The Da Vinci Notebooks at sacred-texts.com
The virtue of gratitude is said to be more [developed] in the birds called hoopoes which, knowing the benefits of life and food, they have received from their father and their mother, when they see them grow old, make a nest for them and brood over them and feed them, and with their beaks pull out their old and shabby feathers; and then, with a certain herb restore their sight so that they return to a prosperous state.
The toad feeds on earth and always remains lean; because it never eats enough:-- it is so afraid lest it should want for earth.