Landbird population trends in parks of the Sierra Nevada Network: 2011–2019 synthesis

Ray, Chris; Wilkerson, Robert; Siegel, Rodney; Holmgren, Mandy; Haultain, Sylvia

doi:10.36967/nrr-2293643

Long-term monitoring of landbird populations within the National Park Service (NPS) Sierra Nevada Inventory and Monitoring Network (SIEN) began in 2011, with the goal of detecting trends to inform the conservation and management of landbirds and their habitats. Here, we used 2011-2019 monitoring data from 2408 point-count stations to analyze landbird population trends in several SIEN parks: Devils Postpile National Monument, Sequoia and Kings Canyon National Parks and Yosemite National Park. Survey crews detected 166 species within these parks during the monitoring period, including 134 detected during formal counts and 32 detected during other crew activities. Point-count data, which included information on potential covariates of abundance as well as distance and time to detection of each bird, were combined in a Bayesian hierarchical modeling framework (Kéry and Schaub 2012) to derive trends in population density for 62 species with sufficient counts to support these models. The number of species declining in density over the monitoring period (up to ten species) was more than matched by the number increasing in density over the same period (up to 14 species). Most populations varied directly with spring temperature and almost half varied inversely with precipitation-as-snow. These relationships, combined with a series of dry winters and mild spring temperatures in these parks, might have helped stabilize landbird densities during the monitoring period. Population density also varied with elevation for many species, but evidence was mixed for temporal trends in population density with elevation. These results align with previous analyses of similar point-count data from national parks in the Pacific Northwest’s North Coast and Cascades Network which suggested that landbird populations within parks might be exhibiting more positive population trends than those reported for the surrounding region. A qualitative comparison of our results for SIEN parks with California-wide results from the Breeding Bird Survey suggests that several species have fared better in these parks than across the state over the past decade. Our results also suggest reduced snow cover may play an important role in the recent, perhaps ephemeral, stability of landbirds within Sierra Nevada national parks. The timing of snow melt might affect breeding site success and bird densities in the following year, such that recent warming and snow drought has led to higher recruitment in some populations. If the depth and duration of snow cover continues to decline, however, negative impacts of drought on primary productivity and breeding habitat quality could lead eventually to negative effects on avian abundance. Indeed, for most species we found a negative effect of year on abundance, indicating an underlying temporal decline that was not completely ameliorated by the apparently favorable climate during recent years in these parks. Another cause for concern is an apparent disparity in trends between parks: negative trends in population growth were observed predominantly in Sequoia and Kings Canyon National Parks, the southernmost parks in this analysis. Parks with a range of relatively natural habitats distributed at a variety of elevations might offer refuge for landbird species sensitive to changes in climate. Although we did not find strong evidence that bird density is shifting by elevation within these parks over the past decade, our study design is stratified by elevation to facilitate estimation of any future elevational response in landbird densities, and to infer distributional shifts from differential changes in density across elevational strata.

Landbird population trends in parks of the Sierra Nevada Network: 2011–2019 synthesis

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