Assessing bird assemblages along an urban gradient in a Caribbean island (Margarita, Venezuela)

Several studies have shown that urbanization usually leads to severe biotic homogenizing, i.e. the local extirpation of many native species and the expansion to regional scales of a small group of urban-adaptors, some of them exotics. Margarita Island (Venezuela) is a tropical Caribbean island that has undergone an accelerated urban development in the last 40 years. Because the island also has a high bird diversity with several endemic and threatened species, we evaluated the effect of urban development on bird species richness, assemblage structure, seasonal changes and feeding guilds. We defined an urban gradient from areas with high vegetation cover (remnant woodlands) through areas with intermediate vegetation (traditional towns) to areas with next to no vegetation (recent suburbs). Each experimental unit was replicated 3 times and birds were surveyed during the dry and rainy seasons. Richness decreased as urbanization increased, being severely depleted in the recent suburbs. The bird assemblage consisted of native species, including six endemic sub-species, but only one exotic, Columba livia. There were no seasonal changes in assemblage structure. We identified the species most tolerant and most sensitive to urbanization. Omnivorous birds were common along the gradient and granivores were also tolerant to urban development. Specialized insectivores and frugivores were the most negatively affected groups. The considerable amount of native woodland and other vegetation present in the traditional towns we evaluated, and their proximity to natural protected areas, favors the persistence of native bird species in these urban areas on Margarita Island.