Fishes and birds are Escher’s favorite animals. Or, at least, that is what his work seems to suggest. When he was experimenting with tessellations in the late 1930s, he arrived at these shapes quite soon. They lend themselves very well to the juggling act that is needed for this technique. Which is why they keep popping up in his work. Individually, like in Day and Night, Sun and Moon, Liberation, Fishes, Swans, Depth, Three Worlds and Whirlpools. In combination with other animals, but often also together. Consider for example Sky and Water I and II, Metamorphosis II, Predestination and Two Intersecting Planes.
That last one, Two Intersecting Planes, is from January 1952. By then, Escher had a firm grasp on these shapes and in this particular print, he also made them three-dimensional. It results in a combination of two of his major fascinations: tessellations and creating the illusion of three-dimensionality on a flat surface. In this woodcut, he created a jigsaw puzzle with his favourite shapes. As Escher described it in 1959 *:
‘Two thin, flat rectangular boards intersect each other at a slight angle. Holes have been sawn in each board leaving openings shaped like fish and birds. The holes in one board can be filled up with the remaining parts of the other board. The jigsaw pieces of the one are mirror images of those in the other.’
[*] The 2013 reissue by Taschen GMBH of M.C. Escher. The Graphic Work. Originally published by Royal publishing house J.J. Tijl NV, Zwolle 1959, page 15.