The Nightmarish Bird Whistle doesn't seem to be working.

I gave the item a test myself, and found out that it's not meant to stop the BFB from landing when it's flying overhead. What it does do, however, is stop it from swooping in and landing ever again at the player's own discretion.

You can enable the BFB flying and landing once more if you play the normal Bird Whistle.

Here's a wiki link, if needed: https://dontstarve.fandom.com/wiki/Nightmarish_Bird_Whistle

Scaring the bird away when it already appears would be expected I suppose if you made it blind, but yes the purpose is to stop spawning altogher.