To Koester it is about "capturing the sunbeams falling in through the foliage and […] performing a splendid dance on the animals' colorful plumage. Animal and surrounding appear […] as a unity, as an impression submerged in light and color. [Koester's delightful animal scenes] show us […] that the physical aspect of the individual animal is not overlooked in the face of such an abundance of reflexes. The character of the cackling and wagging ducks is as well depicted as the yellow sunlight and the soft purple shades, the brown rocks shimmering in the sunlight, the pond reflecting the blue sky, the brown branches and the rootage on the water's edge. Taking great pleasure in sun and color, everything is rendered with a broad and juicy stroke of the brush of which every single one delivers proof of the artist's spirit and sense of color." (Dr. Paul Kühn on the exhibition at the Leipzig Kunstverein in 1904, quote after Stein 1988, pp. 51 f.).
Alexander Koester's painting were acquired by, among others, Emperor Wilhelm II, Luitpold, Prince Regent of Bavaria, Victor Emmanuel III of Italy, the Belgian king and Raymond Poincaré, Prime Minister of France.
Important works by the artist are in possession of, among others, the following museums:
Kunstsammlung, Chemnitz
Anhaltische Gemäldegalerie, Dessau
Joanneum, Graz
Neue Pinakothek, Munich
Frye Art Museum, Seattle
Österreichische Galerie, Vienna