Researches around wireless sensor network (WSN) were very prolific recently. However, traffic modeling related WSN research was poorly less. In this paper, source traffic dynamics in a simulated target tracking WSN scenario are explored. We find the source traffic arrival process doesn't follow the usually considered Poisson model. Instead, an ON/OFF model is found to be capable of capturing the burst nature of source traffic arrival. Further, we find the measured ON/OFF periods follow the generalized Pareto distribution perfectly. Mathematical analysis also shows a surprising fact: all ON/OFF period distributions in the experiment exhibit short-tail property, which is a nice property that could be exploited by applications such as anomaly detection and node failure detection. Categories and Subject Descriptors I.6.4 [Simulation and Modeling]: Model Validation and Analysis; G.3 [Mathematics of Computing]: Probability and Statistics--distribution functions; C.2.4 [ComputerCommunic...