Drop them into boiling water first? It's more humane (well, quicker) than some methods used & if the wee beastie is dead before it reaches the hens does it matter what they do to them? :-\
As for too much for a chicken... I had 17 Blue Tits fighting over one lightly poached slug today. ??? That was ,until they spotted a small bird that had been turfed from the nest. :( (It lasted 90 seconds. )