To kill and leave prey can have different causes. Killing for sport which is usually when the weasel is completely new to the area so it must develop a food chain in your area or something is disturbing it. Such as a dog barking or movement. Another thing I can think of is that it kills the bird and tries to bring it down the hole but it can't fit it down the hole and will leave it. Weasels tend to live in small families. If you're going to trap them I would put eggs inside the live trap or the breast meat from the chickens that it has killed. Let us know how it works.
Edited by Birdrain92 - 11/24/15 at 8:23pm