Effect of Density Reduction on Overwinter Survival of Free-Ranging Mule Deer Fawns

Abstract

Understanding how overwinter survival of fawns changes as a function of density of deer is a critical relation for managing mule deer (Odocoileus hemionus) populations. We examined change in overwinter survival of fawns in response to intentional density reduction by radiotracking fawns on control and treatment areas. Deer density on the treatment area was lowered about 75%, mostly from antlerless harvests in December. There were 7 years of pretreatment data, 4 years of harvest, and 3 more years of posttreatment monitoring. Fawn survival rate on the treatment area during the 3 winters after density was lowered averaged 0.16 higher (P = 0.001) than the control area. After density was lowered, body mass of fawns on the treatment area in November-December averaged 0.8 kg more than the control (P < 0.001). A parallel decline in deer density began on the control area 2 years after initiation of the intentional density reduction on the treatment area. This decline was unexpected and the cause unknown, which left unanswered what the differences in fawn survival and body size between the 2 areas might have been if the control population remained high.