The bird's feet are grasping the window tightly.
The bird may let go of the window.
Grasping the window tightly does not necessarily mean that it may let go of the window.
exists e1 s1 x1 x2 x3.(n_window(x3) & n_foot(x2) & of(x2,x1) & n_bird(x1) & a_tightly(s1) & Manner(e1,s1) & Theme(e1,x3) & Actor(e1,x2) & v_grasp(e1))
exists e1 p1 x1 x3.(n_window(x3) & n_bird(x1) & exists e2 x2.(Theme(e2,x2) & Actor(e2,x1) & v_let(e2) & of(x2,x3) & n_go(x2)) & Topic(e1,p1) & Actor(e1,x1) & v_may(e1))