Suitable distance to observe Red-crowned Cranes: a note on the observer effect

In order to assess the effect of the observer distance on the behavior of Red-crowned Cranes (Grus japonensis), we studied their time budgets with a gradient of observer distances in Yancheng, China from December 2008 to March 2009. Observer distance was classified into four levels, i.e., < 200 m, 200–300 m, 300–400 m and > 400 m. The results indicated that cranes increased their vigilance when the observer was less than 300 m away. A safe distance for the observer was often as far as or more than 300 m. Cranes increased scan frequency when observers were within 200 m, whereas they prolonged the duration of their scan when observers were between 200 m and 300 m away, indicating two distinct vigilant strategies, given the observer distance. We suggest an observer distance of at least 300 m, in order to minimize the observer effect.