Biodeg. yes & no
Well, White Flyer Bio's are obviously biodegradable, that's how they're advertised. That would tend to indicate that the "traditional" clay bird is "non-bio" by comparison.
Have had a couple soaking 24 hr. Neither a Remington Blue Rock, broken or unbroken, nor a Sporting Clays White Flyer (badly chipped, so I deemed it served both ways) showed the slightest sign of decomposition of any sort from the immersion.
My conclusion: Don't throw/shoot clay birds where you don't want the pieces to remain, for a considerable period, unless you are using the "bio" type. And don't use 'em where hogs feed, as per instructions on the box. Maybe they do disappear into the soil given enough time, but in the meantime they are an unsightly mess, with sharp edges to boot.
Having said all that, I have to add, They sure are fun to shoot!
God Bless America