The rising of birds in their flight is the sign of an ambuscade.
Chang Yü’s explanation is doubtless right: “When birds that are
flying along in a straight line suddenly shoot upwards, it means
that soldiers are in ambush at the spot beneath.”
Startled beasts indicate that a sudden attack is coming.
An example of 覆 _fou_^4 in the meaning of “ambuscade” may be found
in the _Tso Chuan_, 隱 9th year: 君爲三覆以待之. In the present
passage, however, it is to be distinguished from 伏 just above, in
that it implies onward motion on the part of the attacking force.
Thus, Li Ch‘üan defines it as 不意而至, and Tu Mu as 來襲我也.