I'm confused about the placement of "also" here. If it's modifying Marcus, doesn't that mean the parrot was killed more than once?
Yes, that is why I opened this page too. How can one specify that Marcus is killing the parrot (as well as the cat)? (Rather than both Marcus and Stephanus set out to kill the parrot).
In the first case you would say: Marcus psittacum quoque interficit.
Ah, but we don't know that what's at issue here is "the" parrot. It could well be "a" parrot. Maybe the preceding sentence was "Vesperi Brutus psittacum interfecit".