20-Year Trends for Riparian Birds in Utah

Riparian systems make up \u3c 1% of land area in Utah, but riparian systems are arguably the habitat in greatest conservation need. Over 70% of Utah’s birds use riparian habitat, yet little was known about population status and trends of these species when Utah DWR initiated a 20-year riparian monitoring project. To get baseline data and evaluate trends, Utah DWR conducted point counts at 37 riparian sites across Utah from 1992 to 2011. I estimated statewide density of 38 species for each year of the study using the Conventional Distance Sampling and Multiple Covariate Distance Sampling (MCDS) engines in DISTANCE. We estimated trends using a Bayesian framework that provides estimated probability of a 25% change in density over 25 years. The variable affecting detection that appeared most often in the best-supported MCDS models was ‘Year.’ Yellow warbler (Setophaga petechia) had the greatest density with 4.5/ha (95% CI: 4.3 - 4.6), followed by Spotted Towhee (Pipilo maculatus) with 1.6ha (95% CI: 1.6 - 1.7) and Warbling Vireo (Vireo gilvus) with 1.6/ ha (95% CI: 1.6 - 1.7). Although there were annual fluctuations in density, Bayesian trend analyses indicated only 4 species had strong (0.7 \u3c P \u3c 0.9) or very strong (P ≥ 0.9) evidence of a 25% decline over 25 years; whereas 9 species had evidence of an increasing trend. Species with declining trends were Black-billed Magpie (Pica hudsonia), American Goldfinch (Spinus tristis), American Robin (Turdus migratorius), and House Wren (Troglodytes aedon). These trends were generally consistent with Breeding Bird Survey trends for Utah during the same time period. It is encouraging that riparian bird populations in Utah are not declining at alarming rates. Nonetheless, protecting and restoring riparian systems remains important for keeping population trends steady in the face of drought, fire, exotic species, and human activity, all of which may be exacerbated by climate change