The killing cone
This is a plastic cone attached to a fence post or something sturdy, with its narrow end cut off so you can insert a bird into it. You can then decapitate it, knowing the bird is safely held and calm (thanks to a natural phenomenon called tonic immobility where the bird remains calm and still due to it being upside down).
Another option is stun the bird with a blow to the head before placing it into the cone so it is unconscious. You can then cut its throat – make sure to use a very sharp knife. It can take up to 3 minutes to drain the body completely of blood.
With the bird’s body in the cone, you don’t have to worry about it flapping around due to nerve reactions as it is securely held, and blood can drain onto the ground or be collected in a bucket.
A plastic cone with the top cut off, turned upside down, is a good size for holding a bird.