Keep birds away from the window sill

This system consists of round stainless steel mounts (pins) with diameters of 3.5mm (for pins with an M4 thread) or 4mm (for push-in pins). The thicker version has a diameter of 5mm and an M4 thread. The individual pins are available in different lengths (between 25 and 300mm) and different thread lengths (between 5 and 50mm). A 0.54mm thick stainless steel rope is held between the pins, tightened by a spring. The pins are screwed or hammered into place or glued with special mortar. 

Pro: Barely visible, no significant impact on appearance.
Con: Only for use as preventative measure, cannot cope with severe bird problems. 

Keep birds away from the window sill