They look like oak tree seedlings to me, but there is a shrubby tree that looks similar who's name esacpes me at the moment. Any squirrels in your area? They love planting acorns and hazelnuts etc in pots of compost!
An oak tree is the best tree for wildlife. It supports more insect species than any other. I consider mine to be the main reason I have so much wildlife in the garden. I have no doubt that without the oak, I would see a lot less birds, and probably the bats would disappear.