G'day,
I have seen this song named as the oldest known song in the English language (early 13th cent), however in a book I recently acquired, "Medieval English Lyrics", there are two older ones.

Here is how I've heard it sung ("modernised"):-

Summer is a-comin' in
Singeth loud cuckoo
Groweth seed and bloweth mead
And springeth the woods anew
Ewe bleateth after lamb
After calf the cow
Bullock rouseth and buck brouseth
Merry sing cuckoo
Sing cuckoo, sing cuckoo, sing cuckoo.

Then sung as a round.

The last part of the old version, not used in the "modern" version translates as: Cuckoo, cuckoo, well singest thou cuckoo, nor cease thou never now.

Cheers,
Alan