Seasonal dynamics of range expansion in South American thrushes

Species ranges are shaped by a variety of ecological and environmental factors that are inherently dynamic, fluctuating in response to climatic, biotic, and anthropogenic influences. Dispersal plays a key role in range shifts, allowing species to adapt to changing habitats and exploit new regions. In this study, we examined the dispersal processes of four thrush species (Turdus amaurochalinus, T. chiguanco, T. falcklandii and T. rufiventris) that have expanded their ranges in recent decades, with a focus on the interaction between spread and seasonal movements.

We collected eBird records from 2003 to 2023 to create heat maps that illustrate changes in densities of reported occurrences between seasons and over the years. We also evaluated how bioclimatic and land cover variables influenced the observed variations.

The four thrush species have shown significant range expansions, with initially distinct seasonal distributions, which became increasingly similar over time, leading to significant overlap in their breeding and non-breeding habitats. Temperature and precipitation associated with the presence of the four species varied over time and between seasons. Additionally, all four species exhibited shifts in habitat selection, both seasonally and across years.

The changes of range are likely driven by a combination of climate and land-use change, and resource availability, which also influence seasonal dispersal patterns. At the same time, thrushes perform very well in urban environments, which offer stable resources and may contribute to their reduction in seasonal movements. Overall, these findings highlight the dynamic nature of thrush species' range shifts and their adaptation to environmental changes.