It’s been a weird week. I was motoring down Pushaw Lake in Old Town last Thursday, enjoying the unseasonably warm weather and fall foliage, when I spotted a long line of dark ducks a half mile away. Black scoters. A hundred scoters were loafing in the middle of the lake, presumably resting.

Scoters are diving ducks that nest on freshwater in northern Canada. They winter on saltwater along the eastern coast of the United States, especially along the coast of Maine. There are three species: black, surf and white-winged. Black scoters are all black with bright yellow bills. Surf scoters have white patches on the front and back of the head. White-winged scoters are larger and have — wait for it — white wings.

The migratory movement from northern Canada’s Hudson Bay to the Gulf of Maine means that many of our wintering sea ducks stop over in our freshwater lakes. It’s not common to see them, but it’s not a surprise either. It happens more often in Aroostook County, but it can occur anywhere.

Black scoters are nicknamed “butter bills,” and the yellow bills of these Pushaw Lake ducks shone brightly in the low autumn sun. Their identity wasn’t a mystery but their behavior was. Rather than bunching up in a tight flock, they were strung out in a long row. I see black scoters do that a lot, but I don’t fully understand why. I can hypothesize situations where that might be reasonable, say, when they are riding along a tidal eddy, but the tides in Pushaw Lake are notoriously imperceptible.

Does this alignment give them an advantage in spotting incoming predators? Perhaps they are less likely to crash into each other in an emergency take-off? Maybe it’s just their style. Even in flight, they play follow-the-leader more than most ducks, and they habitually dive together. This behavior is so diagnostic that it’s a key identification clue, even when the ducks are a mile away.

I did spot a couple of females in the crowd. Female black scoters are brown, with whitish cheeks. However, this Pushaw flock was 98 percent male. Another mystery: Do the genders migrate separately? This was becoming harder to understand than a midnight tweet from the Oval Office.

Two days later, I was onboard the Bar Harbor Whale Watch boat, Friendship V, as it made its annual lighthouse tour from Bar Harbor to Bangor. As we ventured through Eggemoggin Reach in Blue Hill Bay, four white-winged scoters flew by the boat. This was not shocking. Scoters nest in the subarctic, but a few non-breeders stick around Maine in the summer. They don’t bother to make the hazardous flight north if they are too young or disinterested to make babies.

What happened next is the most astonishing bird-thing I’ve experienced this year. An enormous flock of dark ducks was visible in the distance as we approached the Deer Isle Bridge. Some were sitting on the water. Some were flying about in tight flocks, like shorebirds. There were hundreds. I looked left, and there was another huge flock. I looked right, and there was another. There were thousands of scoters packed into the bay, acting very un-scoter-like.

These were surf scoters, and they were unusually restless. I’ve seen big flocks before, but never a congregation this colossal. I can’t imagine there is enough food in the channel to feed that many ducks for the winter, so I assume they had just arrived from Canada and would soon disperse. Scoters do not generally feed on fish. They prefer mollusks, crustaceans and sea worms, augmented by a little vegetation. It couldn’t have been a large school of fish that had drawn them into the channel. Weird things happen during migration season.

The truth is, we really don’t know that much about scoters. They nest in remote, inaccessible, freshwater wetlands south of the Arctic Circle. You can’t get there from here. We know they are not terribly defensive about their breeding territories. Chicks quickly learn to feed themselves, and the role of mothers is limited to protection. Babies often get mixed in the flock, ending up with different mothers. The flocking behavior intensifies in migration and away from breeding territories.

In Maine, scoters are not hard to find in winter, and there are some reliable spots even in summer. Scoters, eiders, buffleheads, goldeneyes, mergansers, long-tailed ducks, grebes and loons are gathering in our saltwater bays right now, and that makes winter a fine time to bird, assuming winter ever gets here.

Bob Duchesne serves as vice president of Maine Audubon’s Penobscot Valley Chapter. He developed the Maine Birding Trail, with information at mainebirdingtrail.com. He can be reached at duchesne@midmaine.com.