Beaks never grow out of the back of the head, its an illusion caused by the duck being behind the front one. There is a space between them visible though.
Yup, having shot thousands of bird images over the years, I am well aware of the "illusion" factor and that beaks don't really grow out of backs of heads, but the illusion makes it looks like they do to the casual observer. This is one of many of the types of illusions that have been referred to as "pokies" - things sticking into, or out of, an image, or parts of an image where they probably shouldn't be. How many times have you seen folks take casual images of their friends only to find a lamp post, or a tree branch poking out of their head or the side of their torso. Sometimes they can be cropped out and sometimes not.
It is just that when you have two or more birds in an image, it is not usually the best thing to have them appear to intersect with each other. I have done it and a lot of folks just starting to photograph birds, including me, years ago, do it without even noticing it has happened until someone points it out. Fortunately, I had Art Morris to point this out to me on a workshop many years ago.
For those interested in bird photography, there is a web-based book, free for all to read. Its table of contents can be found here -
Secrets of Digital Bird Photography
Hope this helps.
WesternGuy