Apparently, it's used for more aquatic birds, like ducks as well.

But they also sort of mean the same thing. Bills are typically rounder.

sorry, I don't know (and Google doesn't seem to either: http://isearch.avg.com/search?cid={20e2c62e-2... )

It seems you can say either.

Thanks to both of you - it seems that both terms fit the bill.

@1: How about "Time to Rise", one of the few poems out of Robert Louis Stevenson's "Child's Garden of Verses" that I still know by heart? Definitely not about an aquatic bird, I'd say ;o)) (IIRC, the illustration featured a blackbird)

A birdie with a yellow bill
Hopped upon my window sill,
Cocked his shining eye and said:
"Ain't you 'shamed, you sleepy-head!"

These two sites, which seem very good as far as I can judge, both use "bill":

http://www.teara.govt.nz/en/penguins
http://www.penguinworld.com/index.php

(The latter repeatedly in the descriptions under "Types of Penguin.")

bill und beak sind Synonyme, wobei "bill" für mein Gefühl das allgemeinere Wort ist welches anatomisch nicht nur das Horn sondern auch den Knochen einschliesst.
Webster's New Encyclopedic Dictionary sagt dazu "bill: 1) The jaws of a bird together with their horny coverings..."
Ausserdem wird dort nochmal hingewiesen: "In popular usage BEAK is applied especially to the strong triangular pointed or hooked shape associated with striking, tearing or crushing (an eagle's beak) while BILL is applied to the structure in any bird (a duck's bill)"