Bird Communities in Transition: The Lago Guri Islands

We report on the bird communities of a set of 12 7-yr-old forested land-bridge islands in Lago Guri, a 4300 km2 hydroelectric impoundment in the State of Bolivar, Venezuela. Birds were censused on all islands and at mainland control sites by spot mapping in 1993, and via point counts in 1995. Instead of orderly “nested sets” of species on landmasses of graded size, the species composition of small (≈1 ha) and medium (11–12 ha) islands was highly variable. Spot mapping substantiated the occurrence of 58 species of forest-nesting birds, collectively, on the 11 small and medium islands, which supported means of only 9 and 12 resident species, respectively. No species was found on all islands, and only five species were found on as many as 7 of the 11 small and medium islands. The mean number of islands per resident species was 3.0 for the 11 islands. Transients of many species were detected on all islands, but were more frequent on near (≤0.5 km from a larger landmass) than on far islands (≥1.0 km from a larger landmass). A large majority of the populations on the 1-ha islands consisted of a single pair, implying that few such populations could have survived since isolation without new colonizations to replenish individuals lost to dispersal and mortality. Notwithstanding low species numbers, both census methods indicated that avian densities were approximately twice as high on the 1-ha islands as on the mainland. In contrast, two of three medium-sized islands supported anomalously low densities of approximately one-fifth the mainland level. Both islands retain relict populations of capuchin monkeys (Cebus olivaceus). One hundred percent of artificial nests set out on one of these islands were raided, whereas no more than 30% of nests were lost on any of the other islands or on the mainland. For small and medium islands, we conclude that the founding communities present when the waters of Lago Guri reached their final level have already collapsed and been reconstituted. Species loss on some islands may have been accelerated by relict populations of predators, such as capuchins. Changes in composition thus appear to have been driven by a combination of biological (nest predation) and stochastic processes (high turnover).