Birds have a habit of wiping both sides of their beaks on a perch or other handy object — a quick left-right, or a quick right-left. This serves two purposes, the most obvious one being to keep the beak clean. It also helps keep the beak trimmed properly, since the action slowly wears down the beak material, which keeps growing. Some bird's beaks grow too much, and need to be trimmed (vet's offices do this, if they're familiar with birds). A perch made of hard material, like concrete, can also help keep a beak trimmed.

Darjeeling, shown in this photo, usually keeps herself quite clean. But she was snacking on soaked seeds when I interrupted her to take this photo. I'm sure she wiped her beak clean once I closed the door and walked away.