RESPONSES OF UNDERSTORY BIRDS TO ANTHROPOGENIC EDGES IN A NEOTROPICALMONTANE FOREST

In a Neotropical montane forest in southwestern Colombia, we investiga ted how the distribution of understory birds changed from forest edge to forest interior (0-10 m, 30-40 m, 60-70 m, and 190-200 m from the e dge) and how these changes were influenced by edge age (three old [>40 yr] and three young [<12 yr] edges) and month sampled. Capture rates of frugivores were highest both at the forest edge (0-10 m) and forest interior (190-200 m); for insectivores, capture rates were highest at the forest interior; for nectarivores, they Here highest at the fores t edge. Distance, edge age, and month interacted in various ways. Frug ivores were more abundant at the forest interior than at the edge duri ng the dry months. Insectivores were more abundant at new edges than a t old edges during the wet months. Seventeen out of 25 abundant specie s (greater than or equal to 21 captures), including the Tangara spp. a ssemblage, exhibited a non-uniform distribution, showing either an inc rease or decrease from forest edge to forest interior, or bimodal dist ributions. Extremely sparse species (one capture) were found more ofte n than expected near the forest edge (0-10 m). Edges resulting from la rge-scale, anthropogenic disturbances influenced the distribution of u nderstory birds in complex ways. Significant interactions between dist ance, month, and edge age suggest that ''edge effects'' change over va rious temporal scales. Instead of emphasizing ''depth'' of ''edge effe cts,'' future studies should emphasize edge dynamics, i.e., how proces ses taking place at edges change over time, and how organisms can modi fy any ''edge effect.'' In particular, changes in the distribution of frugivores suggest that seed dispersal may be influenced by the presen ce of edges, leading to changes in the structure and location of edges through time. This might be particularly true in our study area, wher e transient corn fields, pastures, and second-growth areas of various ages are embedded in a forest matrix.