Even although the seed has been bought from a petshop as bird food
Didn't know that. He didn't say. You obviously know more about this particular 1 2/3 Ha of sunflower than me.
So it wasn't a commercial crop? I wonder why someone would plant up that much to be eco-friendly and then use non eco-friendly plant types.
Perhaps a 'seed-eating bird lover' and not a thinking environmentalist? Please tell us more about this particular sunflower patch.