Hi Guys,
No interest in shooting anyone down, but since Mournings & Connecticuts both breed in the woods behind my house and they are a bit of a hobby of mine, I hope I can share some observations and an opinion.
First on behavior, the Mournings are listed as skulky in the guides, but aren't bad on the breeding grounds. They hop around trees all the time, albeit usually say 1-6 feet up, can be a bit noisy, and you if know their territories, not tough to find. Connecticuts are hard to find even when you know exactly where to look, they make Mournings look like exhibitionists in comparion. They stay in the deep cover, almost always under 3 feet in height. They do hop when going from branch to branch, but along the same branch or the ground they walk on those looong legs of theirs. I've never personally seen a Connecticut behave like Nick describes, and it would be unusual to be able to photograph one at that angle without being down on the ground.
For appearance and structure, it helps to be used to seeing birds of both species. In Nick's photo I see a bird with a thin eye ring broken on the entire upper right third, not big-headed like a CT, the throat looks on the light side and subtly yellow tinged in spots (confirmed using eye dropper and saturation tests in PSP), and the yellow underparts look on the dark side, All those characteristics add up to female Mourning in my opinion, particularly in combination with the behavior described.
I remember reading some bander theorizing that most Connecticuts seen out east are actually female Mourning Warblers with particularly strong eye rings, based on comparing RBA report ratios to banding station ratios. I do know that some of the (unmistakable) female Mournings that I see have what appear to be quite distinct and full eye rings, more so than this bird.