Had a sparrow hit my armored windshield at 500kts (EA6B at 500' AGL on a military training route) - completely shattered, nearly penetrated into cockpit. I can assure you that hitting a bird can quickly become a safety of flight emergency in an aircraft. Here's a few pics to see what birds can do:



Even if damage to engine is contained, you often lose redundant electrical and hydraulic systems due to compressor / turbine blades cutting lines etc. Leaking hydraulic fluid on hot section of engine often then leads to inflight fire.

I can only imagine what a 2lb quad full of metal motor casings and lipos would do.