Stepping stones sustain bird connectivity at intermediate forest cover: implications for tropical forest conservation

Context. Theoretical frameworks propose that animals make movement decisions by balancing resource acquisition and movement costs. However, there remains a shortage of empirical studies testing these trade-offs in the context of functional movement thresholds—particularly for tropical forest birds. Objectives. We investigated how functional connectivity influences gap-crossing behavior of forest-dependent birds in fragmented landscapes, aiming to identify forest cover thresholds that balance movement costs and benefits. Methods. The study was conducted in 180 landscapes within the Atlantic Forest–Cerrado transition in southeastern Brazil, distributed across 15 regions spanning gradients of forest cover, stepping-stone density, and patch isolation. Using a modified fixed-point method, we recorded gap-crossing movements of birds and manually digitized trajectories in Google Earth. Landscape metrics were calculated at multiple spatial scales (50, 100, and 200 m). Results. We recorded 2,424 gap-crossing events from 110 species, 45.8% of which were forest dependent. Movements rarely exceeded 100 m from the nearest forest patch, though distances varied among trophic niches and lifestyles. Edge and patch density emerged as key predictors, with shorter and straighter trajectories in more fragmented landscapes. For frugivores, we detected an ecological threshold, with peak gap-crossing at ~14% forest cover (50 m scale). This pattern highlights the role of stepping stones in sustaining connectivity at intermediate forest cover, where movement costs and resource availability are balanced. Conclusions. Our findings provide a practical basis for conservation and restoration planning in fragmented tropical landscapes, emphasizing the importance of fine-scale structural elements to maintain functional connectivity and sustain key ecological functions such as seed dispersal.