Sonya had no means of 'human' communication once in hawk form. She merely stood waiting while Zira stretched out his wings.
"I normally don't get to either, I'm too much of a workaholic-or one by nature at any rate. Stubborn, maybe, but anyone who knows me will say that I'm the same way-if not more. I can't speak 'human' in hawk form, just to forewarn you."
Then she began her own transformation. There was no incantation, potion or amulet: she simply closed her eyes, focusing on letting the transformation take place. In less than a minute, Sonya's form was surrounded by blue/green sparks of light. When the sparks burn out, there's a hawk where Sonya once stood, her feathers brown and white, but there's a slight green/blue sheen to them that appears when the light hits her feathers at an angle.
The hawk lets out a shrill cry, stretching out her wings as she lifts herself up into the air, flying horizontal until she's completely off from the roof.