I'll show a "cheat" and I have never, ever in all discussions about this photo stated anything other than it is a cheat.
the two birds in this photo are adult and chick , both were photographed at this exact spot, on this exact hole in the tree, on the same day, just not at the same instant.
the only manipulation was to cut the chick (and the surrounding shadow of the hole out of one photo and place it in the other. the alignment of the beaks was pure chance.
I will admit, that even though this pose is possible, (i have one from a previous year that is similar but a little blurred due to low light) it is a cheat.
Now, when it comes to other cheats, touching up photos to remove blemishes etc, i think the issue would be, if it is done with the subjects permission then no, but if it is done without, then yes. Also if manipulation is done to imply something, then even if done with the subjects permission, it may be considered a cheat. i.e. removing blemishes to promote products related to skin appearance,/ blemishes would be a cheat.
I will separate here art, advertising and such from accurate reporting. reporting should not involve any manipulation that adds either color, or adds or removes image content. I will accept lighting to isolate background but thats about it.