I cannot quote the references - but from memory :
Birds can and do fly in IMC. This is particularly true of the migrating versions. IMC / Cloud penetrations are unavoidable and they will climb before making IMC so that they can use what I believe is termed a Dihederal Glide. Notwithstanding that birds have a sense of the Earth's Magnetic Field (proven) to a sensitive enough degree to resolve the DIP component is unproven.
But the glide is a fact - they will hold their wings at an angle and sense sideslip on the primaries and correct. This ensures that they keep the Blue Side Up even in IMC. If the IMC duration is prolonged they go into a power / glide / power / glide routine. Bad turbulence is a problem, but lift in moderate nimbus activity is a bonus I would suppose.
There are lots of birdstrikes and sightings in IMC and at night. But then if we really understood the mechanics of bird flight my Boeing would probably have feathers.
MG
