I love the idea of having a nighttime/nocturnal mode. A good replacement for the day time predators like the blue birds could be some sort of owl like creature which could perch in/on trees and otherwise works the same way stealing baby niches if they have one gem and are left unattended. For the bearyena replacement I'm not sure... I would suggest a fox like creature kind of like the arctic ram foxes but a bit different. It could attack adult nichlings kind of like a bearyena, but if it were to attack a baby nichling it would try to run away with it and you would have to chase it and kill it to try to save the baby. If it got away into a fox den or something then it would then "eat" the baby nichling. I know this would be extremely difficult if not impossible and would probably take a very, very long time being that you would basically be remaking the entire game to work at night time and replacing all the predators, but if it were to happen these would be my suggestions for possible predators that could work in a nocturnal environment.