Chestnut-backed Chickadee

I thought chickadee immediately when i noticed two fluttering birds with red brown color on their back, black head cap and white cheek. They ate the seeds from the pine cones , were very shy and fast and I was surprised that my many pictures showed any bird at all. Obviously the rather grey belly seemed white in the full sun.
The only thing that confused me was that they seemed too large for a chickadee, I would guess they were 15 cm long