A magnificent bird and a great example of evolution. That "form follows function", although a phrase attributed to modern architecture, is in clear evidence here.

There is surprisingly little information to be found on the internet about this bird.
But for probing deep into it's habitat for food, I can see where a curved beak would be most handy. With a straight beak, the bird would have to bend it's neck at an extreme angle to reach food that wasn't on the surface, while tilting it's rump upwards and it's head downwards. That curved beak allows it to maintain a normal balanced posture while reaching deep into crevices or moss to forage for insects etc. A truly fascinating bird.