As I was saying to Fred, I had a couple of cockatiels.
One morning, my main squeeze of the time neared the cage; the birds went beserk. I gave the guy a proper talking to, believing he'd tortured them - they were terrified of him.
Later that evening I saw both birds calmly perched on the side of his hand.
The next morning as I watched him approach the cage, the birds flappped and screeched.
After several episodes, we discovered it was his neckties - particularly diagonally striped ones. Turns out that some birds are instinctively afraid of snakes and snake-like things. After this, we draped ties over the curtain rods to keep the birds from perching there.