Fine-scale GPS tracking reveals variable stopover duration and use of multiple winter sites in Pooecetes gramineus affinis (Oregon Vesper Sparrow)

Abstract Recent improvements in tracking technology have enabled researchers to better understand nonbreeding season movements in migratory birds. Pooecetes gramineus affinis (Oregon Vesper Sparrow) is a grassland-specialist temperate migrant of conservation concern. Knowledge of migratory stopover and wintering sites, and potential threats originating during the nonbreeding season, are key information gaps needed to target conservation actions. Our objectives were to identify locations used during fall migration, winter, and spring migration, including land cover and ownership; and describe migration phenology and stopover patterns. In 2020–2024, we deployed archival GPS tags on 42 adult P. g. affinis in the Rogue Basin and Willamette Valley, Oregon, USA. Novel geospatial data retrieved from 10 males revealed a variety of migration strategies, including a combination of rest-roost (1 day) and multi-day (2–12 days) stopovers. Some individuals migrated rapidly in fall, reaching initial wintering sites in 2–6 days, while others used 1–3 secondary wintering sites for up to 2 months before arrival at final wintering sites in mid-December to early January. Lengthy stops blurred the lines between migratory stopovers and multiple wintering sites, a phenomenon not previously described in a short-distance temperate migrant. Wintering sites ranged from 34°56'28.2"N to 39°33'50.4"N in latitude, mostly in the Sierra Nevada foothills of central and southern California, USA. The majority of nonbreeding locations (78.7%) were grassland land cover; 7.9% were on cultivated croplands. Only 2.2% of nonbreeding season locations were on protected lands. Our diverse results for fall stopover duration, wintering latitude and number of wintering sites, and potential examples of facultative migration behaviors, suggest that our knowledge of the range of P. g. affinis nonbreeding season movements is still incomplete. This study is an important first step towards delineating the subspecies-specific winter range, and the ability to integrate full life-cycle spatiotemporal information into targeted conservation efforts.