In this month, the ground softens and earthworm casts reappear, inviting the return of robins. Tribes from further north knew this as the full crow moon, when the cawing of crows signaled winter's end, or as the full crust moon, after the crust of ice snow develops from thawing by day and freezing at night. Another variant was the full sap moon to mark the time for tapping maple trees.