Stop bird reflection battles
Several species of birds that tend to nest near human habitation will suddenly discover their reflection in a window. In these cases, the bird (often a male robin or cardinal, sometimes a goldfinch) thinks it’s seeing an intruder, so he spends fruitless hours trying to drive the potential competitor out of his territory.
This behavior is also seen at rearview mirrors on cars, which have a handy “perch” for the bird to stand on while it pecks relentlessly at the mirror. And because the bird is highly stressed, it tends to poop a great deal, too (this happens at house windows, as well).
It’s annoying for humans and highly stressful for the birds, which may end up injuring themselves if it goes on too long.
The standard advice is to place a piece of cardboard on the outside of the window (or car mirror), and if the bird moves to a new area of glass, move the cardboard there. This is effective and is a kindness to the bird. If he stops trying to battle an intruder he can get on with spring’s nesting cycle.