Hedwig's large, round, amber eyes gazed at him reproachfully over the dead frog clamped in her beak.
‘Come here,’ said Harry, picking up the three small rolls of parchment and a leather thong and tying the scrolls to her scaly leg. ‘Take these straight to Sirius, Ron and Hermione and don't come back here without good long replies. Keep pecking them till they've written decent-length answers if you've got to. Understand?’
Harry Potter and the Order of the Phoenix
I guess "if you've got to" here probably means if you have any chance or if necessary. But I have no confidence at all. What does it mean here?