Why these specialized fish-eating ducks are known as the sawbills of the water

Nature · Published 3 October 2026 Swipe more like this →

In short: Mergansers are unique diving ducks equipped with serrated bills designed to grip slippery prey. While most ducks favor mollusks, these birds are specialized hunters. Their name, meaning diving goose, reflects their aquatic prowess, yet their history spans millions of years, from ancient fossil records to modern conservation challenges.

The genus Mergus, established by Carl Linnaeus in 1758, encompasses a group of highly specialized fish-eating ducks. These birds are distinguished by their long, thin bills featuring serrated edges, a physical adaptation that earns them the nickname sawbills. Unlike other ducks that forage for mollusks, Mergus species are dedicated divers that submerge completely to pursue fish. Anatomically, they share a rare skeletal trait with related genera like Bucephala and Mergellus: their sternums contain holes surrounded by bone rather than the typical notches found in other Anseriformes.

The genus currently includes four living species, though their status varies wildly. The Brazilian merganser is among the world's most threatened waterfowl, with estimates suggesting fewer than 250 individuals remain in the wild. In contrast, the common merganser, or goosander, maintains broad populations across the northern hemisphere. In the United Kingdom, the goosander has expanded its range significantly since first breeding there in 1871, spreading from Scotland into northern England, Wales, and the southwest. This expansion has occasionally led to conflict with human interests, as the birds' appetite for salmon and trout puts them at odds with local fishermen.

The evolutionary history of the genus is complex, with genetic analysis clarifying relationships that were once misunderstood. For instance, the smew was previously grouped with goldeneyes due to natural hybridization but is now recognized as basal to other mergansers. Fossil records provide further depth, with species like Mergus miscellus appearing in the Middle Miocene, approximately 14 million years ago. While some historical classifications—such as the Early Oligocene Sula ronzoni—were later debunked as misidentified, the lineage remains a fascinating example of avian adaptation to riverine and marine environments over millions of years.

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