During a drought, G-d commanded ravens to bring food to Eliyahu (the prophet Elijah). Way back in parshas Noach, we are told that the raven Noach sent was brought back "until the waters dried up" (Genesis 8:7). Rashi there quotes the Midrash that the raven was relieved of duty in favor of the dove and assigned another task - to bring food to Eliyahu at a time when "the waters dried up."