Trends in riparian bird populations and habitat conditions along the Madison and Missouri Rivers from 2004-2015
Long-term assessments of the distribution and abundance of populations are central to evaluating the potential effects of human activities on wildlife. Since 2004, the University of Montana (UM), with support from Northwestern Energy (formerly PPL Montana) and Bureau of Land Management (BLM), has monitored bird populations and riparian vegetation along over 500 miles of the Madison and Missouri Rivers. This program meets Northwestern Energy’s Federal Energy Regulatory Commission (FERC) license requirements for hydroelectric operations on the river system by monitoring system-wide bird distributions and population trends as an indicator of wildlife habitat conditions, identifying critical habitats for wildlife based on spatial analysis of bird habitat use, measuring bird and vegetation responses to management efforts, and evaluating project benefits for wildlife to inform future project priorities within the system. This report summarizes analyses of bird population and vegetation trends across four annual surveys events between 2004 and 2015, and provides recommendations for future efforts. To date, our monitoring efforts have resulted in 1,276 point-count surveys and detection of 23,723 individual birds of 139 species, including seven BLM Sensitive species, 16 Montana Species of Concern, and 14 species ranked as a continental priority by Partner’s in Flight. For 33 of those bird species, we obtained sample sizes that were sufficient to generate precise annual estimates of density with the use of distance sampling methods.
We found statistically significant declines in densities of 11 bird species and increases in densities of six species across time. Declining species have a broad range of nesting and foraging requirements, and include both generalists and riparian specialists. Patterns we observed largely correspond to long-term trends documented across the region based on the North American Breeding Bird Survey (BBS; Sauer et al 2014). Such similarities suggest the drivers of population declines along the river system are likely operating across large spatial scales. We also documented significant changes in riparian habitat conditions since 2004, which are likely influencing habitat suitability for bird populations. Those changes include aging riparian forests, declining shrub cover, and increases in non-native plant species.
A review of bird data gathered since 2004 indicates that current monitoring effort (i.e. 1 visit to 223 sample points) is sufficient to generate precise estimates of density for many riparian species that occur along the Madison and Missouri Rivers of Montana. Those species include both common and uncommon species and species of significant management and conservation interest. Nonetheless, greater sampling effort is required to obtain more precise estimates of densities of rare species, which may be of greater management and conservation concern. Based on analyses of the tradeoff between sampling frequency and power to detect population changes, we recommend conducting monitoring every other year to effectively monitor population trends of birds in this system. This program provides a direct measure of the status of wildlife within riparian areas across a large stretch of the Madison and Missouri Rivers, and is currently the only monitoring effort targeting riparian birds in Montana. Although monitoring spans over 10 years, our findings should be viewed cautiously, since inferences are based on surveys during only four years. Future monitoring will build on this dataset, providing a more complete picture of changes in wildlife populations over time.