'A fairly accurate predictor'

In Flora Britannica Richard Mabey says that the first full blooming of Mayflower (Cardamine pratensis) is generally a fairly accurate predictor of the first hearing of the cuckoo. He also refers to its common name of Lady's Smock, from the slightly cupped or 'frocked' flowers, but explains that smock 'was once a less than complimentary slang for a woman'. Anyway, always a welcome sight!