Wagtail

Hi, we had this little fella arrive in the garden about 6 weeks ago. Don't know if its a yellow or grey wagtail. I have a pond in the garden and I have been taking the cover off my filter to let it get at the wee flies that hatch, so plenty of nice food. It keeps banging into the bifold doors intentionally, been doing it for hours today. Don't want it to hurt itself; how can I stop it? My cats are also watching :( 

Should it not have moved away to somewhere else. We are in Larbert, Central Scotland.

Thanks, Gareth