The way the wood has been taken out looks like rodent marks,the only rodents I know of with teeth that big are coypu and beaver,but beaver don't live in trees.I'm not sure about coypu though.
Can't see Coypu being arboreal but could be very wrong!
Trying hard to do this without 'google' help.
Bird wise i may have went for Goosander but think that's been ruled out.