Since the verse in question is
'I was on the spot, i was deep in thought,
I had scarse commenced my woe
When the blackbird spoke from the bower scutch
My mind he seemed to know
Saying if you could fly, just the same as I
Your wings and your voice would strain
You'd be whistling shrill on the window sill
Surmising Sarah-Jane'
I personally would go with the idea of the coarse grass, which is the sort of thing I've always imagined when singing the song.
BTW Lorraine, I don't know if you've had the same problems as I did with the last verse when I was learning the song, just in case you have it is
Now the pain it is decreasing daily, and a roving she might go
She may call at 'Liza Keeley's as she passes through Myroe
She may drink from a bottle of the best, and drink to the poets name,
And I hope always she'll have happy days,that maid called Sarah-Jane.
Liza Keeley's was a bar/ale house in Myroe as was explained to me by Frank himself. It's a great song, nice to know you've picked it up.