The July COTM uses Birch in the neck.  
 
What type of tone does Birch impart?

Great question.  Seems all Brown Basses have birch laminates (including mine).  Not sure specifically what tone they impart. I'd say my neck is dominated by the larger mahogany pieces, but I never asked Mica what the theory was behind adding two birch lams around a central walnut lam (aside from looking incredibly cool and woody).
Bill

I love the look of Birch.  
 
Does it add to the sustain?
 
How stable is birch in a bass neck?

Apart from having wonderful sustain and (so far) stability in the neck of my BB, I have no idea how birch helps or hurts these things.  But inquiring minds want to know...

A quick google suggests that Birch is similar to Maple; not as bright; more mellow.

Thank you Dave