Variation in the herd composition counts of sika deer

Herd composition counts (HCC) are commonly used to assess population status in deer. We evaluated the reliability of HCC of sika deer (Cervus nippon Temminck) using repeated counts and by comparing estimated sex ratios and calf-to-female ratios of marked deer on Nakanoshima Island, Hokkaido, Japan between April 1999 and October 2000. Although both total counts and sex and age ratios fluctuated greatly by month, seasonal changes showed a relatively small variance. This suggested that seasonal changes in behavior within sex and age classes contributed to biased ratio estimates obtained from HCC. Route counts should be used as a relative population abundance index with estimates of detection probability, especially for females. Adult sex ratios and age ratios from HCC were unbiased during the rutting season (October–November), and age ratios in spring could be used if yearlings are counted as adults.