Divergent species richness and vocal behavior in avian migratory guilds along an elevational gradient

Climate change is expected to disrupt the distribution and behavior of montane birds. Monitoring these impacts will be essential because the ecological effects of climate change are likely to be complex. Hiking trails that traverse montane regions provide an opportunity to efficiently survey bird diversity along elevation and other ecological gradients, and these data can be used to model climate-related vulnerabilities of avian communities. In 2010, we surveyed a 697-km segment of the Pacific Crest Trail in northern California, USA. We conducted point counts of birds at 404 sites during the breeding season when birds were readily detected by song and other vocalizations. To bolster our sampling effort, we left automated recorders at approximately half of the sites to make recordings for later interpretation of bird vocalizations. Using a multispecies occupancy model, we investigated how relationships between richness and elevation and between vocal activity and daily temperature differed among three migratory guilds-residents, altitudinal migrants, and Neotropical migrants. We found that richness decreased with increasing elevation for residents and Neotropical migrants, whereas it increased for altitudinal migrants. As temperature increased, residents and altitudinal migrants curtailed their vocal activity, but Neotropical migrants did not reduce vocal activity even though this behavior is energetically expensive on hot days. We also found that total species within each of three elevation zones was greatest at middle elevations (1200-1900 m). Altogether, these findings suggest that as global temperature rises there may be greater competition among birds previously separated by elevation and that Neotropical migrants may be at greater risk of heat stress during the breeding season. Furthermore, the conservation of structurally complex, middle-elevation forests could provide birds a refugium to the impacts of climate change.