With a song to guide them.   At the edge of the forest, at their mother's gate, Bird began to sing. "Two little cygnets," she sang, "starting on their way." It was from an endless nursery-rhyme song their mother used to sing when they were small. "The path was dark,/And they went astray."             "We won't go astray," said Summer. Bird didn't answer, but with a twist of the pink backpack, she stepped ahead of Summer, and entered the forest first. Excerpted from Summer and Bird by Katherine Catmull All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.