Source identified1
“Hope is the bird with its dim sensations; The shell that keeps it alive is Patience.”
MacDonald pictures patience as the protective structure around hope: hope may begin weakly and imperfectly, while patience gives it time to mature rather than demanding immediate proof.
Source note: “Hope and Patience,” in The Poetical Works of George MacDonald in Two Volumes, Volume II (1893), Project Gutenberg #9984. The bird-and-shell image is a poetic metaphor rather than a literal psychological model.