Radiotracking refines the role of food supplementation on overwinter survival of the Dark-eyed Junco (Junco hyemalis)

Resource availability, particularly food, can limit the size of bird populations during the nonbreeding season. Resident and short-distance migrant birds, wintering in colder environments, should show strong effects of food limitation on winter survival. Here, we introduce a novel experiment testing for the effect of supplemental feeding on the winter survival of the Dark-eyed Junco (Junco hyemalis), a short- to medium-distance migrant in North America. Color-ringed juncos were monitored by resighting in the winters of 2011, 2012, and 2013. Winter supplemental feeding significantly increased junco survival within winters. However, it is possible that juncos not resighted underwent within-winter dispersal rather than died. Thus, in the winter of 2014, very high frequency radio transmitters were attached to 29 juncos from unsupplemented sites to ascertain the fate of birds not resighted. We found that movement beyond the study area was minimal and most juncos maintained a discrete home range. Of the radiotagged birds, 27.5% were not resighted during observation periods, but were located and confirmed alive via radiotracking. A correction factor for within-winter dispersal was applied to previous years’ data and the effect of food supplementation remained statistically significant for two of the three winters; the trend was the same, but not statistically significant during the warmest winter. The present results contribute to a developing conceptual model of migration-specific effects of winter food limitation. Dark-eyed Juncos are an iconic backyard birdfeeder species that have shown a steady population decline for decades. Supplemental feeding during the wintering period may regularly elevate population recruitment by increasing winter survival in avian species that winter in thermally challenging environments.