Century-long shifts in body size and proportions of lowland rainforest birds

Climate change is having profound impacts on biodiversity, with notable effects on the distribution, and morphology of species. Many studies have documented changes in body size and shape in response to changing climatic conditions, with a widespread trend of decreasing body size observed in temperate regions. Despite the growing body of evidence on morphological changes in birds associated with climate change, no study has assessed temporal trends in body size and shape over a century in tropical lowland areas. Using data from museum specimens, we examined morphological trends of 23 bird species in Barbacoas, Nariño, a lowland site near the Pacific coast of Colombia, over the past 109 years. Our results revealed mixed trends in body size. Hummingbirds exhibited an overall decrease in body size, with strong evidence of reduction in Florisuga mellivora (White-necked Jacobin). In contrast, non-hummingbirds showed an average increase in body size, with strong evidence of increases in three species: Microbates cinereiventris (Tawny-faced Gnatwren), Myiobius barbatus (Whiskered flycatcher), and Myrmotherula pacifica (Pacific Antwren); and a notable decrease in Sporophila nigricollis (Yellow-bellied Seedeater). Wing length decreased across most species, while tail length increased in 61% of them (average increase of 4.4%), possibly in response to changes in forest structure associated with climate change. Changes in bill morphology were limited to an increase in maxilla depth for ten species, with no consistent changes in total bill length, width, or maxilla area. These results underscore the complexity of morphological responses potentially linked to climate change and the need for continued research to understand the factors and mechanisms driving these changes, particularly in tropical bird communities.