Hold the plastic bottle upright on a flat work surface; a 2-liter soda bottle is ideal for birds of different sizes. Cut a window into the bottom area of the plastic bottle using a utility knife. Leave a 2-inch border along the bottom of the window, and cut the window at a height of 4 inches to accommodate medium- to large-sized birds.
Unscrew the cap to the plastic bottle. Tie a length of twine around the bottle neck, leaving about 1 foot of twine hanging on either end. Screw the cap back onto the bottle to keep the twine in place.
Puncture a hole on either side of the cut-out window using the utility knife. Push the unsharpened pencil through one hole, and pull it out the other side to form a perch for birds to sit on.
Pour a few inches of bird seed into the bottom of the plastic bottle bird feeder. Tie the ends of twine around a sturdy tree branch in an area where you can view the bird feeder from a window.