Posted: 12/05/2014 at 18:32
That looks a little like French Sorrel If it is that, it is very invasive. Both my bird feeders: Sunflower Kernals and Niger Seed are germinating like crazy after the birds drop the seeds on the ground - just after I sowed a load of flower seeds; Ammus Major and Orlaya Grandiflora. I can't identify anything as don't know what my seedlings are supposed to look like.
I did some research on this last year and ALL seeds for birds are supposed to have been sterilised! Well, quite obviously they aren't.
Are you feeding the birds a mixture of seeds? I have found that each species of bird will simply discard any seed that is not to it's taste. I have less of a problem, but still a problem, just feeding 2 types of seed in separate containers.