Birds start screaming at 4am like they just won the lottery. They didn't. They're just awake and want everyone to know about it. That's the whole secret behind the dawn chorus—it's basically nature's version of posting "up and at 'em" on social media before the sun even thinks about clocking in.
Scientists say birds chirp early because the air is clearer and sound travels better. Sure. Or maybe they're just competitive little weirdos who found out that being the loudest gets you noticed. First one up wins the worm, brags about it, then judges everyone else who rolls out of bed after sunrise like some kind of failure.