Status of Nearctic and Austral migratory birds in the Brazilian state of Acre, Southwestern Amazon: insights from citizen science contributions

Migratory birds undertake seasonal movements between breeding and non-breeding areas in search of favorable climatic and food conditions. In South America, two major migratory systems are recognized: Nearctic and Austral. The Brazilian state of Acre, located in the southwestern Amazon, serves as a strategic stopover for both groups, yet increasing anthropogenic pressures threaten these corridors. Understanding migration dynamics is therefore essential for conservation planning. This study analyzed temporal patterns of Nearctic and Austral migratory bird species in Acre and assessed their degree of temporal overlap. Data was obtained from citizen science platforms, with photographic records filtered by species and month and visualized through heatmaps. Records were classified into reproductive and non-reproductive months, and differences tested using the non-parametric Mann-Whitney U test (p < 0.05). A total of 1554 images representing 61 migratory species (31 Nearctic and 30 Austral) were analyzed. Solitary Sandpiper (Tringa solitaria) showed the highest frequency of records among Nearctic migrants, whereas Vermilion Flycatcher (Pyrocephalus rubinus) prevailed within the Austral group. Phenological analysis revealed distinct seasonal windows: Nearctic migrants occurred mainly from September to March, whereas Austral migrants were recorded from March to September, with minimal overlap. Some species, such as Chivi Vireo (Vireo chivi) and Crowned Slaty Flycatcher (Empidonomus aurantioatrocristatus), were observed nearly year-round, suggesting complex strategies including partial migration or coexistence of resident and migratory populations. These findings highlight Acre’s role as a dynamic migratory corridor and demonstrate the value of citizen science in monitoring avian biodiversity in remote regions, reinforcing its importance for conservation strategies.