Chris,
Comparing these two pictures, except for the common angle of the bird, one would think they were hardly the same creature, but two distinct species!
The change in form is simply a stunning adaption. The intervertebral disks must be elastic to allow the neck to elongate so! But that's unlikely to be the whole story.
Rather I expect that the neck is already almost that long but curved back on itself in the first picture, so the leverage of the bird on its perch is minimized. To catch a fish the neck is straightened and then the extra “muscle” force of gravity further elongates the neck, stretching the skin and giving that so different appearance!
But I expect you have figured this out already, many years back!
Both you and Doug Herr, as well as Peter Dexter do a fine job of sharing the fruits of your labor. I cannot say enough as to how much this is appreciated!
Without loving and feeling for these amazing creatures, how can we hunters respect that the land must be shared?
Asher