Seasonal drivers of mixed-species flocks from tropical savannas: insights from the Pantanal wetland

Birds participate in different types of multi-species groups (e.g., mixed-species flocks) occurring from temperate to tropical forest and also savannas. However, the effects of seasonal variation in flocking dynamics and formation in tropical savannas, especially in seasonally flooded wetlands, have received comparatively less attention than tropical forests and temperate ecosystems. Because multi-species groups can reflect specific aspects of the bird community (e.g., temporal dynamics), the present study aims to describe the structure, organization, and seasonal variation of mixed-species flocks in the Pantanal, the world’s largest tropical wetland. During the dry, flood, and rainy seasons of 2019 and the flood season of 2020, we observed flocks within the natural mosaic of grassy-shrubby vegetation and patchy forest, and recorded the number of species, individuals (per species), and diversity. Seasonality had a strong influence on flocking structure and organization. During the rainy season, flocks had significantly fewer species, individuals, and diversity, whereas during the dry and flood seasons, flocks had more species, individuals, and diversity. Moreover, we suggest Polioptila dumicola and Casiornis rufus to act, respectively, as the nuclear and the sentinel species of flocks. These species were present in flocks during the year, but they were less frequent during the rainy season. The breeding period during the rainy season may negatively influence the formation of mixed-species flocks, whereas the access to food resources and predator protection may instead facilitate their formation in the dry and flood seasons, respectively. Our results help fill a knowledge gap on how mutualistic interactions among bird species are organized and vary in seasonal and heterogeneous environments, and also provide a starting point to subsequently understand the species role and the effects of human-induced environmental changes on mixed-species bird flock dynamics in the Pantanal and other world’s savannas and wetlands facing rapid landscape conversion.