With the usual caveat about the specific regs applicable to a given type, here is the relevant section for birdstrike....
Sec. 25.571 Damage--tolerance and fatigue evaluation of structure.
.....
(e) Damage-tolerance (discrete source) evaluation. The airplane must be
capable of successfully completing a flight during which likely structural
damage occurs as a result of--
(1) Impact with a 4-pound bird at Vc at sea level to 8,000 feet;
Vc is the annotation used for the structural design speed which is usually coincident with Vmo, although they can be different if so desired...
VMO/MMO must be established so that it is not greater than the
design cruising speed VC
So, to answer the question:
Is 8-10.000ft the official certifiation altitude limit when it comes to bird strike, or is it an altitude chosen by DeHavilland?
8,000ft is a specific certification requirement. The 'illogical' nature of the Vmo cutback isn't the fact that it has a given value at 8,000ft - it's the fact that the speed can suddenly increase at 8,001ft. Logic would suggest that some kind of ramped Vmo limit would be consistent with the (PRESUMED) declining probability of bird encounters between, say 8000FT and 12000FT. But, of course, the reg as written and applied has worked, so why mess with it I guess.
Now, I don't know the certification issues of the various types, but:
And if I understand you correctly, a B737, A319 or Lear Jet have wind shields that can withstand a bird strike at, say, 300KIAS? Then I wonder why the Q400 could not have been fitted with stronger windows?
Be very careful in assuming that because the Q400 has a lower limit, its windshield is less strong. There are many instances where, due to more stringent regs, or more stringent interpretation of regs, a later-certified aircraft may have limits applied that an older types does not - even if the "real" strength of the windshield is actually better for the more modern type. You can't reverse engineer the design characteristics from the limits without knowing a LOT of background ....