I find lead to be the safest, care of a 22 air rifle. So as to ensure a humane kill, I use a bait of peanut butter to lure them as they have to eat it at the feed station and cannot grab the food and run.
I put the food out when the birds go to bed for a couple of days so that the rats get used to it. On the third day, I lie in wait. If I do then have a runner that was not a clean kill, if the dogs or cats (or Red Kite) find the carcass, at least it will be safe for them.