with any camera, if you look closely enough, you’ll see grain.
there’s not a single factor to overall image quality. if you want a bridge camera like the FZ1000, i think the only ones that will produce better images are the FZ2500 and the Sony RX10 models, but the RX10 IV is 6 years old at this point, and the FZ2500 is 8 years old. Sony has said that the Alpha line is the intended successor to the RX10, and it’s unclear if Panasonic will ever have a successor to the FZ2500 (though i doubt it).
that said…
because, with birds, you’re often dealing with a distant fast-moving object the better the sensor, the better the lens, the better the autofocus, the better the everything, the better your photos will turn out. it’s just a question of when is good enough good enough? oh, and how much money do you have to spend?