The kinetic energy of the bird hitting an object is
E = 0.5*m*v^2
Let's see, speed is about 200 kts and the bird weight is 4 lbs. That is 103 m/s and 1.8 kg, so we get
E = 0.5*1.8*103*103 = 9550 J
Wow... that is some load for a plexiglas. There is "Plexiglas T3" (trade name) available, which is 22 times tougher than standard acrylic sheet. The following link seems to have some design information.
Let's do some additional estimations. If the impact energy is stopped so, that the plexiglas bends 0.2 m (20 cm) and since J = Nm and the energy goes from full to zero, we can now do a raw estimate:
E = 0.5*Fmax*s ->
Fmax = 2*E/s = 2*9550/0.2 = 95500 N ~ 9700 kg !!
Even the average force is about 5000 kg or 5 tons. A very good reason not to have a windshield angle too much with the horizon.
Very hard to say, how thick the plexiglas should be without testing, since the impact angle is also a very meaningfull factor.
* * *