This is normal. When it sits, the oil will drain from the tank, to the bottom of the engine. This is called sumping. When you start the engine, the oil pump can not work fast enough to pump the oil back to the tank, and the pressure, caused by the pistons, pushes the oil out of the breather vent.
Just run the engine till it clears out, then top off the oil, and clean up the mess when it's done.