Your chickens will be in their night house I hope, Im a coturnix quail breeder so I know the light 'issue'.
But then you have to place 2 varifocal next to each other to have them both zoomed in
. Or have motion activated lights. You can't fix zoom with more megapixel. I live in the country side too, but having the property lit does deter people with bad intentions. I haven't seen a single one yet dying the night hours.
It's best to place 2.8mm or 4mm overview cameras, and have narrow (choke) points where you have higher focal length models.
Heck, I even use a 2mp 32mm IR dome indoors on the horse stables just for this purpose, zoomed all the way into the door, where a 4mm above it gives a nice overview Of the stables. So anyone going in, has like 800ppm on its face