The corridors from the Ninomaru Palace entrance to the Grand Chambers (at Nijo Castle) use a specially-devised floor construction. When someone walks, the floors squeak, which is compared to birds chirping.
The mechanism is very unique. When gravity is applied to the floor, a gap created beneath moves the clamps up and down. This causes friction with the nails, producing a squeaking sound. The device functioned as an alarming device, warning that someone is coming.