Data from: Ongoing changes in the avifauna of La Selva Biological Station, Costa Rica: twenty-three years of Christmas Bird Counts

Tropical forest fragmentation influences community composition via differential species-level effects. Avian responses to fragmentation at La Selva Biological Station are, in part, responsible for the particular concern over the fate of understory insectivorous species. However, since the 1990s, much previously deforested land within and surrounding La Selva has reverted to forest, providing an opportunity to test hypotheses explaining ongoing avifaunal change. Analyses of 23 years (1989–2011) of Christmas Bird Counts reveal that 63 of 202 species have increased whereas 44 are declining, with declines occurring more rapidly than increases. Habitat association was an important predictor of population trends, as understory birds continue to decline whereas forest generalists increased. Our results differ from previous work in the tropics by revealing that, at La Selva, insectivores are not currently suffering greater declines than birds of other dietary guilds. Instead, body size was more strongly associated with population change than was diet, with smaller birds having more negative population trends than larger birds. These results suggest that we must consider additional hypotheses that may explain ongoing population declines of tropical birds. In particular, the associations between population trends and body size implicate physiological mechanisms influencing population change, which may result from direct or indirect consequences of changing climates.

热带森林片段化通过物种水平的差异化效应影响群落组成。拉塞尔瓦生物研究站（La Selva Biological Station）中鸟类对森林片段化的响应，是学界格外关注林下食虫鸟类生存命运的部分原因。然而自20世纪90年代起，该研究站及其周边大量此前已被砍伐的林地已恢复为森林，这为检验解释鸟类群落持续变化的相关假说提供了契机。对1989至2011年共23年的圣诞鸟类统计（Christmas Bird Counts）数据进行分析后发现，202个鸟类物种中63种种群数量上升，44种下降，且下降速率快于上升速率。生境关联是种群变化趋势的重要预测因子：林下鸟类种群仍持续下降，而森林广适性鸟类种群则呈上升态势。本研究结果与此前热带地区的相关研究存在差异：在拉塞尔瓦生物研究站，当前食虫鸟类的种群下降幅度并未高于其他食性类群的鸟类。相较食性，体型大小与种群变化的相关性更强，体型更小的鸟类种群趋势更趋负面。上述结果表明，我们亟需考量其他可解释热带鸟类种群持续下降的相关假说。具体而言，种群变化趋势与体型大小之间的关联，暗示了影响种群变化的生理机制，而这类机制可能源自气候变化带来的直接或间接效应。