White Wagtail

If you saw this bird move about in person, its name would be immediately obvious. That long tail of theirs bobs every time it stops moving! It gives them a distinctive movement pattern that’s easy to spot!

The first three photos are on the shore of Lake Ashi in Hakone, all of the same bird. The last was outside the Imperial Palace. I don’t have an environment shot of the area, but there were a good number of wagtails all on patrol there!
Wikipedia
eBird