Combining genomic and field analyses to reveal migratory status in a burrowing owl population

Partial migration, in which migrant and resident members of a species spend part of the annual cycle in the same habitat, is a widely-occurring strategy among animal species. However, studies of this behavior are impeded by problems such as distinguishing migrant from resident individuals and detecting resident-migrant hybridization. We used a combination of genomic sequencing and bird banding to determine the migratory status of individuals in a declining population of western burrowing owls (Athene cunicularia hypugaea) in northern California. We banded individuals for four consecutive years in winter and summer. Each summer we surveyed for birds we had banded during previous winters. Using genomic analysis, we analyzed feathers from birds found in winter and summer to assess migratory status and interactions between winter and summer owls. The data showed a pattern of migration in which long-distance migratory birds were found in areas outside the breeding sites and joined resident owls at the breeding sites in the winter, but disappeared from these areas by the next breeding season. These results fit a pattern of partial migration in which long-distance migrants join resident birds in the winter. Although during breeding seasons we never observed any migrants that we banded in winter either within or outside the breeding sites, genomic analysis showed that some migrants stayed into the summer and bred with resident owls to produce hybrid offspring. This interaction brings different genetic material into the small resident population, a contribution that may benefit this declining population. This work demonstrates the value of combining genomic assessments of migratory status with field data collection to better characterize population structure and inform conservation actions.