Consistent, long-term ecological data is relatively rare, yet critical for examining population trends over time. These datasets are particularly important for investigating species’ responses to long-term perturbations such as climate change. For instance, species are predicted to shift up-slope in relation to a warming climate. We examined the long-term (1999-2011) population trends of common songbird species within the Hubbard Brook Experimental Forest, New Hampshire. This is a mountainous, forested landscape that spans an 821-m elevation gradient and has been relatively undisturbed over the course of this study. We used point count data on singing males collected three times each year throughout the breeding season at 371 sites across the 3160-ha watershed. Using dynamic occupancy models, we calculated year-dependent settlement and vacancy rates to derive trends in occupancy probability, that accounted for imperfect detection. We then tested for distributional shifts across the elevational gradient over time.
Results/Conclusions
Six of 16 species showed significant linear declines in valley-wide occupancy over this period (p<0.05). Rates of decline ranged from 0.9-3.5% per year. These declines appeared to cut across species traits such as migratory strategy and vegetation association. Two species showed slight, but non-significant, population increases, 13 tended to be decreasing, and one species showed little change in occupancy. We found some evidence for elevational range shifts, but interestingly, these shifts were both upwards and downwards in elevation, indicating that perhaps climate may not be the key factor in driving changes in distributions.