Well, of course there are now dragons now in England - all those gallant heroes killed 'em all! But not so the nightingales, which would be a difficult target as they're not easy to spot.
Thanks for the Peter Kennedy reminder, mayomick. I do have the book, so I've had a look. It includes Cornish words which were written by a 20th-century Cornish scholar. The tune seems to have come from St Ives, and the words from either Truro or Plymouth, at least as early as mid-19th century (and both are possibly much older).
But I'm stil puzzled as to why the Cornish would sing about a bird that they never saw or heard - it appears that they're only occasionally found in Devon, and not in Cornwall (Dorset is a long way from Devon, as any holidaymaker who drives down to the south-west will tell you).