Ducks are mysterious, which is part of their attraction.
By species, they can appear wildly different from one another — think florid male wood ducks and more subtly colored male ringnecks.
Their behaviors can vary as well.
Canvasbacks, for example, dive for their food, but gadwalls dabble. And while some blue-winged teal hatched in Minnesota migrate to South America, certain homegrown mallards are content to spend their winters in Minneapolis.
Yet a most peculiar aspect of ducks has emerged only recently, and waterfowl researchers are struggling to understand its extent and ramifications.
At issue is a disparity, most evident in the past 20 years, in North American mallards between the number of adult males in that population vs. the number of adult females — with males being more abundant by a ratio of nearly three to one.
Among juvenile mallards, by contrast — those born in a given year — the split is closer to even.
That this is occurring while the continent's overall mallard population is declining, and while questions are being raised about the longtime methods federal and some state wildlife agencies use to count breeding ducks that return north in spring, only adds to the quandary's complexity.
Todd Arnold, a professor at the U in the Department of Fisheries, Wildlife and Conservation Biology, is one of a relative handful of waterfowl population experts that North American duck managers consult to unravel such mysteries.
"It's not only mallards in which the disparity between males and females is showing up," Arnold said. "It's most pronounced in mallards. But we're seeing it in pintails and wood ducks and bluebills, too."
The issue is important, perhaps even critical, to duck management because the reproductive capability of any species depends on the presence of a critical mass of females.
Yet understanding why adult male mallards so significantly outnumber adult females defies ready understanding, in part because hunters kill far more male mallards than females, especially in mid-continent and southern states, where the largest portion of mallard harvests occur.
"Particularly later in fall when the birds are fully plumed, hunters can readily identify male from female mallards," Arnold said. "They prefer to shoot males, and they do shoot more of them. They always have."
Arnold suspects the loss of female mallards might occur on spring nesting grounds, where they are vulnerable to predation. The reduction in recent decades of Conservation Reserve Program acres, particularly in the Dakotas, where many mallards nest, might have facilitated these losses by reducing the abundance of safe havens hens have to incubate eggs and rear their young.
The hen mallard falloff has raised alarms only in recent years in part because the severity of their decline hasn't been highlighted in spring surveys conducted by the U.S. Fish and Wildlife Service and some state wildlife agencies.
Surveyors conducting these aerial counts often tally as "present'' female mallards they don't actually see.
Minnesota Department of Natural Resources waterfowl specialist Steve Cordts explains.
"As we start our spring survey in May, we see some male and female mallards paired," Cordts said. "But as we go on, we almost never see the females, because they are on nests."
In response, the accepted method to survey mallards, Cordts said, is to count up to five females for a similar number of males seen together without hens. If more males than five are spotted together — say 10, for instance — those males are counted, but only five "seen'' (but actually unseen) females are counted.
Another scheme called the Lincoln method uses harvest and band-recovery data to estimate duck population sizes. Regarded as accurate, this method nevertheless only provides rear-view population estimates, because it requires input of Fish and Wildlife Service harvest data collected from select hunters during fall seasons.
Which points to another variable in the hen-mallard puzzle.
"It's possible," Arnold said, "that some hunters selected by the Fish and Wildlife Service to send in wings of ducks they shoot send in wings only of drakes rather than of any hens, because of a bias they have about what they 'should' shoot given their pride as hunters."
Another issue is "fecundity," or reproductive capability.
It's well known in wildlife science, Arnold said, that reproduction increases as available habitat is optimally utilized — meaning that if excess numbers of male mallards use habitat that otherwise could be occupied by drake-and-hen pairs, reproduction could be adversely affected.
Though perhaps counterintuitive, "We know productivity goes up when fewer ducks come back in spring," Arnold said.
One way to increase mallard production would be to return the adult mallard sex ratio closer to even by increasing hunters' limits on male mallards.
Currently, the daily duck limit is six in the Mississippi and Central flyways, which are primary mallard incubators. Four of the six can be mallards, two of which can be hens.
Allowing instead six male mallards to be killed and/or reducing the hen limit to one could help even the species' sex ratio.
But for hunters who have complained in recent years of seeing too few ducks over their decoys, increasing the mallard limit could be a tough sell. In Minnesota last year, for example, the wood duck harvest almost exceeded that of the mallard, an outlier by historical standards.
Yet if available waterfowl nesting habitat continues to be lost in Minnesota, the Dakotas and elsewhere, increasing duck production might be one way, and perhaps the only way, to sustain duck populations at or near their long-term averages — which is where mallards are now.
"If the sex-ratio disparity is true, and it appears it is, and if we do nothing," Arnold said, "then we'll have a less productive mallard population than we could have."
