Minnesota's breeding duck population declined 31 percent from last year -- falling to an estimated 507,000 birds -- and state officials aren't sure why.

The decline continues a trend: The state's breeding duck population has fallen four of the past five years. It's the third-lowest estimate in the past 26 years. There's no easy explanation for this year's decline, said Steve Cordts, Department of Natural Resources waterfowl specialist. Wetland conditions weren't bad, he said, though it was dry in east-central and southern survey areas when the agency conducted its annual aerial waterfowl survey.

Duck numbers in those dry areas appeared low, he said, and "in other areas they were pretty good, but not good enough to offset the dry areas."

The number of wetlands was 318,000, down 2 percent from last year but above the long-term average of 248,000.

The DNR's long-term goal is to have 1 million breeding ducks in the state, a number that is getting more distant, based on recent survey results.

Here are the numbers:

• The mallard breeding population is estimated at 236,000, 6 percent above the long-term average of 224,000 but 21 percent below last year and 19 percent below the recent 10-year average.

• Blue-winged teal numbers declined 11 percent from last year to 135,000 and remained 39 percent below the long-term average.

• The combined populations of other ducks, such as wood ducks, ring-necked ducks, gadwalls, canvasbacks and redheads, decreased to 170,000, 5 percent below the long-term average.

The Minnesota duck situation contrasts sharply with the broader continental view. Last week, the U.S. Fish and Wildlife Service reported that the midcontinent breeding duck population increased 13 percent from last year and is 25 percent above the long-term average.

Cordts said excellent water conditions in the Dakotas could have drawn ducks to nest there this spring, instead of Minnesota. But he's not sure how to interpret the overall decline in breeding duck numbers. "I try not to get too concerned for a one-year change, up or down," he said. "I look at the survey as more of a trend."

But the trend, he acknowledged, for the past few years isn't good.

Still, he said the bulk of ducks Minnesota hunters harvest each fall are migrants, and Cordts figures there should be good numbers of birds available this fall, assuming duck production is good in Canada, the Dakotas and Minnesota.

The DNR's Duck Recovery Plan says 2 million acres of additional habitat needs to be restored to achieve the 1 million-bird breeding population level. "We are committed to hitting the 1 million-bird target," Dennis Simon, DNR wildlife chief, said in a news release. "That means focusing on a long-term strategy to improve the quantity and quality of wetlands and grassland through the combined efforts of many partners."

The DNR has added wildlife habitat in recent years, Cordts noted, but that likely has been offset by loss of lands enrolled in the federal Conservation Reserve Program. "It points to the long-term need in Minnesota for habitat," Cordts said.

The DNR's waterfowl survey has been conducted in early May since 1968. The survey -- designed to provide an index of breeding duck abundance in about 40 percent of the state that includes much of Minnesota's best remaining duck breeding habitat -- is timed to coincide with peak nesting activity of mallards. The report can be viewed at mndnr.gov/hunting/waterfowl.

Officials noted that the Legislature recently appropriated $21.5 million for wildlife habitat enhancements and acquisitions.

Meanwhile, this year's estimate of 285,000 Canada geese remains similar to last year's estimate of 289,000. That's above the DNR's goal, but the breeding population has stabilized and is no longer increasing.