That's wheat, out of the bird food perhaps
If you don't want this to happen, there are treated bird foods that will not germinate, or you can cook the bird seed for 2 minutes in a microwave which stops it germinating and makes no difference to the birds - as long as it is cold when it goes out!
Thanks guys! Didn't know wheat was included in bird food. it is in my son's garden and I know he's keen on feeding the birds. Now he knows what it is he can pull it up before it gets to this stage.
There is often a lot of wheat in cheap bird food. I got caught by a big bag of bird food from Aldi. A lot of it ended up on the floor. False economy!