Disclaimer: There are several other methods to cull ducks and ducklings. If the other methods work for you feel free to use them. However, if you have the daunting task of having to cull a bird and can't emotionally handle the other method this may work for you.
5 Dollars at most pet stores and Walmart.
5 Dollars at most retail stores. I do NOT advise the clear ones. It can make the experience much harder emotionally for some people.
Call around for pricing. It's around 10 dollars for 10 pounds around here. You shouldn't need more than 3 pounds if your only culling one bird. In most areas you must be 18 to purchase it.
Step 1: Open your storage bin. Many people advise putting newspaper down for easier clean up.
Step 2: Fill your Critter Keeper full of the Dry Ice.
Step 3: Set your Critter Keeper inside the bin to one side.
Step 4: Say your final goodbyes to your bird.
Step 5: Place your bird to the opposite side of the Critter Keeper.
Step 6: Lightly set your lid on the bin. You do NOT want it to air tight. That can cause the lid to explode off.
Step 7: Come back in a few minutes. The amount required will vary by the individual.
Step 8: Your bird is now over the rainbow bridge. Now you can say your final words and bury your bird.
Culling a bird is not something that should be taken lightly. Also, if your can pull up the strength I would consider the other options of culling as well. The Co2 should put your bird into a sleep that it never wakes up from. This method is often used for culling smaller animals such as mice.