Novel nesting behavior in a population of Sooty Fox Sparrows ( Passerella iliaca fuliginosa ): white feathers as the common lining material on Mandarte Island (British Columbia, Canada)

The use of feathers as avian nesting material serves multiple functions, including thermal insulation, signals of parental quality, and antimicrobial defense. However, the functional significance of feather use varies considerably across species and remains poorly understood in many taxa. We document a novel nest construction behavior in a resident population of Sooty Fox Sparrows (Passerella iliaca fuliginosa) on Mandarte Island, British Columbia, Canada. This population uses white gull down from the local colony of Glaucous-winged Gulls (Larus glaucescens) as nest lining, a behavior that has been observed since their colonization in 1975. To characterize the distribution of this behavior, we examined Fox Sparrow nest composition across Western North America using field observations, museum specimens, images from the Macaulay Library, and published literature. We quantified nesting material of 37 nests from the Western USA and Canada. Of these, we found only one additional instance of a Fox Sparrow nest lining that included white feathers, from St. Lazaria Island, Alaska (USA), which is also colonized by seabirds. Our findings reveal substantial intraspecific variation in Fox Sparrow nest construction and suggest that local ecological conditions, specifically seabird presence, may drive this behavioral divergence.