The Groove-billed Ani is similar to the Smooth-billed Ani but has a smaller bill with a smooth curvature to the culmen and grooves (often difficult to see). They are best told apart by voice. Grackles have much thinner bills.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.