Effects of roads on movements by understory birds in mixed-species flocks in central Amazonian Brazil

Roads through tropical forest create linear disturbances that have unknown consequences for forest birds. We studied how a narrow, rarely used road th rough otherwise undisturbed Amazonian forest affected the movements and are a requirements of understory birds that form mixed-species flocks. Differen ces in road maintenance led to two distinct treatments along the same road. Trees along the "closed" road formed a partial canopy connecting the two s ides of the road, although the roadway itself was kept open. The "open" roa d was regularly maintained, making a complete opening 10-30 m wide. We foll owed 15 flocks, 5 each in interior forest, along the open road, and along t he closed road. These flocks were led by Thamnomanes antshrikes, and each f lock had a discreet, permanent territory. Flock territory size (mean = 8.5 ba) did not differ among the three locations. The open road formed the terr itorial boundary for all five flocks, although birds moved within a few met ers of the edge of the road. Tbe closed road was less of a barrier: 2 of 5 flocks used both sides of the closed road. Playback experiments showed that flocks readily crossed the closed road to approach agonistic vocalizations . Along the open road, even though birds responded to playback by becoming agitated and moving to the extreme edge of the roadside vegetation, they we re less likely to cross the road and did so only after a longer duration of playback. Our results suggest that flocks respond to a road as they would to a long linear gap. They use the vegetation along the edges of the road, but because they are unwilling to cross the open area, it becomes a flock t erritory boundary. Similarly, as in forest gaps, successional change along the closed road produced suitable habitat for flocks. Although this suggest s that roads are a trivial problem, we caution that this result applies onl y to narrow roads that are not accompanied by deforestation or other distur bance.