Grey Wagtail

This bird was around 9-11 inches long. It would constantly fly around, and was quite scared of me. When it flew, it had two long feathers sticking out it's rear end-a bit like a swallow. I have been around the area it was found for maybe six years, and have no memories of this bird. It did not seem to be making much noise. It's underside was yellow, and it's back was grey. It was in a public garden/pond.