If you have chickens, you must lock your coops at dusk.
Do not leave feed out, do not leave water out if you can help it.
Rats (and mice) can spread disease as well as eat ALOT of feed, and thus reproduce.
Big roof rats can and will dine on baby birds especially at night when the hen is blind in darkness.
Not to mention that racoons, possums and stoat & mink are right there to help the rats dine.
A coop left open at night is also subject to owls.
I have a friend in BC that tells the story of sending her daughter (10 years old) out to the coop to collect eggs one winter morning at about 6 am.
The girl returned saying there was a giant mean bird in the coop !
She went out to see what the girl was talking about, and there indeed , was a great horned owl (she says about as tall as her breasts !) and in one clentched claw was a hen, still shrieking...and the owl walked out of the coop, hen on 1 foot...she backed away and let the owl go, off he flew into the darkness.
Moral of the story is, even if you have a great fence around your coop...predators will still get in !
There is food in there !
LOCK your coop at dusk !
(everyone loves chicken !)