Actually if these notes from the '69-'79 series are still in very good condition, there is no reason why the bank of canada would destroy them. It still happens occasionally that the odd 1954 $20 gets spat out. I still see Lawson-Bouey '79 $20's come out on occasion.
According to the Bank of Canada, "old series" notes are unfit for circulation and should be destroyed:
"A note is considered "unfit" for circulation when it is damaged, worn, soiled, discoloured, or mutilated. A note is also unfit if it is heavily creased or has a tattered appearance, indicating that the fibre of the paper is broken and that disintegration has begun. Dominion and Province of Canada notes,
old-series Bank of Canada notes, and Bank of Canada $1 and $2 notes are also classified as unfit for recirculation."
I have to wonder what they consider "old series", since plenty of Birds notes are still around.
Source: http://www.bankofcanada.ca/publications/review/r973b.pdf