Please please help me !!!! Whats the secret?

We have a secret, just we three, The robin, and I, and the sweet cherry-tree; The bird told the tree, and the tree told me, And nobody knows it but just us three. But of course the robin knows it best, Because she built the—I shan't tell the rest; And laid the four little—something in it— I'm afraid I shall tell it every minute. But if the tree and the robin don't peep, I'll try my best the secret to keep; Though I know when the little birds fly about Then the whole secret will be out.