While outside cutting wood the other day, I kept hearing something growling, but when I scanned the forest, I saw nothing more menacing than a couple of gray squirrels performing acrobatic stunts and death-defying leaps among the branches.

Squirrels are not just extraordinary aerialists but amazing vocalists that alternately bark, chitter, coo and groan — but the more I listened, the more it sounded like a guttural croak.

Tree frog? Nope, way too early in the season.

Raccoon? Fox? Coyote? Fisher cat? Bobcat? Maybe — but the noise emanated from the direction of the lake. I had seen a pair of minks scamper along the frozen banks a few weeks earlier, so speculated they might be the source.

Then I noticed a flock of hooded mergansers splashing around in a patch of water surrounded by ice. Hmmm — mergansers are a species of duck, and I expected them to quack, not growl.

Time to check YouTube.

Sure enough, a video of the male’s mating call made it clear what I had been hearing.

Hooded mergansers don’t just growl when trying to attract a mate, they raise a white crest on their heads that make them resemble a cross between a Roman centurion and Mr. T of the 1980s TV show “The A-Team.”

Among the smallest duck, the merganser is the only one with a serrated bill for catching fish — thus its nickname, “sawbill.”

A few years ago, my son Tom and I launched a five-day paddling expedition on Sawbill Lake in northern Minnesota’s fabled Boundary Waters Canoe Area Wilderness, where hooded mergansers abound, but I don’t recall seeing any then. It was late in the season — our campsites were covered in frost most mornings — so perhaps the birds already had taken wing for the winter.

Hooded mergansers migrate from the Great Lakes and other points north to ice-free waters throughout the East Coast, venturing as far south as the Mississippi Delta while nesting in tree cavities. It’s entertaining having them visit, even temporarily, because they have more attitude than the mallards, teals, canvasbacks and domesticated Pekin ducks we often see swimming in local waters.

Those are called dabbling ducks, displaying “duck butt” when they dip down to feed.

Hooded mergansers, though, are diving ducks that disappear beneath the surface to grab fish and other small prey. They pop up, swim around for a few seconds, and dive back down, over and over. I got tired just watching them.

The flock also seemed indifferent to other birds in the vicinity and didn’t flinch earlier this week when a pair of bald eagles landed on the ice a few yards away. I’ve heard other birds screech in terror at the approach of such giant predators, spreading the alarm, but the mergansers appeared too busy even to glance in the eagles’ direction.

By the way, one of my favorite eagle encounters occurred a few years ago on the Hudson River roughly midway through a 300-mile kayak trip from the Canadian border to the Statue of Liberty.

While paddling through Saugerties, N.Y., I stopped to watch a great blue heron lurking near a shallow pool that was brimming with shiners — the avian version of hog heaven. The heron was preparing to chow down on the small fish when an eagle sailed by, jammed on its air brakes and swooped down to the feast.

The heron wasn’t about to share, though, and the two birds got into a flapping, pecking and screaming battle that lasted several minutes.

Finally, the heron ruffled its feathers and flew off with an angry squawk, and the eagle began gorging itself in earnest.

Anyway, the eagles I watched earlier this week must have been bored, lazy or satiated, and ignored the mergansers. It’s as if they were sending a message: “Don’t mind us — we’re just taking a short break.”

Sure enough, the eagles soon sprang from the ice and soared away. The mergansers never even looked up.

I know all birds are warm-blooded, but that was real sangfroid.

Who knows — maybe the mergansers will stick around. Why bother flying all the way back to Minnesota when you can hang around Connecticut all year?