culmen

Culmen is a term used in zoology, most often in ornithology, to describe the dorsal midline ridge of a bird’s upper beak (the upper mandible). It denotes the length and contour of the upper bill from its base near the nostril area to the tip, and is a standard measurement in beak morphometrics. The culmen is important for identifying species and assessing variation in bill shape among individuals and populations.

In practice, culmen length is measured along the midline of the upper bill, typically from just behind the nares to the tip. The shape of the culmen—whether straight, curved, or angled—contributes to the overall form of the bill and can reflect ecological adaptations, such as feeding strategy or prey type. Along with other measurements like bill depth and culmen width, culmen morphology helps explain how birds interact with their environments and differentiate closely related species.

Etymology of culmen traces to Latin, where culmen means “summit” or “top,” a sense that reflects the ridge-like appearance of the upper beak. While most common in birds, the word historically carries broader uses in anatomy and geography to denote a top or apex.

See also: beak, bill, culmen length, bird morphology, morphometrics.