Everyone has probably heard the phrase “as scarce as hen’s teeth.” It means almost exactly what you think. Birds don’t have teeth, so it’s a way to describe something exceptionally rare. It’s a characteristic that sets birds apart from other creatures, such as mammals.

There are some birds, however, that come closer to having teeth than their other beaked counterparts. Mergansers would definitely fall into that category. The mergansers are a type of duck known collectively as “sawbills,” a reference to their long, thin bills with serrated edges that help them grip prey. The “sawbills” come closer than any of our other birds in having teeth, although not in the same sense as mammals have teeth.

Although classified as sea ducks, the mergansers are more at home in river habitats. There are six closely related species in three genera: Mergus, Mergellus and Lophodytes. The latter two genera have only a single species, the Hooded Merganser (Lophodytes cucullatus) and the Smew (Mergellus albellus).

The other mergansers in the Mergus genus consist of four species, Common Merganser, Brazilian Merganser, Red-breasted Merganser and Scaly-sided Merganser. The latter is an endangered species with only about 2,500 adult birds in the worldwide population. These remaining Scaly-sided Mergansers are found in the border regions of China, North Korea and Russia.

Another species, the Auckland Merganser, became extinct in the early 1900s. Sadly, the last evidence of this species dates back to Jan. 9, 1902, when the last wild pair was shot. After that time, this bird native to the Auckland Islands was never seen again.

In recent weeks, a couple of Common Mergansers spent a few days on the Watauga River in the Lynn Valley community. I saw them on a couple of occasions and also managed to get some photographs of the two handsome drakes.

In Europe, the Common Merganser is called a Goosander, probably a nod to its large size that makes this bird superficially more similar to geese than ducks. Early naturalists such as John James Audubon also provided this bird with a different name, referring to it as the “Buff-breasted Merganser.”

For many years, the Common Merganser was one of my target birds. Finally, more than 10 years ago, I saw my first Common Mergansers during a visit to Middlebrook Lake in Bristol with Reece Jamerson, Gil Derouen and the late Howard Langridge. Despite the word “common” in its name, this merganser isn’t particularly common in Northeast Tennessee. Its relatives, Hooded Merganser and Red-breasted Merganser, are much more regular visitors to the region.

In addition to Bristol’s Middlebrook Lake, I have seen Common Mergansers on Watauga Lake and on the Holston River in Kingsport.

The Common Merganser, particularly the males, are easily identified. Apart from their large size, which is about 26 inches long for males, males of this duck have a dark green head and upper neck. The lower neck, breast and underparts are creamy-white with a varying amounts of a pink or reddish wash. The back is black, while the bill, legs and feet are red. Females are similar to female Red-breasted Mergansers but show a clearly defined white chin patch lacking in their close relative.

According to the website Ducks Unlimited, Common Mergansers breed from Alaska, the southern Yukon, Labrador and Newfoundland south to central California, Arizona, New Mexico, southern Chihuahua and east of the Rockies to Minnesota, Michigan, New York, New England and Nova Scotia.

They are also one of the biggest of North America’s cavity-nesting birds, utilizing natural cavities in trees, as well as man-made nesting boxes. They will also nest on the ground.

Common mergansers feed mainly on fish, amphibians, crustaceans, mollusks and other aquatic organisms. The last extensive population surveys of Common Mergansers took place during the 1970s, when the population in North America was estimated at 1.5 million birds.

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While today’s birds, even mergansers, all lack teeth, that hasn’t always been the case. About 80 million years ago, a bird known as Hesperornis (“western bird”) swam the inland seas that stretched over areas from Kansas to Canada. They swam through those ancient seas because they could not fly.

The body plan of Hesperornis was similar to modern loons and even the mergansers. Instead of a serrated bill, however, this ancient bird had actual teeth in its long beak. Just like today’s loons and mergansers, it probably fished for its food.

At almost six feet in length, however, it dwarfed our modern mergansers and loons and probably fed on larger piscine prey. So, why don’t today’s birds have teeth? The best answer I have found is that teeth (and other solid bones) were lost in order to make the avian form more streamlined and lightweight.

The power of flight demands a great deal of energy, and teeth are an unnecessary weight. As a result, birds grew hollow bones and lost their teeth.

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I have been taking part in the Great Backyard Bird Count and have enjoyed some interesting observations that I’ll discuss in future columns. I would love to hear from readers. Just post comments on my new blog at ourfinefeatheredfriends.wordpress.com. You can also reach me on Facebook or send email to ahoodedwarbler@aol.com. Please share the link to the blog with others who might be interested in the topic of birds and birding in Northeast Tennessee.