There are many different ways to build a bird feeding shelf or windowsill feeder. Here is one that is easy and straight forward.
1. Attach a one-half inch thick piece of cedar or pine the length of the window sill to the outside of the window.
2. Attach a one-inch high lip to the exposed side and ends of the shelf to prevent the seed from blowing off and being kicked off by the birds.
3. By making the long side of the lip a little shorter than the length of the board, leaving a two-inch gap at both ends, cleaning will be much easier. It will prevent seed from collecting in the front corners and will provide a “gate” to brush the old seed and shells out, which will help in keeping the feeder clean.
4. Drainage is also an important factor. The feeder can be installed with a little tilt to allow water to drain away from the window, through the corners which have been left open. Or another solution is to drill holes in the feeder at strategic points to allow drainage.
5. Treat the feeding shelf with a bird friendly sealer to increase its longevity.
6. A seed bin can be rigged up at one end of the feeding shelf to hold seed and allow the seed to be replenished as it is eaten.
For example: a plastic container can be screwed onto the shelf through the bottom of the container. Cut an opening on the side at the bottom of the container to allow the seed to be slowly dispensed. Fill the container with seed and snap on the lid. The seed is protected from the elements and is slowly made available to the birds as they eat it.
7. Suet can also be made available very easily by attaching a suet feeder to the shelf. Attach a board vertically to one end of the windowsill bird feeder. Next staple a small piece of wire mesh onto the inside of the end board, making a pocket big enough to insert a clump of suet into it. (See how this can be done here.) Or a suet cage can be purchased and attached to the vertical board.
If you like the idea of watching birds up-close at the window but don't want to make one, then explore Window Bird Feeders.