What mammal or creature dropped this? (Warning: Gross Pictures)
You can see a similar pile of bird poop seen below a barn swallow nest: 



 Source: deepmuckbigrake 

Alternatively, if it really did appear overnight, perhaps a pile pf pigeon poop was pushed out of a nesting area in the soffit?



 Source: Scranton Times Tribune 

If it's pigeon poop, you might want to be careful.