological Society’s (AOS, formerly American Ornithologists’ Union) Committee on Classification and Nomenclature—North and Middle America. The Committee has continued to operate in the manner outlined in the 42nd Supplement (Banks et al. 2000). During the past year, Andrew W. Kratter left the committee.

Avian migration has long captured human interest, but the causes of the evolution of migration remain unclear due to limited study of the full spectrum of migratory strategies, including short‐distance and intratropical movements. We examine the climatic drivers of migration across the roughly 1300 species of suboscine birds, a group containing many intratropical migrants. Comparative analyses confirm that migratory behavior in temperate‐breeding suboscines evolves in association with temperature seasonality. The evolution of migration in the tropics, however, has a more complex association with climatic variables including precipitation and greenness seasonality. Projections under future climate scenarios show that suboscines will experience average lower temperature seasonality, potentially favoring the loss of migration, but higher precipitation seasonality, potentially favoring an increase in short‐distance migration. The divergent impacts of climate seasonality on the evolution of different migratory strategies highlight the complexity of climate‐movement associations and the challenges of projecting responses to climate change. We used comprehensive data on phylogenetics, migratory behavior, and past and future climate to test the climate‐migration associations in a large radiation of roughly 1300 suboscine passerine birds. We demonstrate that while longer‐distance migration is influenced overwhelmingly by seasonal fluctuations in temperature, tropical migrations have more complicated associations with the environment, including precipitation seasonality and fluctuations in vegetation greenness. Our future climate projections (2080–2100) suggest that long‐distance, temperate‐breeding migrants will experience conditions that might reduce migratory propensity on average, while tropical breeders might experience conditions that either increase or reduce migratory propensity.

Although species limits of North American birds are relatively well delineated, discrepancies among global lists identify species complexes that are subject to differences of opinion. As part of our work with the North American Classification Committee (NACC) of the American Ornithological Society, here we assess species limits in 11 such species complexes of North American birds: Spruce Grouse Canachites canadensis, Band-tailed Pigeon Patagioenas fasciata, Antillean Mango Anthracothorax dominicus, Greenish Puffleg Haplophaedia aureliae, Black Oystercatcher Haematopus bachmani, Hook-billed Kite Chondrohierax uncinatus, Sharp-shinned Hawk Accipiter striatus, Elegant Trogon Trogon elegans, American Three-toed Woodpecker Picoides dorsalis, Golden-olive Woodpecker Colaptes rubiginosus and Olive-throated Parakeet Eupsittula nana. We update information on the taxonomic history of these species, and recommend revised taxonomic treatments by using published works, analysis of museum specimens and citizen/ community science databases. This work can provide a foundation for future taxonomic research in these species complexes.

Excerpt: This is the 24th supplement since publication of the 7th edition of the Check-list of North American Birds (American Ornithologists’ Union [AOU] 1998). It summarizes decisions made between April 25, 2023 and April 30, 2024 by the American Ornithological Society’s (formerly American Ornithologists’ Union) Committee on Classification and Nomenclature—North and Middle America. The Committee has continued to operate in the manner outlined in the 42nd Supplement (Banks et al. 2000). During the past year Oscar Johnson joined the committee, and J. V. Remsen, Jr. and Kevin Winker left the committee.

Giant constrictor populations in Florida and the Caribbean have top-down impacts on prey that are still poorly understood. We report opportunistically documented predation events by introduced Boa imperator (Central American Boa Constrictor) on four species of native birds on St. Croix, U. S. Virgin Islands, compare these to more detailed dietary studies from Aruba, and encourage detailed studies of Boa imperator diet on St. Croix to better understand population and community-level impacts of predation by this invasive snake.