Open a field guide and you'll find one mallard—one description, one range map, one set of habits. But follow mallards season by season with GPS transmitters, and a more nuanced picture emerges. The mallard working a flooded rice field in California's Sacramento Valley and the one dropping into an Arkansas oxbow lake in December are living different lives—same species, same orange feet, completely different relationships with the landscape. New research tracking more than 1,600 hen mallards tagged with GPS transmitters in California and West Tennessee has revealed the most detailed portrait yet of how mallards in each of these populations spend the year.
The Homebody
California mallards, which breed and winter in and around the Central Valley, Sacramento–San Joaquin Delta, and Suisun Marsh, are remarkably faithful to place. In fact, more than half of mallard hens followed in this study remained within about two miles of the previous year's nesting areas during the breeding season. From November through January, these birds remained within nine miles of where they had spent their time during the same period the previous year. Even in summer, most of these California mallards returned to the same lakes and marshes in the Great Basin and Klamath Basin to molt, with many birds using the same habitats in consecutive years.
The Nomad
Most mallards in the midcontinent population are produced in the Prairie Pothole Region and winter somewhere in the Central and Mississippi Flyways. Unlike their California counterparts, however, they show much greater variability in where they go from year to year. Hen mallards marked with GPS transmitters in Tennessee showed a median annual displacement of more than 125 miles on their northern nesting grounds. In November, researchers found that roughly half of these Tennessee mallards were located more than 500 miles away from where they were a year earlier.
Measuring Site Fidelity
The term “site fidelity” refers to a bird’s tendency to return to a previously used migration or wintering location. It can be measured in two ways, and each method answers different questions.
The first treats this behavior as a biological process that plays out over the course of the entire season. Take every location a bird used one winter and compare it with every location it used the next. Did this bird, at any point, put its feet down in places it had visited the year before? California mallards in this study came within a median distance of about 550 yards of locations they had visited the previous winter, and they did so repeatedly. Tennessee mallards also showed fidelity to specific wintering areas, with 25 percent of these birds returning to the same habitats in subsequent years. Overall, Tennessee birds returned to areas within a median distance of 125 miles from the previous year’s wintering locations—still close at the flyway scale but much greater than the median distance for California mallards.
The second method of measuring site fidelity looks at where an individual bird is located at a single moment compared to where it was on that date the year before. In terms of biology, it’s a blunter measurement, but it’s perhaps more relevant to what hunters see on the landscape. We hunt the mornings that we can get away, and the question then is not whether birds will return to a marsh at some point during the season; it’s whether they are there now. This measurement tool also explains the 500-mile November gap for Tennessee mallards, which follow the same migration corridors and eventually return to their previous wintering sites, but often at different times from one year to the next.
Two Landscapes, Two Strategies
The differences in behavior between California and midcontinent mallards have less to do with the ducks than with the landscapes they inhabit. Think of a California mallard’s range as a bathtub, walled in by the Sierras to the east and the Coast mountains to the west. Nearly all of the wetlands are on the valley floor, and the climate reliably provides habitat at certain times each year. Waterfowl encode productive locations in memory and often return to these locations based on prior experience, somewhat independently of current conditions. A California hen mallard does not return to a familiar marsh because food is present there now; she returns because she remembers that food was there the year before. That is why restored wetlands with good food and water can see lower use by ducks in the initial years after restoration.
The situation for most midcontinent mallards is completely different. Their selection of breeding and wintering grounds is driven by weather and habitat conditions that can vary dramatically across their range. A hard freeze can push birds hundreds of miles in a few days, and last year's reliable oxbow can be this year's frozen pond. At that scale, returning to a fixed address carries real risk, and across the vast midcontinent landscape, birds that react to current conditions will generally fare better than those that depend on a smaller number of habitats.
What This Means in the Field
At a daily scale, the movements of California and Tennessee mallards are similar. The median daily displacement for California mallards during the season was roughly 400 feet, compared to a quarter mile for Tennessee mallards. That means the ducks that work your decoys in the morning likely roosted nearby the night before. The divergence grows with time. By season's end, 92 percent of California birds remained within 185 miles of where they started in October, while 49 percent of Tennessee birds exceeded that distance during the same period.
The consequence is one that experienced waterfowlers recognize. California hunters largely pursue the same mallards from October through January, so pressure falls on a fixed group of birds, and it compounds as the season wears on. In contrast, hunters in the Mississippi and Central Flyways pursue mallards that arrive on wintering areas on different schedules from October through December, giving hunters a crack at new birds as they arrive in a series of migratory pushes throughout the season.
Two Ways of Being a Mallard
As this research has revealed, the mallards in these two populations have arrived at different strategies for survival. California mallards succeed by remaining in a relatively small, stable landscape that they know well. Tennessee mallards, and their midcontinent brethren, survive by ranging widely and reacting to changing environmental conditions across a large portion of North America. Both strategies work. The differences only become visible when you follow individual birds throughout the season. Same species. Two worlds.
Dr. Cory Overton is the science director for the California Waterfowl Association. Dr. Brad Cohen is an associate professor in biology and Cory Highway is a doctoral student at Tennessee Tech University. Jamie Feddersen is the migratory game bird program coordinator for the Tennessee Wildlife Resources Agency.
Telemetry Research Partners
California telemetry research was funded by the California Department of Fish and Wildlife, California Rice Commission, Ducks Unlimited, California Waterfowl Association, and California Department of Water Resources and managed by the US Geological Survey. Research in West Tennessee was supported by the Tennessee Wildlife Resources Agency, US Fish and Wildlife Service National Wildlife Refuge System, and Tennessee Tech's Center for the Management, Protection, and Utilization of Water Resources.
