Feathers white as refracted light, a neck curved like a sigmoid line. Territorial, yet moving with grace - what creature rules a mirrored place?

What kind of ship has two mates but no captain?

I write without ink, disappear with a swipe. I’m held in a hand, slim and white. What am I?

I once was sap in forest shade, then heat transformed what nature made. Now I may bounce, erase, or roll, a stretchable thing with many roles.

No matter what you say, I'm peak. No cap, except ice on me. When snow sloughs off me, don't trip, My melting streams are pure drip.