​Imperceptible Change

I know John Keats has said it’s so,
Romanticist, you’d think he’d know.
The nightingale just sings a tune
He sang before, as in last June.
In his famous “Ode,” he claims
Old kings had heard the same refrains,
That birds had sung in ancient song
They sing today. He could be wrong.
Some say the birds sing different notes,
With changes smaller than the motes
That on their feathers cling unseen,
Impossible for birds to preen.
Slight changes in the tune we hear
Will go unnoticed year to year,
Much as each culture subtly moved,
Rejecting what it once approved.
The values sung in ancient songs
As worth our praise are now deemed wrongs.
As offspring change a note or two,
The old ways slowly change to new.
The birds appear to look and sing
As they once did for ancient king,
But now you know the truth at last 
That nothing in a culture’s past
Is just the same as it had been.
The present time is no past’s twin.
Thus, in your time you, too, have made
A subtle change in what you played.
Your song today will slightly vary
From the tune you used to carry.
Now add your changes to the others’;
That’s all your sisters and your brothers. 
The sum of all is a mutation
To any culture or a nation.
And once the singers changed their tunes,
Those songs they sang in former Junes,
Just like the sounds the old kings heard
Are not those sung by modern bird.