A small flock of Common Mergansers, some 13-15 individuals, stayed with us on the pond for most of the winter, leaving just a few weeks ago, presumably for its nesting area further north (I actually saw it go). It was replaced by another, of similar size. Then another. There may even have been a fourth — it was hard to keep track. Once or twice, there was overlap, one flock arriving before the other left.

These graceful, diving ducks were presumably attracted to the pond by the warm winter weather, the absence of pond ice, and the ample supply of small fish they feed on. Despite the recent, massive die-off
of these fish (Bluegills, a type of Sunfish), there are apparently still enough to sustain the ducks.

(Which leads me to wonder, could the fish be victims — not of pollution, or oxygen depletion, as some of us had suggested — but of their own success, the die-off being due to over-population?)

Common Mergansers will dive down and grab a slippery fish under water with a serrated bill. They often hunt non-stop, even while moving along, diving quickly and re-emerging seconds later some ten or twenty feet further on. While in this hunting mode, they exhibit obvious nervous energy, darting here and there quickly, and often moving across the water at great speed.

Interestingly, the latest flock to arrive does not seem to employ this traditional, serial-diving method. Instead, they use a “snorkeling” technique, plowing along with their heads partially submerged, looking down into the water for prey. This obviously saves them energy, but the drawback is equally obvious: their range of vision may be limited in turbid water.

On April 7, 2012, several males came close to my shore and gave me a private demonstration. They moved very fast, however, coming and going before I could get off more than a few shots. The top and bottom photos show them with their heads in the water as they swim along — what I’ve dubbed “snorkeling.” So far, I’ve not seen the females engage in this behavior.

This is a subject crying out for more information. Is this a unique behavior, or can it be seen among other Common Merganser flocks, or even among other duck species?

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