Mountain bluebird?

[Joe Austin]

Photographed 3 weeks ago along the Madison River in Yellowstone. Looks like the gray mountain bluebird illustrated in the Sibley guide to me but I'm far from certain......

 

[Daniel Cadieux]

That is a Mountain Bluebird for sure. "Gray" is possible, but also possibly just simply a female or young one just about to lose the "scaling" on the belly feathers.

 

[Joe Austin]

Thanks, Daniel. What would be the clues that would rule out/in the 'Gray'?