The first Bird in Space was made from marble; bronze and plaster versions followed in the years and decades to come. In 1923, Brancusi refined the figure of the bird to its most concentrated form. Of the nine existing bronze versions of Bird in Space, no two are identical. In order to make each sculpture, Brancusi began by creating a plaster cast based on a marble version of the sculpture, manipulating the plaster to alter its proportions, and then casting the result in bronze. This ensured the uniqueness of each Bird in Space.