Where is this Painting of a Spix's Macaw Located?

The Spix Macaw, Cyanopsittacus spixi, by Karl Plath and published in the April 1931 edition of Aviculture, the journal of The Avicultural Society in London, England [larger view].

A reader is wondering where this painting of a Spix's Macaw is located; does anyone know?

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Have you tried the Brookfield Zoo in Chicago (formerly the New Chicago Zoological Park) where Plath was curator of birds until 1961? (Although I wouldn't be surprised if it is actually in the British Museum)