The animals need a flower to complete a May Day ritual.

Unwilling to steal ritual flowers, but unable to speak and explain their need, the animals leave gifts in exchange --- among them a Roman glass tear bottle, found by the mole in a tunnel and filled by the rabbit with fresh May dew. Intially bewildered, Miss Susan [the nearly blind women who tends the garden] finally rubs her eyes with that dew and finds that she can again see 'the feathers of the birds, the petals of the flowers.'

Alison Uttley (1884-1976)
"Grey Rabbit's May Day"
found in House and Garden, March 2000 p. 78