Reversing songbird decline: The effect of food supplementation on the overwinter survival of the dark-eyed junco (Junco hyemalis)

Abstract

Decline of songbirds is widespread in North America. Habitat loss, fragmentation, and degradation are hypothesized to be a primary cause of avian decline. The Dark-eyed Junco (Junco hyemalis), a granivorous, migratory sparrow, has declined annually since 1966. Juncos nest in the boreal region; boreal species have become particularly imperiled in recent decades. Decline of this ubiquitous bird is alarming and important, as it indicates widespread habitat degradation. It is possible that lessened food availability during the nonbreeding season can limit junco populations. We introduce an experiment that indicates supplemental feeding increases the survival of juncos during the overwinter period. Using mark-resight methods, we captured and uniquely color-banded 208 juncos, and found that supplemental feeding significantly increased site fidelity in three winters (2011, 2012, and 2013). This effect was exacerbated in cold winters with high average daily snow-accumulation. However, it was unclear whether juncos absent from unsupplemented sites represented within-winter dispersal or mortality. In the winter of 2014, we attached VHF radio-transmitters to 29 unsupplemented juncos to ascertain the fate of birds absent during resighting periods. Utilizing the mark-resight methods employed in previous years, and the addition of radio-transmitters, we found that movement beyond the study area was minimal. Juncos occupied the home range previously described by literature. Of the radio-tagged birds, 27.5% were not resighted during observation periods, but located and confirmed alive via radiotelemetry. A correction factor for within-winter dispersal was applied to previous years data and the effect of food supplementation remained statistically significant. Supplemental feeding during the wintering period may elevate population recruitment by increasing winter survival, migratory success, and reproductive fitness. Therefore, large-scale overwinter food supplementation may reverse decline.