Watership Down (1978)

Continuity mistake: The rabbits have a close encounter with some rats in a shed on the cemetery. A spade falls over. In the next shot the spade lays in a wheelbarrow.

Hazel: Lord Frith, I know you've looked after us well, and it's wrong to ask even more of you. But my people are in terrible danger, and so I would like to make a bargain with you. My life in return for theirs.
Frith: There is not a day or night that a doe offers her life for her kittens, or some honest captain of Owsla, his life for his chief. But there is no bargain: what is, is what must be.

Trivia: Occasionally the rabbits use terms which are not explained in the movie, but which are featured in Richard Adam's novel, along with their meanings. For interest, some of these are: "owsla"=a term for a selected band of rabbits responsible for both security and raiding parties (for vegetables, mostly; organization varies with the warren); "hraka"=rabbit droppings or something similarly offensive; "tharn"=mad or paralyzed with fear; "elil"=any rabbit-killing/-eating enemy; "hrududu"=generic term for motorized vehicles; "zorn" (which Holly cries out right before he joins the others near the new warren)=catastrophe; "Frith"=the sun; "Inle"=the moon, or death; "hlessi"=a wandering rabbit.

Question: I'm trying to remember the song that was made famous by this film. Can anyone help me?