Update 9.Nov. 2016:

Because of my own experience and the feedback of my friends and relates i developed this update.
It prevents spilling the content which mostly happens while removing the roof for filling.
I developed a ring, which also provides additional seats. Just open all ropes and lead them through each hole then fix them with the screws. Those screws also can be used by the birds as waiting loop and a funny swing. Place the ring close to the top.

The idea:

While the weather is getting colder here, it's time to support our little feathered fellows outside with additional food.
I don't give them orange juice, as the header let's you suppose. But I thought why not combine it all:

• Recycle empty orange-juice bottles
• make some printed parts
• support the wildlife.
• saving printing material

The (reverse-)engineering part was the thread for the bottle. Also it may be the the critical part of it to fit other bottles worldwide.
I used the bottles from Aldi Germany. Bottles from EDEKA work too. The bottles from Lidl have a smaller necks and do not fit.
Just try your bottles and tell other members which will fit too. Feel free to create a remake with other threads. At least it has to be wide enough to let the seed easy pass through.

How it works:

The feeder is a silo type and I didn’t claim I had invited it. It.s another (more intelligent ;) edition.
Silo feeders offer only a specific amount of food, while the rest is kept save and dry in the cylinder. Another advantage is, unlike in bird-feed houses, the birds sit and poop not in their own food. The pot in the seat unit has drainage holes to let water ran out which has entered due rain, dew and molten snow.
The roof has two functions: Once to keep all dry and twice for reversed use as a funnel to make it easy fill up.
Finally it's designed to fill it on place without unscrew the bottle or remove it from the tree. By the way you have here not to grab in the birds poop.

How to Set Up:

First clean an empty juice bottle and let it dry it inside. Cut now a hole about 5cm / 2 " in the middle of the bottom of the bottle. Keep the cap for later use.
Screw the bottle thread in the seat_unit and place three wires, thin para-cords for example, in the three holes of the ring.
Knot the wires on the top of the bottle (the top is now the bottles bottom) far enough to have space for the roof and long enough to hang it later on a tree or hook.

Place it cat-save and in a clearance where you can fill it easy. Now it's time to fill it:

1. Place the roof upside down in the previous cut hole and put the wires in the gaps of the roofs bevel.
2. Pour the seeds in until the bottle is filled mostly. Hold the bottle with one hand, to prevent it from accidentally spilling
   
3. Turn the the roof back - as roof - on the bottle.
   The important part is now the "rope trap"
   This handy ring is to keep the roof stable in place.
4. Run the wires in the three slots of the ring and move it down over the roof's thread.
   5.Fix it with a cap of the bottle. The wires hold tight now the roof.

Yore are done now and can wait for the first birds..

RepRap

mendel

Doesn't Matter

No

0.3

20%

The seat unit is shipped with build in supports where can get ripped away after print. I decided to create my own, instead of let my slicer babe fill all up with a huge mesh.
The roof will print w/o supports up to 0,3 mm layer height.

I recommend to print the new ring and branches in 0.2 for good threads
Print the 3 screws (branch.stl) as provided with the knob on the plater with a brim to prevent them falling 5 minutes before finish.

After a longer print test serial with fails I figured out, that the thread and the rest of bottle threads world, are not single pitch threads like used on screws and nuts!
In this case a a three-pitch is used. More pitches have more chances to fit the cap while closing it. Also the pitch can designed more higher that the bottle closes wit less turns. For further explanations about bottle threads please refer to our good wikipedia.
If you plan to design a adaptor for bottles you have to measure the pitch of one thread, the count and distance of the whole threads.
I figured out, that it's not a good idea to remove the roof of a filled feeder. The bottle will tilt upside down and the seeds get lost. I thought many hours to make a decision between a central or three wires outside. The central wire is bad while filling, the wires outside keep the unit not stable while the roof is removed but made it easy to fill.