Oscar Johnson

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.

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.

Understanding the factors that govern variation in genetic structure across species is key to the study of speciation and population genetics. Genetic structure has been linked to several aspects of life history, such as foraging strategy, habitat association, migration distance, and dispersal ability, all of which might influence dispersal and gene flow. Comparative studies of population genetic data from species with differing life histories provide opportunities to tease apart the role of dispersal in shaping gene flow and population genetic structure. Here, we examine population genetic data from sets of bird species specialized on a series of Amazonian habitat types hypothesized to filter for species with dramatically different dispersal abilities: stable upland forest, dynamic floodplain forest, and highly dynamic riverine islands. Using genome-wide markers, we show that habitat type has a significant effect on population genetic structure, with species in upland forest, floodplain forest, and riverine islands exhibiting progressively lower levels of structure. Although morphological traits used as proxies for individual-level dispersal ability did not explain this pattern, population genetic measures of gene flow are elevated in species from more dynamic riverine habitats. Our results suggest that the habitat in which a species occurs drives the degree of population genetic structuring via its impact on long-term fluctuations in levels of gene flow, with species in highly dynamic habitats having particularly elevated gene flow. These differences in genetic variation across taxa specialized in distinct habitats may lead to disparate responses to environmental change or habitat-specific diversification dynamics over evolutionary time scales.

Do related populations that are separated by barriers predictably evolve differences from one another over time, or is such divergence idiosyncratic and unpredictable? We test these alternatives by investigating patterns of trait evolution for 54 sister pairs of Andean forest birds that live in similar environments on either side of the arid Maranon Gap, a strong dispersal barrier for humid montane species. We measured divergence in both sexual (song and plumage) and ecological (beak size and beak shape) traits. Sexual traits evolve in a clock-like fashion, with trait divergence positively correlated with genetic distance (r = 0.6-0.7). In contrast, divergence in ecological traits is uncorrelated or only loosely correlated with genetic distance (r = 0.0-0.3). Thus, for geographically isolated Andean montane forest birds that live in similar environments, divergence is predictable in sexual traits, but not for ecological traits. This means that sexual trait divergence occurs independently of adaptive ecological divergence within the mega-diverse tropical Andean avifauna. Last, we show that variation in genetic divergence across a biogeographic barrier is associated with traits that are proxies for species' opportunities for dispersal (low elevation limit and elevational niche breadth), but not with traits that are proxies for species' dispersal abilities (hand-wing index and foraging strata).

We documented a Northern Mockingbird (Mimus polyglottos) with a bill deformity consistent with Avian Keratin Disorder (AKD) from West Texas. However, bill deformities remain underreported in the literature and, as such, we also reviewed photographs deposited in participatory science databases for the genus Mimus to estimate rates of bill deformities for the group. We found individuals with deformed bills in Chilean (M. thenca), Galapagos (M. parvulus), Bahama (M. saturninus), Patagonian (M. patagonicus), Northern (M. polyglottos), Whitebanded (M. triurus), and Tropical (M. gilvus) mockingbirds. Within the genus Mimus, we found that Chilean Mockingbird had the highest proportion of individuals that showed evidence of a bill deformity (2.16%).