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 mountai ns of north-central Costa Rica. We grouped the capture data from 20 samplin g sites into five Holdridge life zones that represent a gradient in elevati on 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. W e recorded a total of 235 species, with species richness increasing downslo pe 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 (We t Premontane Transition Forest) was the most diverse per unit of mist-netti ng 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 similarit y 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 sample s as delimited by Holdridge's life zone system, suggesting a concordance be tween the life zone system and the distribution of the region's diverse und erstory avifauna. Elevational migrants made up similar proportions of the a vifauna in ail five zones, whereas the diversity of long-distance migrants was inversely related to elevation. The distribution of species of conserva tion concern did not track the pattern of overall diversity. Our results pr ovide several lessons for land managers and present a strong case for the n eed to preserve large areas with elevational diversity in mountainous regio ns of the tropics.