Tosca, "Death to All Humans!" Psychobird.

My cockatiel, Tosca, had been my only pet  for years.  He seemed happy when I was around, but what about when I was at work?  Was he lonely?  Sad?  Cockatiels are social birds.
Certainly he'd appreciate a buddy.
I picked out a friendly yellow bird, took him home, and set him up in a nearby cage.
I figured they'd soon be best buddies:

Tosca seemed interested in his new neighbor....

Then it happened.
Tosca's flocking instinct went berserk.   In his mind, he was the leader.  The new bird was an underling.   I was a non-bird: The Enemy!
Overnight, he went from being Tosca, Mild Mannered (if shrill) Bird:

 

To Tosca, "Death to all humans!" Psychobird.

He hissed, squawked and lunged when I got near his cage.  I thought he was bluffing. 

 

Certainly he wouldn't bite me when I fed him, right?

This was no play bite or warning nip.  This was a hatchet chop with beak grinding.  My fingers were bleeding.  I'd expected him to like the new bird more than he liked me.  I wasn't expecting attempted murder.  How was I supposed to give him food and water?  Or clean his cage?
And what kind of example was he setting for the new bird?
Hey kiddo, here's how to treat human beings!

I asked for advice on a cockatiel message board. "He's probably jealous," they observed. "Give him some extra cuddles..."
Do YOU want to give this guy some cuddles?

Tosca has never been a cuddler.  He's strictly a "I'll stand on your finger if you don't try to touch me" bird.
Except now it's more like "Get near me and I'll rip your damned hand open"

I draped a towel on the side of his cage.   I figured maybe if he couldn't see the new bird he'd calm down.  Will it work?  He seems a bit less aggressive...

... but he also seems to remember who put the towel up. The current vibe I'm getting from him is "I still hate you, but since you bring me stuff to eat I'll let you live.  For now."
You're welcome, Tosca.