That spruce will burn great.
Nice Demonstration
It would be nice to track one birch split's weight cut now VS what it weighs 2 years from now.
I learned that 2 yr old seasoned birch burn significantly better than 1 yr seasoned birch.
It has allot of water to shed before it's close to the 10% the spruce shows.
Don't know what the moisture reading is on the birch I've cut this spring & summer
but I know it's allot heavier than the 1 & 2 yr old splits in the shed. I bet close to your 40%.
1/2 a cord is a good load on my trailer. (almost 1/2 the weight is water
)
Another reason to leave it S&S where it is till late Fall, it'll be allot lighter & easier to move
Makes sense why birch logs & rounds will start to rot, It's got no where to breath except the ends with the water tight bark.
Also spruce dries much quicker & has less % moisture to start with.