Twenty-year changes in riparian bird communities of east-central Oregon

Abstract

Riparian zones in the semi-arid West are disproportionately important habitats for both breeding birds and agricultural operations. Despite growing interest in studying avian–habitat relationships to inform land management decisions, few studies have described temporal changes in riparian bird communities. We compared indices of avian abundance and diversity from 3 streamside vegetation associations in east-central Oregon during May–June 2014 with indices from 1993–1994. Our objectives were to identify patterns of change in the avian community with a focus on riparian shrub-dependent species, re-examine previously reported relationships between avian abundance and vegetation volume, and identify possible causes of declines in bird abundance and diversity. We combined historical field protocols to survey birds and measure riparian vegetation with modern analytical techniques. We found few differences in species richness between study periods but documented approximately 30% species turnover. Increases in diversity were driven by increased detections of grassland and wetland bird species and increased evenness due to precipitous declines in detections for 2 of 3 riparian shrub–dependent focal species (i.e., Yellow Warbler [Setophaga petechia] and Willow Flycatcher [Empidonax traillii]). Detections of Song Sparrows (Melospiza melodia), our third focal species, declined by a smaller margin. Large changes in detections of riparian shrub–dependent species did not reflect trends in mesic shrub cover or volume, which had been identified as likely drivers of obligate species abundance. Focal species' declines reflected regional Breeding Bird Survey trends, corroborating our finding that their declines were not a result of changes in local site conditions. Our findings suggest that managing lands to increase wetness and extent of riparian zones can be beneficial for grassland and wetland bird species. However, managing for riparian shrub cover or volume, important metrics of grazing intensity and riparian system health, may be insufficient to conserve riparian shrub–dependent birds because other unidentified local and/or regional factors are likely contributing to habitat suitability of riparian shrub–dependent birds.