However, I will agree that your argument about the density/compressibility of a typical LiIon battery has some merit. It would be interesting to see a more definitive study of the relative dangers of greater mass vs. higher density vs windscreens, rotor blades, etc.
Already done, the publicly released report is below. I don't think it has all the details in of comparison to birds but the results are fairly straightforward.
https://www.gov.uk/government/publications/drones-and-manned-aircraft-collisions-test-results
Non-birdstrike Certified Helicopter Windscreens
5.2 The non-birdstrike certified helicopter windscreen proved to have a low resistance to
all the classes of drones tested, with penetration through the windscreen shown to
occur at speeds well below the normal cruising speed of a helicopter of that type. For
the fixed-wing drone, which is itself capable of a significant speed in flight, it was
found that the drone could penetrate a helicopter windscreen of this type even if the
helicopter was stationary.
5.3 As general aviation (GA) aircraft do not have a requirement for birdstrike certification,
the result from the non-birdstrike certified helicopter could be read across to GA
aircraft with comparative severity in the result of a collision.
Birdstrike Certified Helicopter Windscreens
5.4 The birdstrike certified helicopter windscreen was found to be much more resistant,
but it was found that the quadcopter drones could penetrate these windscreens when
the closing speed was similar to the helicopter’s typical cruising speed. The speed
the fixed-wing drone can itself reach meant that it could penetrate the windscreen if
the helicopter was moving at a speed significantly below the normal cruising speed.
When the helicopter was stationary, however, it was shown that a fixed-wing drone,
when flying at its maximum speed, was unlikely to penetrate this windscreen.
Helicopter Tail Rotors
5.5 The modelling of helicopter tail rotors showed that they would be vulnerable to
impacts with all types of drones. Due to the very high speed of a rotating tail rotor
blade, it could be critically damaged by an impact with any drone.
5.6 Again, it should be noted that although the most accurate properties available were
used, the helicopter tail rotor results were based on modelling only, with no live
testing to calibrate the model.