The coincidence of the pale and delicate Cuckooflower (Cardamine pratensis) coming into bloom at the same time as the first Cuckoos arrive back from over-wintering in Africa is the likely origin of this wildflower’s common name, though several other wildflowers also bloom at this time and, with the climate changing, the flower’s blooming and the bird’s arrival no longer coincide very precisely. Perhaps the alternate Lady’s-smock and Milkmaid are more appropriate names.
Another wildflower that is known in some areas as Cuckoo flower, as its flowers open around this time, is Arum maculatum. I know it best as Lords-and-Ladies, but many call it Cuckoo-pint (rhymes with mint), for which there is a somewhat more risqué explanation: pint is short for pintle, meaning penis. I’m sure you can all see why.