What's Fred barking at? Nothing by the look of things! Nothing?! Open your eyes man! Behind that bushes Jock with my bone!
Fred has grudge about Jock stealing Fred’s bone; therefore, Fred just barked at Jock as warning not to steal his bone.
I know there are no reason for those dogs barking at nobody out there. Sheeshh! That is so annoying! Barking nuisance!