I used to have too many pigeons in house. If by chance one of the baby falls out of the nest we used to put it back using gloves.
Usually, if we touch them with bare hands, the other pigeons usually kill the baby. I want to know why they do that?
This is a story I have been told as well when I was a kid. Usually this is related to the foreign smell that the humans leave on the chick. However, this seems to be an urban legend, as birds have not a great sense of smell.
Snopes says about this:
What can happen is that the birds returning to their nest which find the nest disturbed compared to the situation when they left might cause them to temporarily or permanently abondon their nest.