The aircraft was being operated at between 500 feet and 1000 feet above ground level. The pilot was in frequent communication with motor cyclists on the ground. After about two and a half hours one of the motor cyclists observed the aircraft enter a shallow descent and a slight left turn. The aircraft continued in this attitude until the left wing struck a tree, it then rolled rapidly to the right before the right wing and nose struck the ground about 25 metres beyond the tree. An inspection of the aircraft did not reveal any faults which could have contributed to this occurrence. The pilot was recovering from an upper respiratory tract viral infection at the time of the accident. It is possible that his physical condition combined with the relatively high workload involved in sheep spotting caused a degredation in perception which resulted in an inadvertent loss of altitude.