Wintering Artemisiospiza sparrows: patterns of segregation between Sagebrush Sparrow ( A. nevadensis ) and Mojave Bell's Sparrow ( A. belli canescens ) across Lower Colorado Desert vegetation assemblages, with evidence for differential migration in Mojave Bell's Sparrow

Bell’s Sparrows ( Artemisiospiza belli ) have only recently been recognized as distinct from Sagebrush Sparrows ( A. nevadensis ), and the “Mojave” subspecies ( A. b. canescens ) shares an overlapping wintering distribution with Sagebrush Sparrow in the Mojave and Sonoran deserts of southeastern California and western Arizona. We lack understanding of the two species’ respective wintering habitat preferences and the degree to which they interact or segregate on their wintering grounds due to the difficulty in separating them in the field and to their previous classification as one species. We captured and sampled 74 Artemisiospiza sparrows from 5 sites across western Arizona, supporting field identifications with genetic analyses of mitochondrial DNA to confirm species and molecular sexing of sampled individuals. Bell’s Sparrows and Sagebrush Sparrows segregated into different habitat types across our study area, with only one species detected at four of five study sites. Bell’s Sparrows comprised 82% (n = 33) of Artemisiospiza sparrows captured at the 5th site at Robbins Butte. Broadly, Sagebrush Sparrows were found in more upland, well-drained locations that were less vegetated with xerophytic scrub. Bell’s Sparrows were found in more vegetated locations with halophytic Mojave seablite ( Suaeda nigra ) and saltbush ( Atriplex ) adjacent to mesquite and tamarisk woodlands. Bell’s Sparrow sex ratios were significantly female-biased (binomial test: n = 56, observed k = 48 females, expected k = 28 females for assumed p = 0.5, Pr [ k < = 8 or k = > 48] < 0.0001; 95% CI = 0.369 – 0.631 for assumed p = 0.5) at Fort Mohave and Robbins Butte, the 2 sites where Bell’s Sparrows were found. Our observed sex-ratios and well-documented year-round presence of Bell’s Sparrows on and near the breeding grounds suggest that Bell’s Sparrow males and females employ different migration strategies, a phenomenon not previously documented for this taxon.