Barty Crusoe and his Man Saturday by Frances Hodgson Burnett

Barty's bed of leaves was so comfortable that he slept all night like a dormouse and never rolled over once. There is no knowing when he would have opened his eyes if he had been left to himself, but when the sun had risen and begun to make the blue sea look as if it were sparkling with diamonds, he suddenly awakened and sat up to listen to something he had heard in his sleep.

What he had heard was Blue Crest. There she sat on the edge of the cave window, whistling the calling song she had learned from him the day before.