Crows do not hunt. They are opportunists that feed mostly off plant matter an carrion At times they've been spotted raiding nests for eggs or defenseless hatchlings, and a mouse can make a nice snack, but they dont really bother with any adult birds - too much work and theyre poorly equipped for hunting. 
At 25 weeks your birds will be fine, I've never heard of crows hurting chickens.
If you want to chase them off, most crows are easily spooked by noises and movement, things like windchimes or CDs spinning in the wind. There are lots of tips online because they can be quite a nuisance to some people.