The reason that Karana killed these ten birds was that she wanted to make a skirt for herself. She was going to use the birds' skins and feathers to make herself a skirt.
Because she wanted this kind of a skirt, she went to Tall Rock and killed the ten cormorants. She took off their skins and hung them up to dry. Eventually, she used the skins and feathers to make the skirt.
She thinks about giving the skirt to the Aleut girl when she makes friends with her. But she does not and eventually wears it when she leaves the island on the ship.