North American duck populations are changing. Specifically, their sex ratios, measured as the number of drakes per hen, are increasing for many species. This discovery has sparked new research to understand the causes of changing sex ratios in ducks, and the waterfowl management community is now working to refine breeding population surveys and other monitoring programs to account for these changes.
Male-biased sex ratios (i.e., more males than females) in duck populations have long been of interest to waterfowl biologists. In the first textbook on wildlife management, Aldo Leopold wrote that sex ratios in duck populations were “seriously deranged” based on banding data that showed about one-and-a-half drakes per hen. Differences in sex ratios across species and through the seasons provide a window into the unique life history strategies of waterfowl. Sex ratios of swans and geese tend to be nearly even because both males and females incur risks when protecting nests and caring for young. For example, male geese and swans readily drive nest predators like arctic foxes away from their incubating mates.
At the other end of the spectrum are ground-nesting dabbling ducks. Adult populations of these species tend to have uneven sex ratios. The reason for this is simple: Hens are highly vulnerable to predation when laying eggs and incubating nests, while drakes simply hang out on nearby wetlands. If a hen is skillful or lucky enough to hatch her clutch of eggs, she spends the next 45 days shepherding eight to 10 ducklings in hazardous environments by herself, while males have already departed to molt. This unequal distribution of risk makes hens far more likely than drakes to die during the breeding season.
Research in the 1940s, ’50s, and ’60s demonstrated that the sex ratios that we observe in duck populations can vary across both space and time. In those days, most efforts to understand sex ratios took place in early spring as ducks were settling on the breeding grounds, initiating nests, and incubating their eggs. Although this early research was somewhat limited, it revealed important patterns. Researchers learned that sex ratios appear more male biased when nesting hens are hidden from observers. They also discovered that sex ratios vary substantially among species. Studying waterfowl during migration and in winter presented even greater challenges. Differences in the timing of migration, wintering destinations, and behaviors among male, female, and juvenile birds mean that the sex ratios that we observe in the field can change from nearly even to highly skewed over just a few weeks at a single location.
Adding to the challenge of estimating sex ratios are the actual changes that occur as ducks progress through their annual cycle. During the breeding season, more females die from predation than males, causing sex ratios to shift toward more males as summer progresses. The ratio of males to females reaches its peak during fall. During winter, hunters shoot more males than females, and in spring new recruits (ducks that were hatched the previous year) join the breeding population at a ratio of almost exactly one-to-one. This helps to offset the unequal mortality experienced by hens during the previous breeding season.
An understanding of sex ratios in duck populations is important for more than just biology. During breeding population surveys, some hens are already nesting and not observable by survey biologists, so researchers used knowledge of breeding behavior to develop decision rules that could be applied to observed drakes to account for unobserved nesting hens. In general, these rules state that single male dabblers and groups of four or fewer males are assumed to all be paired (with a hen on a nest), while groups of five or more males are assumed to be unpaired. Decision rules for diving duck species differ slightly, based on species-specific nest-initiation patterns. These rules helped address the underlying issue of unobservable nesting hens and provided relevant estimates for monitoring and management, but they were rarely validated and did not account for potential changes in sex ratios.
Research in the early 2000s found that paired drake mallards behaved as the decision rules assumed, but that research also revealed that unpaired drakes spent a substantial amount of their time behaving as if they had a mate. Because of this, a systematic bias in sex ratios across the breeding grounds could lead to over-estimation of duck populations. Researchers concluded that while this over-estimation was noteworthy, it would be inconsequential as long as sex ratios remained relatively constant. In fact, work by Dr. Ray Alisauskas and colleagues in the early 2010s documented long-term increases in ratios of male to female mallards during fall, and updated analyses have shown that these trends are continuing.
Analytical advancements and long-term data sets have helped scientists make more detailed evaluations of spring sex ratios among more than 10 species of ducks. This work is shedding light on assumptions of the past, as well as the current state of duck populations. Preliminary estimates indicate that over the last 30 years, spring sex ratios for mallards have increased from 1.4 drakes per hen to nearly two drakes per hen. Smaller but important changes in sex ratios have also occurred in other species such as blue-winged teal, northern pintails, and even some European duck populations.
Disparate sex ratios are not necessarily a sign of unhealthy populations, but abrupt changes or longer-term trends among several species could be a sign of large-scale environmental changes. Analyses of banding data indicate that changes in sex ratios have been primarily driven by decreases in female survival during the breeding season. This is particularly concerning because the female component of the population is disproportionately important in sustaining populations. Male-biased sex ratios may have other impacts on population dynamics. In extreme cases, too many males may compete with females for habitat resources or expose females to high levels of harassment, impacting the females’ ability to secure resources for breeding.
Because of these discoveries, waterfowl researchers and managers are actively exploring new approaches to account for shifting sex ratios in population estimation methods. More importantly, large-scale research will likely be needed to identify the causes of lower female breeding-season survival and assess opportunities for addressing it through targeted conservation efforts.
These discoveries were made possible by the collaborative analysis of long-term data sets, including band recoveries, harvest surveys, and waterfowl population surveys, all of which depend on the dedicated participation of waterfowl hunters and conservation professionals across North America. While management actions that could help address changing sex ratios in ducks have yet to be determined, it is clear that no other research method can duplicate the continental scope and scientific impact of these data sets.
In addition, the waterfowl management community is constantly looking for better ways to collect and analyze the data that informs our decisions, and never have the opportunities been more exciting than today. Scientists from federal, state, university, and nonprofit organizations are reexamining methodologies used in waterfowl population and harvest surveys, with the goal of building a more modern system of data collection. While traditional methods have served the resource well for over 70 years, new technologies are certain to foster innovation in the future, which will help managers refine how they monitor and manage waterfowl populations for decades to come. Stay tuned.
Frank Baldwin is a wildlife biologist with the Canadian Wildlife Service in Winnipeg, Manitoba. Dr. Thomas Riecke is the James K. Ringelman Chair in Waterfowl Conservation at the University of Montana in Missoula, Montana. Dr. Mike Brasher is senior waterfowl scientist at DU national headquarters in Memphis, Tennessee.
