This thread is old, I know. But today I had occasion to kill a fresh-hatched chick and wanted to pass along information I had gained and, instead of posting another thread, I thought I'd search and append to one which was on point. This thread was on point.
Please read the information that donrae posted a link to (third reply to the OP). It's more detailed than I could be. Basically, it involves killing the animal via CO2 asphyxiation. I had first read about it at
www.waldeneffect.org but several links on that site don't work and it seems to have fallen out of routine maintenance (just my take on things) so I wanted to promote the info here. This thread does 98% of my work.
Basically, I put a small container inside a slightly larger container, placed some baking soda in the smaller container, placed the chick outside the smaller container but inside the larger container, poured vinegar in the smaller container on the baking soda to produce CO2 and lightly placed the lid on the larger container (loosely so that the off-gassing reactants had a place to disburse the O2-laden air inside the container). Less than 1 minute later, the chick was dead.
Read the info that donrae posted a link to; it covers this in more detail including the concentrations of CO2 which may be painful to the chick and how much of each component could be used (I'm sure, having done this before reading this thread, that I used too much baking soda and vinegar and probably achieved a concentration which was painful to the animal).
But, the principle is sound and the idea works and, while any killing is unpleasant, it's better than letting the bird, which was failing to thrive, die slowly over the next few days or weeks.
FWIW.
--HC