Nowadays, many applications demand precise localization information. One of those applications is wildlife monitoring, e.g. tracking of bats. Wireless sensor networks featuring field strength measurements are a promising approach to track tiny and lightweight animals. In this paper a robust grid-based filter for the localization of bats based on field strength measurements is proposed. Therefore, fundamentals of field strength-based direction finding and optimal filtering are presented. A gridbased filtering approach to the problem of field strength-based position estimation of bats is derived and evaluated by Monte Carlo simulations.