Turdus flavipes altitudinal migration in the Atlantic Forest The Yellow-legged Thrush is a partial altitudinal migrant in the Atlantic Forest

Few studies on bird migration address altitudinal migration in South America, a regional-scale movement currently known for only nine species within the threatened Atlantic Forest biome. The Yellow-legged Thrush Turdus flavipes (YLTH) is one of such migrants thus far studied within a narrow altitudinal gradient and for which only fruits are pointed as the likely triggers of its seasonal movements. Here, we expand this knowledge by testing the hypotheses that arthropod and fleshy fruit availabilities, and climatic variables may all play roles in the seasonal abundance of this species along a broader altitudinal gradient (10-1,100 m asl) in the Atlantic Forest. We estimated YLTH density and its food resource availability in sites at three different altitudes. Besides supporting previous findings on the YLTH migratory behaviour, our findings reveal a novel and complex seasonal density variation of the species across altitudes, consistent with a partial altitudinal migration from higher altitudes. Aside from fruit availability, rainfall also partially influenced such behaviour, a new perspective for this species. Besides providing a broader picture for the altitudinal migration of YLTHs, we conclude by stressing the need for further studies on the understudied altitudinal migration system of birds within South America, including the assessment of the general validity of current hypotheses on the mechanisms underlying this interesting behaviour and its evolution.