Our objective is to develop a small tree climbing robot for ecological surveys to protect endangered birds and mammals nesting in trees. By using this robot to climb trees on behalf of people, the risk of a crash incident and thus the risk of nesting abandonment can be reduced. In our study, we first developed a small six-legged robot, the driving force of which is shape memory alloy (SMA) actuators. Because the possibility of nesting abandonment is related to both the size of the approaching object and the volume of its sound, it can be reduced by using SMA actuators, which are very quiet. Because an SMA actuator and spring antagonistic driving mechanism are adopted, the robot can remain at a specific location in the tree without requiring an external energy supply. The size of the robot is L48 x W81 x H78 [mm] and its weight is 4.9 [g]. The robot can walk on a flat soil ground at 0.15 [mm/s] and climb a cedar tree tilted at 30 [deg] at 0.18 [mm/s].