"the sky became suddenly overcast with clouds..."See in text(Chapter I.)
Wilde juxtaposes the beautiful, idyllic English countryside with the gloomy, ominous area around Canterville Chase to set the sinister tone of the location and play into traditional Gothic and horror story conventions.
Rooks are a type of crow with black plumage that nests in colonies in treetops. Such birds are typically associated with ill omens, death, and the supernatural in literature.