Trap fences are a combination of fences (wooden stakes) and in between a trap pit or trap pit with sharp stakes.
The best place to build is near reindeer marshes.
You cannot dig holes in marshes so you have to build it on the border. It is better there because you can use the trees as a natural obstacles when building a fence line. Since the new update (3.20), birds are very common. To increase the odds of catching birds, used mainly for their feathers, the player should invest in making cordage and loop snares. Birds will fly over the fence, but not over the loop snares, thus making the trap fence ultra efficient! The player should keep in mind that he/she would want to go out of the fence, and since the loop snares are in the way of the fence, the player should leave one square empty of any traps (shown above).
A good way to do this is to build an arrowhead like fence situation - so you can herd them into a corner.
Example:
forest ######0#####0#####0#####0#####0 # # # f 0 o # r # e # marshland s 0 t # # # 0 # # #