Rapid and varied responses of songbirds to climate change in California coniferous forests

The global climate is changing faster than previously anticipated. Although scientists expect cumulatively deleterious impacts to birds and other wildlife, effects on individual species are likely more complex. The California Department of Fish and Wildlife has monitored > 100 songbirds across 37,600 km(2) of Northern California conifer forests for close to two decades to facilitate informed, science-based conservation planning. The study area represents 42% of all conifer forests in the state. Autonomous sound recorders were used to survey birds at 1065 randomly distributed sites from 2002 to 2016. The richness of Neotropical migrants declined below 1515 m (90% CI: 1150-1950 m) elevation whereas it increased above this threshold after controlling for changes in tree cover due to forestry and wildfire. This finding suggests an overall upward shift in Neotropical migrant distributions in response to an annual 0.037 degrees C (90% CI: 0.029-0.045 degrees C) increase in mean daily May temperature during the timespan. Residents and altitudinal migrants may be less vulnerable to increasing temperatures, conversely, as evidence of elevational shifting was much weaker or non-existent for them. Yet for individual species, there were both declines and increases in occupancy. Rapid and mixed population trends, in combination with elevational range shifts, suggest that songbirds vary widely in their capacity to adapt to climate change and other stressors. Conservation of structurally-complex and fire resilient forests above similar to 1500 m elevation is paramount in helping to buffer songbirds against rising temperatures. The expansion of biodiversity monitoring across large taxonomic, spatial, and temporal extents is vital to effective conservation planning.