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 through otherwise undisturbed Amazonian forest affected the movements and area requirements of understory birds that form mixed-species flocks. Differences in road maintenance led to two distinct treatments along the same road. Trees along the “closed” road formed a partial canopy connecting the two sides of the road, although the roadway itself was kept open. The “open” road was regularly maintained, making a complete opening 10–30 m wide.We followed 15 flocks, 5 each in interior forest, along the open road, and along the closed road. These flocks were led by Thamnomanes antshrikes, and each flock had a discreet, permanent territory. Flock territory size (mean 8.5 ha) did not differ among the three locations. The open road formed the territorial boundary for all five flocks, although birds moved within a few meters of the edge of the road. The 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 were 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 territory boundary. Similarly, as in forest gaps, successional change along the closed road produced suitable habitat for flocks. Although this suggests that roads are a trivial problem, we caution that this result applies only to narrow roads that are not accompanied by deforestation or other disturbance. Efectos de los Caminos sobre los Movimientos de Aves de Sotobosque en Parvadas Mixtas en la Amazonía Central Brasil Resumen: Los caminos en bosques tropicales crean perturbaciones lineares que tienen consecuencias desconocidas sobre las aves. Estudiamos como un camino angosto, poco usado, en bosque no perturbado afectó los movimientos y requerimientos de área de especies aves de sotobosque que forman parvadas mixtas. Las diferencias en el mantenimiento del camino condujeron a dos tratamientos distintos a lo largo del mismo. Los árboles en el camino “cerrado” formaron un dosel parcial que conectaba los dos lados del camino, aunque la vía se mantenía abierta. Al camino “abierto” se le daba mantenimiento regular, formando una abertura completa de 10–30 m de ancho. Seguimos a 15 parvadas, 5 en el interior del bosque, 5 en el camino abierto y 5 en el camino cerrado. Estas parvadas eran dirigidas por Thamnomanes , y cada parvada tenía un territorio permanente discreto. El tamaño del territorio (media 8.5 ha) no difirió en las tres locaciones. El camino abierto formaba el límite territorial de las cinco parvadas, aunque las aves se movían a unos metros de la orilla del camino. El camino cerrado fue una barrera menor; 2 de 5 parvadas utilizaron ambos lados del camino cerrado. Experimentos con cantos grabados mostraron que las parvadas atravesaban el camino cerrado para acercarse a vocalizaciones agonísticas. A lo largo del camino abierto, aun cuando las aves respondieron a las grabaciones agitándose y moviéndose al borde extremo de la vegetación, Paper submitted April 20, 2000; revised manuscript accepted November 22, 2000.