Diversity and conservation of understory birds in the Tilaran mountains, Costa Rica

Using data from 10,726 captures of birds in mist nets, we document the high alpha and beta diversity of the understory avifauna in the Tilaran mountains of north-central Costa Rica. We grouped the capture data from 20 sampling sites into five Holdridge life zones that represent a gradient in elevation and seasonality of precipitation. Despite the limited size of the study area (200 km(2)) and the relatively small differences in elevation (700 to 1,700 m), major differences were found among each of the five life zones. We recorded a total of 235 species, with species richness increasing downslope on both the Pacific and Caribbean sides of the range. Rarefaction curves showed that the elevational zone of 650 to 750 m on the Caribbean side (Wet Premontane Transition Forest) was the most diverse per unit of mist-netting effort and that the Lower Montane Rain Forest on the continental divide (1,500 to 1,700 m) was the least diverse. Adjacent life zones had similarity indices ranging from 0.30 to 0.47; beta diversity for the region was 0.45. These measures are indicative of a high species turnover among our samples as delimited by Holdridge's life zone system, suggesting a concordance between the life zone system and the distribution of the region's diverse understory avifauna. Elevational migrants made up similar proportions of the avifauna in ail five zones, whereas the diversity of long-distance migrants was inversely related to elevation. The distribution of species of conservation concern did not track the pattern of overall diversity. Our results provide several lessons for land managers and present a strong case for the need to preserve large areas with elevational diversity in mountainous regions of the tropics.