Ledge check: what will actually stay put up there
Answer what you are protecting, what you want it to do and how it is held, and the rail narrows to the pieces that suit that spot.
Pick an answer and the shelves narrow.
Solid surfaces like a window sill, roof edge, balcony railing, fence or gutter need physical barriers to keep a pigeon, crow or sparrow from roosting, while open spaces like an orchard, garden, fruit tree or vegetable patch call for visual movement. The choice between shelves comes down to whether the area is a rigid structure where spikes install with screws or tape, or an open branch or pole where hanging rods, wind spinners and reflective tape move in the breeze.
Run length and surface width must be measured in feet or inches before picking any hardware. For a window sill, wall or ledge, a tape measure laid across the depth shows whether a single strip or several parallel rows are required to cover the area. In an orchard or garden, measuring the spacing between fruit trees or fence sections determines how many hanging rods, discs or feet of reflective tape are needed to cover the line of sight across crops and branches.