Culmen

The culmen is the dorsal ridge of the upper mandible. Likened by ornithologist Elliott Coues to the ridge line of a roof, it is the "highest middle lengthwise line of the bill" and runs from the point where the upper mandible emerges from the forehead's feathers to its tip.^[28] The bill's length along the culmen is one of the regular measurements made during bird banding (ringing),^[29] and is particularly useful in feeding studies. 

The shape or color of the culmen can also help with the identification of birds in the field. For example, the culmen of the Parrot Crossbill is strongly decurved, while that of the very similar Red Crossbill is more moderately curved.