Sawbill, a word that refers to a species of diving ducks, also called mergansers, has several synonyms that can be used interchangeably. The most common one is "fish duck," as their diet primarily consists of fish, which they capture by swimming and diving underwater. Other synonyms for sawbill include "river ducks," as they prefer habitats near freshwater rivers and streams, and "goosanders," a name commonly used in Europe. Another lesser-known synonym is "shelduck," a term used in certain regions of the United States and the United Kingdom. These different terms are useful in providing more variety and clarity to descriptions of these intriguing birds.