I've had the sense that spring is the worst for mammalian predators, perhaps because they have hungry young to feed, but winter is worst for hawks.  We have more hawks coming through during fall migration, but more risky, desperate attempts by hawks during winter, when they are starving.  In winter, they'll come down even when I am out there with the chickens.  Also, the chickens are more exposed to aerial predators during winter, because the leaves have fallen, leaving them with little cover.  I once had one capture a chicken not 15 feet away from me, but the hawk was small and could not lift it and fly off. I ran at it, thinking it would fly away, but it didn't -- I grabbed the hawk and the chicken and pulled them apart.  Yes, it sounds stupid on my part (no gloves), but I didn't think, it was a reflex thing.  The hawk seemed as shocked to have a human snatch away its prey, as I was to see the hawk make such a bold attempt.  Hen was unharmed.  I still let them out during winter, but not for more than a few hours/day, and I have to be very watchful.