A way to send messages for those in the pen and the sound of a song from a feathered friend

found in the sea but used as a greeting

I flow like the sea in a crimson cave, And am often swapped with a date To show the passion that you both crave, But release me to show your hate

Scientists, pockets, engines, and bottles have this in common with me.

I flutter in silence, yet rule with might, wings painted in crimson sunset, or robes edged in light. In gardens I dance, in courts I preside, a creature, a crown, two worlds I divide.