A window bird feeder allows you to observe birds up close. There are two types of window feeders: those attached to a window and those placed inside a windowsill. Feeders attached to a window are almost always attached to the glass via suction cups, but may be attached with Velcro. They are generally smaller than windowsill feeders and will typically fit any size or type of window.
Some windowsill feeders, called solarium feeders, protrude into the house through an open window; other windowsill feeders are secured outside a closed window. A solarium feeder rests in the windowsill and will ordinarily have adjustable side pieces that extend to the sides of the windowsill, closing off the open space much like the sides of a window air conditioner.