The other week, we found two large formations of geese just above a solid cloud cover (base 900 or so, tops 2500). They appeared to be joining up, probably preparing for a leg of the migration. Hundreds of them. Rather impressive, but unfortunately also smack dab in the middle of the ILS for the runway in use of a major airport.
The birds had found a 'valley' in the solid cloud cover, in which they were forming up. Nasty surprise for a jet crew coming out of the wall of said valley on the ILS, to say the least.
Not in cloud per se, but probably would appear to be to anyone unfortunate enough to fly into them.
Regards,
/Fred