I had an idea to make cockatrices "realistic" (as in make a bit more sense) for what ever reason the noble cockatrice has evolved to use other species eggs for its own. It does this by "sitting" on usually a chickens egg. When it does this it inserts it's own embryo into the egg.
The cockatrice embryo proceeds to eat the chicken embryo and uses the nutrients in the egg to gestate. Once it gestates the cockatrice would scamper away into the wilds.
Would a parasitic embryo like this work? Could a cockatrice replace a chicken's embryo with its own?