Luckily, I wound up with a few shots that hit both of these themes. The two best options, in my view, turned out to be quite similar. Here are the mockups I slapped together (for other covers in the series, see here). You can click to expand:
For the original higher resolution shots, see here (left) and here (right) — also a fun one here. That's a great blue heron (I think) with the iguanas on the left and a Darwin finch (female medium ground finch, I think) on the right (kinda cool that both species on the right cover are endemic to the Galápagos).
Any preferences? I can go with a different color too, of course.