There’s a feather hanging below the peak of Piero’s beak:
Only a value of -0.997 and higher for the influence on the hair/feather density of the texture which primarily controls the hair length (= body feather size) makes it disappear.
When looking at that texture in the “Image Editor” (set to “Paint Mode”), everything looked fine with the all black beak especially at the red circle in the image below pointing at the peak of the upper inner beak (with the upper throat at the right):

The same position in the “UV Editor” shows a more pointed “UV Map” which reaches into a white area:

Can anybody here explain this?
PS: Even painting the whole texture black didn’t make this feather disappear, painting the whole texture white, made this feather bigger.