Cathy is Mitch's kid sister—and being a kid sister is 100 percent her character. She's cute and winsome and … cute. She adores Mitch, and she likes Melanie because Melanie brought her some lovebirds.
She, of course, wonders why the birds are attacking and looks to Mitch for the answers.
Cathy suffers through most of the major bird attacks and has to witness her beloved teacher being pecked to death. But, she refuses to become a bird bigot. When the family flees the house to take Melanie to a hospital, she insists on bringing the lovebirds with her.
Veronica Cartwright, who played Cathy in one of her first film roles, went on to have a lengthy career in film and television, including a couple of other major horror roles in Invasion of the Body Snatchers (1978) and Alien (1979).
Cartwright started out being terrified and decided that she'd keep on keeping on.