Just about any change in regular diet to something completely new can cause unusual bowel movemetnts, this is where the "runny poop" rumors come form.

JCPs eat birds in the wild, but when they eat nothing but rodents in captivity for a few years, their bodies don't digest the avian proteins as well. Or at least this is my understanding.

However, chicks still smell like good food to most JCPs, so scenting a rat with a chick is sometimes a good trick.