We have developed an ultra low power wireless sensor node with continuously monitoring of activity for animal health care. Intermittent operation of body temperature measurement is enough for health care and effective to reduce the power consumption. However, in activity measurement, the intermittent operation is not suitable because the change of activities is fast. In this report, it is shown that an ultra low power method for continuous activity measurement with a developed custom LSI which works at about 320 nW of calculated power consumption at standby state and a MEMS piezoelectric micro-cantilever. We also demonstrate the node which is applied to a chicken health monitoring system for avian influenza surveillance in poultry farms.