IF you've got a good-sized flock of flighted guineas who are allowed to free-range within a limited yard of about an acre or so and who stick within that territory for the most part, then yes, they can help with keeping daytime aerial predators at bay and sounding the alarm on everything else. They especially hate hawks and ravens who try to roost nearby to look things over and will typically gather en masse under wherever such predators are sitting on and start screaming their heads off at them. The noise alone usually drives these birds off. They will also follow and scream at anything doglike...foxes, coyotes, actual dogs they don't know...and will alert and yell some at mink and weasels, but aside from being annoyed by being discovered, ground predators won't be much deterred by them. Snakes are an exception. I always knew when the guineas had found some poor garter snake trying to cross the lawn portion of my yard because I'd see them clustered together and circling around in a ring, yammering and working up their collective courage to attack. I'd go break it up and there'd invariably be a freaked-out snake in the center of the ring, partly coiled up and striking out in a desperate attempt to save itself. Don't know how guineas would do with large snakes. Ours are all small and were always in far more danger of being killed by the guineas than vice versa.
Guineas are brave birds and one of my males, who was accompanying two females who each had a clutch of several-weeks-old keets, once stood up to a goshawk who flew down at them and landed to try and flush the families into flying and scattering. Didn't work. The hens and keets just scooted into the nearby weeds and crouched, and the male guinea came forward to confront the hawk, doing that hatchet hack thing with his head to show that he was willing to fight. The hawk was so flummoxed by this unexpected behaviour that he just stood there until I came running up to chase him off. I would have found it hard to believe that a guinea would offer to fight with a hawk who stood as tall as himself, but now I know that there was at least one who would, and as far as I know, that particular hawk never bothered my guineas again.
