On the Move
By mid- to late June, with successful hens incubating eggs and the calendar ticking down to the time when renesting is no longer feasible, canvasback hens without broods and drakes gather in flocks on larger food-rich ponds and then gradually trickle away from the pothole country. Most head north to larger lake-marsh complexes in the southern boreal forest where permanent water and reliable food supplies can sustain them during the coming month or so when they will molt their wing feathers and cannot fly. Here, once again, in the journey through their annual cycle, canvasbacks become birds of the big water.
By September as the aspens start to glow and the morning wind begins to bite, both molting adults and maturing young are on the wing. Little by little the birds gravitate toward staging areas where food is abundant. These, by no coincidence, are places of waterfowling legend: Beaverhill Lake, Alberta; Lake Winnipegosis and Delta Marsh, Manitoba; Lake Christina, Minnesota; Lake Onalaska and Stoddard Pool, Wisconsin; Lake St. Clair and Long Point, Ontario; Keokuk Pool, Iowa; and many more.
On these marshes, as well as on the big waters of their wintering grounds, conservation of canvasbacks must be about sustaining or improving the health of whole watersheds. With changing water quality, canvasbacks have adapted their diet to clams and other invertebrates in many places, but historically, migrating and wintering canvasbacks relied on sago pondweed, wild celery, arrowhead (duck potato), and other tuber-forming plants. The common prerequisite for growing these plants is clear, productive water. Sedimentation, eutrophication, common carp, and pollution can all greatly limit the abundance of these plants and their capacity to support hungry ducks. Securing and maintaining the quality of North America's lakes and rivers is an enormous challenge, and the outcome will materially affect the future of canvasbacks as well as people.
A scarcity of good breeding and wintering sites for canvasbacks may contribute to the relatively low numbers of these birds that have been tallied since the 1950s. In 2010 the canvasback breeding population was estimated by the USFWS and Canadian Wildlife Service to be 585,000 birds, down some (12 percent) from 2009 but still slightly above the 1955-2010 average of 570,000 birds. Both of those apparent differences, however, are within statistical sampling error.
Because canvasbacks are less abundant than many other ducks and unevenly distributed across a vast geographic area, estimates of annual breeding population size tend to be relatively imprecise. Looking over the long term, the pattern of year-to-year population changes in canvasbacks (see graph) appears to be more erratic than in mallards, which have had rather well-defined periods of scarcity in the 1960s and 1980s with offsetting peaks in the 1950s, 1970s, and 1990s. Of course mallards are much more abundant and widely distributed throughout the surveyed area than canvasbacks.
Surveying issues aside, canvasback numbers may indeed vary widely from year to year as dramatic annual changes in water levels and nesting success have been observed on their prairie breeding grounds. For example, data combined from three different studies conducted over a 30-year span in southwestern Manitoba found that canvasback nesting success in this region changed abruptly from 0 percent in 1977 (a dry year) to 70 percent in 1978 (a wet year). Several other years show changes of similar magnitude (see graph). I suspect that episodic drought like this may impose substantial limits on canvasback population growth.
Fortunately for canvasbacks, the widespread degradation of upland-nesting cover, largely as a result of agricultural intensification, has not been a factor. So we haven't seen a declining trend in canvasback nesting success as has been observed for some upland-nesting dabbling ducks. What is clear, however, is that conserving the array of prairie wetland basins on which breeding canvasbacks depend will be vital to sustaining their populations.
Call to Action
Speculate as we may about canvasback numbers in the distant past, 55 years of data are enough to convince me that a rational goal for waterfowl managers would be to hang on to the canvasbacks we currently have—a modest but still huntable population of this extraordinary game bird. North America appears capable of supporting an average population of about 500,000 to 600,000 canvasbacks as long as we do not suffer significant losses of the remaining wetlands vital to the birds.
With conservative regulations in place to help ensure a sustainable harvest, fewer people hunt "King Can" than once did. But because of their restricted distribution and the challenge of hunting their wide-open haunts, cans were never everyman's duck like mallards are. Yet everyone should witness once in their lifetime the thrill of a flock of cans ripping over decoys on a crossing wind. The startling roar of wind through a hundred locked wings. The surprising snap of feathers straining against the air and the momentum of a duck trying to change direction fast. Dazzling white birds against indigo water in the evening light.
These ducks and their pursuit embody all that is great in our hunting heritage: the beauty of wildfowl on the wing; the thrill of "bringing 'em close;" traditions of canoes and duck boats, handmade blocks, and old friends; the irresistible pull but tinge of danger that comes with hunting big water; celebration of table fare without equal; and the commitment to conserve and sustain a precious resource.
May we always have the opportunity to meet these legendary birds up close and personal! This shall be so only if we look after the wetlands that sustain canvasbacks across our continent. Like a flight of cans arrowing through an autumn sky, our course is clear. Let us not fail the birds, or future generations, in our resolve.
For more information on DU's efforts to conserve key habitat for canvasbacks and other waterfowl, go to www.ducks.org and www.ducks.ca.
Dr. Mike Anderson is senior conservation advisor at DU Canada national headquarters at Oak Hammock Marsh.