Well, it starts with the boom and bust cycles of prairie precipitation. Driven partly by El Niño, La Niña, and other major oceanic influences, atmospheric circulation patterns can differ dramatically from one year to the next, with accompanying swings in annual precipitation of 50 percent or more. In fact, the prairies have the most variable precipitation of any region in North America.
These annual changes in precipitation, especially in winter and early spring, greatly affect the abundance, distribution, and nature of wetlands across the Prairie Pothole Region (PPR)
. The result is that pond numbers can vary dramatically from year to year and place to place. Since systematic surveys began, pond numbers in prairie Canada and the north-central United States have shown three generally wet periods, in the mid-1970s, the mid-1990s, and 2010−2012, with dry periods in the early 1960s, late 1980s and early 1990s, and the early 2000s. Total duck numbers in the areas traditionally surveyed by the U.S. Fish and Wildlife Service (USFWS) and Canadian Wildlife Service have followed much the same pattern, leading most scientists and waterfowlers to agree that as goes the PPR, so go populations of many of our most important game ducks.
It's not surprising that the number of ducks settling on the prairies in spring largely tracks the number of ponds, which in turn is determined mostly by the combination of carryover water from the previous year and winter precipitation. But what is it about changing water conditions that is so important to the ducks? A variety of factors come into play.
First, the plants and animals that thrive in prairie wetlands are beautifully adapted to wet and dry cycles. Most wetland plants that grow above the water's surface require the damp exposed soil of moderate drought conditions to germinate and restore healthy plant stands. Seeds can lie on the bottom of wetlands for years awaiting just the right conditions to sprout. Many (though not all) wetland invertebrates can survive drought, too, by encasing themselves or their eggs in water-tight structures and waiting for water to return. The combination of new wetland plants, annual seed-producing plants, and increasing invertebrate populations in newly flooded ponds means that wetland landscapes coming out of drought conditions are often dramatically more productive for breeding waterfowl than are the relatively static, open-water wetlands that typically exist after an extended wet period.
For diving ducks that nest over water on platforms made of old cattail, bulrush, or sedges, drying ponds leave the birds nowhere to build their nests and nowhere to hide. When it gets really dry, many ducks simply won't nest. And young ducklings, of course, need cover to escape bad weather and things that eat them. Again, flooded cover is key; without it, the risks to prospective breeders often outweigh potential rewards. Experienced canvasbacks, redheads, and their kin are likely to move on to better places or simply don't attempt to breed in really dry years.