Looks to me like the same problem I've been struggling with lately, Wayne. I don't think these are actually JPG artifacts
per se, they are the result of colour compression - fitting the very large number of colours in the original scene into a much smaller number of colours. JPGs do this, of course, but you can get exactly the same sort of problem with any other file format when you have a small enough number of colours - PNG for example, or even TIFF.
This happens all over the image, but you can only see it when there is (a) no other detail to mask it (such as plumage), (b) not a lot of noise (random noise masks it well), and (c) enough colour graduation for the effect to show.
Printers (as I recall) don't have as fine a control over colour as a good screen does, so it's reasonable to expect problems when printing.
One of the causitive factors is an uneven histogram. You can start with a picture that is perfectly OK and make a JPG no worries. Then apply some PP that changes the histogram (curves, levels, shadow/highlight, even noise reduction in some cases) and make a second JPG. BAM! The problem is there.
I assume that it's worse again when you put the image through conversion to a different colourspace - which is exactly wht happens when you print, of course.
The cure is to return to the original image and do your PP differently ... if you can! Otherwise, you have to do some heavy-duty Photoshop stuff ..... which is the point at which my slender river of knowledge dries up and fades gently away into the sand.