I live in the country and don't have double glazing. Its very obvious that the overall *amount* of bird song changes through the year.   I can quite see a poet assuming (rightly or wrongly) that the nature of the songs changed as well and country people being aware that in some cases (including cuckoo and robin mentioned above) it did.

Is interpreting a folk song on the basis of "I haven't noticed this so is the composer is making it up or referring to something of deep significance" normal practice?