The Slate mutation was first discovered in 1933 by British breeder H. Watson, who bought the original bird from an unknown dealer. Unfortunately, it died before it could produce any more Slates. Two years later, it appeared again under the British breeder T. Bowman. The mutation, although sex-linked, was slow to develop and generally ignored in favor of the modern Grey Green mutation. The variety nearly faced extinction but was revived in the 1990s, becoming the more widespread (but still rare) Slate we see today.
Birds with this mutation are a blue or green-infused grey color. They are more easily identified when combined with Blue (pictured second), where their coloration is darkener. Slates are difficult to identify in their green forms, but look like Grey Greens with violet cheek markings (a comparison is pictured first).
