What does Sarah say or do that makes Caleb think she will stay?

From the text:

"For the birds," said Sarah. "They will use it for their nests. Later we can look for nests of curls."

"Sarah said 'later,' " Caleb whispered to me as we spread his hair about. "Sarah will stay."

"I'll come, too!" cried Caleb. "Sarah said winter," he said to me. "That means Sarah will stay."