I call it '1 Cap' because I have a 3 cap version that I made first that works really well. Found in my Flickr Set: Flickr Photos - Homemade Birdfeeders

This feeder was inspired by billr's How to build a "Flexi-Perch" Squirrel-proof birdfeeder for $10 or less (Thanks Bill!)

Step 1: Tools & Materials

TOOLS
Utility knife
File, round
Small saw with fine teeth
Wire cutters
Fine black marker

MATERIALS
1 water bottle, any size (the cap needs to be as deep as possible to hold on to the bottle. Try to find a cap made of soft plastic, the white cap was very hard to cut.)
2 cable ties, any colour but black ones are stronger
2 small cup hooks or screw eyes, 3/4"/2 cm
4 pieces of wire 8 inch / 20.3 cm long (not shown)
Bird seed - nyger, finch mix, any small seed, etc. (not shown)

Gather your materials and tools. Almost all the materials and tools can be found around the house or bought at a dollar store! Bird seed can be found at a bulk food store, grocery store, farm supply store, home renovation store, etc.

I'm using 8 inch / 20.3 cm cable ties cut down to 5 inch / 12.7 cm.

Step 2: Prepare the Cap

Take the bottle cap and mark the bottom as if you are cutting a pie. Draw lines up the sides of the caps where you want the openings to be. Position a cable tie (indicated in red) on the bottom of the bottle cap so that they exit out the middle of the 'doors', outline them with the marker. Put a dot where the hole of the cable tie is. Where the cable tie exits, cut another slot at the side of the cap.

Step 3: Cut the Sides of the Cap

Use the utility knife to cut down to the 'top' of the cap (now the bottom). Cut away half of the cut portion to make an opening for the bird to access the seed. Bend it outward a bit. Remove the inner ring to allow the cable tie to pass through the bottom.

Step 4: Cut the Bottle Opening

Screw the cap onto the bottle, use the marker to write on the exposed bottle neck. Remove the cap. Make four vertical cuts with the saw and use the file, or knife, to clean away the burrs.

Step 5: Add the Cable Ties

Slide a cable tie into the bottom slot and out the side slot. Screw in a cup hook through the cable tie's opening into the bottle cap until it stops. Repeat for the second cable tie. I use the cup hooks because they are easy to manipulate by hand, rather than using a screw and a screwdriver.

Step 6: Prepare and Add the Wire

Cut the wire to length. (This will depend on the size of bottle you choose. For a small bottle with a channel around the middle put the collar wire there. For a larger bottle, 1-2qt/1-2L, make the collar wire smaller to fit around the neck of the bottle. Add an extra wire near the 'top' to keep the wires from slipping sideways.) Take 3 of the wires, make a small loop at one end and thread the collar wire through them. Wrap the collar wire around the bottle, slide the other wires until they are evenly distributed around the bottle and twist the collar wire closed. Pull the support wires up and twist them together and form a hook.