After some googling I came across the excellent passage written on regulids in HBW, and which is handily reproduced on their website here:
In summary, it states that the song development of all regulids is highly influenced by learning process, and are especially sensitive up to the 47th day after hatching. In captive studies it was found that Firecrests and Goldcrests could learn each others song from tape recordings. Individuals have also been found in the wild which give intermediate song – both in northern Spain and also in southern England, and this was explained because in either area both species are at low breeding densities, thus they learn the closest song to their own.
Ok, that is perhaps a little over-simplified, but that is the jest.
I also came across this fascinating image of a presumed Firecrest, however it looks a bit odd...
Aren’t regulids brilliant!
If you have no idea what I’m on about take a look here.