A test of long-term fecal nitrogen monitoring to evaluate nutritional status in bighorn sheep

We analyzed 23 years of monitoring data from a bighorn sheep (Ovis canadensis) population to determine whether fecal nitrogen (FN), expected to reflect diet quality, can be used to track population nutritional status over the long term. We considered 3 measures of FN: its maximum value each spring (FN peak), the date of the peak, and the area under the curve relating julian date to summer FN (FN-total). We first determined the sources of variation in these 3 measures. Population density had a strong negative effect on FN-total while summer precipitation was positively related to FN-total, suggesting that diet quality declined with increasing density and improved with precipitation. Most sheep were recaptured every year, allowing us to assess FN as an indicator of nutritional condition by examining the relationships between summer mass gain and both FN peak and FN-total. The value of FN peak was not related to summer mass gain for any sex-age class, but FN-total was positively related to Summer mass gain of nonlactating females and yearling females. Our results suggest that FN can be used as an index of forage quality over several years. Over several years, FN also reflects aspects of bighorn sheep body growth and is correlated with changes in density that may ultimately affect population performance. Short-term monitoring of FN, however, may not provide much useful information.