I assume you are talking about a "pole trap" designed expressly for catching birds that landed on top of an elevated perch. They are basically a European trap made mostly in the UK that were used heavily in the 1800s until banned in 1904.
Pole traps range in size from the tiny kingfisher trap (2inch jaws) to the large bird (eagle, heron, hawk) traps with 8 inch jaws. They could have smooth or serrated jaws with a single curved flat spring on small - medium size traps and double springs on the large traps. Their bases are mostly round with some square designs in the larger sizes. Many have brass triggers. Early pole traps caught most birds by the feet. The later, more humane versions with high striking jaws caught the bird mid-body thus dispatching it immediately.
Pole traps that used eggs as bait are called "egg traps" and have a ring-shaped brass pan that holds an egg upright...this trap is set off by lifting the egg out. Egg traps usually caught the bird (crow or magpie) by head or neck.
Throughout history, several different types of killer traps have been mounted on tops of poles/perches to catch nuisance birds, but were not made solely for that purpose like a true "pole trap". Genuine antique pole traps in good condition are highly collectable and have significant value; particularly the smallest (kingfisher) and the largest models (heron, eagle) which may fetch several thousand dollars.
