The trouble with your example is that it conflates common knowledge and expert opinion.
No one would question a parakeet trainer now if he claimed dogs respond to Pavlovian conditioning, because it is common knowledge that dogs respond to this type of conditioning.
However, if Pavlov had been a parakeet trainer and simply asserted dogs respond to Pavlovian conditioning rather than discovering and researching it, it would not have been an ad hominem argument to state he doesn't know what he's talking about... because he didn't until he studied and researched it.
"He trains parakeets, so I don't find him a credible source of dog behavior" is not.