COS 113-2 - Patterns of sooty grouse occupancy and abundance in western Oregon

Abstract: Western North American forested landscapes have been increasingly altered by logging, wildfire, and urbanization, resulting in habitat loss and alteration. Understanding species’ responses aids management and conservation efforts. Adjusting hunting pressures for game species populations provides an additional management tool. Sooty grouse (Dendragapus fuliginosus) is a large game bird that occupies montane forests in the Pacific Northwest. The NA Breeding Bird Survey indicates population declines for this species (-1.8%/yr, 1968-2015). However, they are inadequately represented by BBS due to sampling coverage, and low density and detectability. Advances in quantitative ecology have led to the widespread application of hierarchical models to simultaneously address both the observation and ecological processes associated with monitoring wildlife populations. From 2011-2021, we surveyed for sooty grouse at 113 sites across western Oregon representing four ecoregions: Coast Range, Klamath, East Cascades, and West Cascades. We compared trend estimates using occupancy and abundance models, and different data formats (spatial versus temporal replicates). Trend estimates differed by analysis method and data format, with abundance models showing more declines, and space for time methods showing higher detection. Given the variability in the trend estimates, we used simulated data (where truth is known) to test which dataset format and modeling approach provide the most accurate trend estimate. Occupancy models did not perform well estimating trends on simulated data. However, the abundance-based models both had fairly accurate trend estimates, with the binomial mixture model having the most precise trend estimates. Trend estimates from the binomial mixture models indicate that sooty grouse populations are declining significantly (from -2.98 to -4.76 %/year depending on the scale of the sampling unit used.) Detection probability estimates also varied across models and sampling units, with the binomial mixture models producing the highest estimates (0.51). Next steps include exploration of the effects of environmental factors on occupancy, abundance, and their dynamics. Preliminary results indicated that sooty grouse are more likely to occupy steep slopes and high elevations, although the elevation effect varied by ecoregion.