The imagery in the opening paragraph of "The Devoted Friend" indicate that the story is a fable. Describing the animals and their surroundings through colorful descriptions, Wilde signals to the reader that the story—or at least some portion of it—will take place in a lively setting populated by animals with strong, human-like personalities:
ONE MORNING the old Water-rat put his head out of his hole. He had bright beady eyes and stiff grey whiskers, and his tail was like a long bit of black india-rubber. The little ducks were swimming about in the pond, looking just like a lot of yellow canaries, and their mother, who was pure white with real red legs, was trying to teach them how to stand on their heads in the water.
This paragraph draws a connection between the ducklings and yellow canaries. Initially, this doesn't seem to be a particularly clever comparison—especially for Wilde, who makes active use of interesting figurative language. After all, he is comparing a group of small, yellow birds to a group of slightly different small, yellow birds. The only real contribution this comparison makes is to indicate that the baby birds are yellow and energetic. However, although the comparison fails to contribute a great deal of meaning on its own, it does offer the reader more information when juxtaposed with the description of the rat. Whereas the rat's body looks like a dark, inanimate material (rubber), the lively birds look like lively birds. The comparison's relative simplicity and its contrasting use of visual imagery (color, to be specific) gives the reader the impression that the ducks are simpler and more accessible than the slimy rat.
The imagery in this paragraph establishes a contrast in the characterization of the Water-rat and the Duck (and her ducklings). Whereas the former is stiff and dark with beady eyes and a rubber-like tail, the latter is bright and soft. As the reader will discover in subsequent paragraphs, these contrasting visual descriptions align with the characters' respective personalities, behavior, and approaches to life.