M. C. Escher’s Day and Night

It is hard to find words for. At first sight
There’s a skewed chessboard of fields slanting right,
Juxtapositions of dark squares and white.

But look again. There’s more to it than that.
As the eye goes up, shapes cease to be flat.
Become lines of birds, a black or white slat,

Fly south for the winter, no, east and west.
The skeins wind both ways, do not come to rest.
White birds in night, black in day seek a nest.