what does the nightmarish bird whistle do?

[Sacco]

After the new beta klei added the nightmarish bird whistle, a nightmare version of the traditional bird whistle, which calls the BFB.

The problem is... what does the nightmarish bird whistle do?

the description says "strikes fear into the even biggest birds", but it doesn't do anything when i use it and there is the BFB on land, i tried to use it in other situations but nothing happened too.

You also HAVE to kill the ancient herald to craft it, because the crafting recipe require a bird whistle and two dark tatters, so it's a mid/end game crafting recipe.

I have not found anything on the internet yet, so does someone know what this thing do?

Thank you.

[Szczuku]

Apparently it stops the natural spawning of the BFB

[oCrapaCreeper]

It's supposed to just stop the BFB naturally spawning together after you use it once, then if you wanted to spawn him on purpose you'd just use the regular bird whistle. 

Maybe the nightmare whistle should also chase it away if it's already following, if it doesn't already?