Editor’s Note: This is the third of our four-part series examining in detail the problems facing ducks, duck hunters and waterfowl managers. The author, Mickey Heitmeyer, one of the world’s leading authorities on mallards, provides unique and extraordinarily perceptive insights and solutions. We highly commend it to your attention. Because of its semi-technical nature, it is recommended that you first print the article, then read from the printed page.

Many things kill waterfowl. However, survival of a duck, goose, or swan is mostly related to a bird’s ability to avoid being eaten by something, whether it be fox, man or microbe. Consequently, understanding a duck’s life is in large part understanding what kills (or seeks to kill) it, and when it gets killed. Generally, the killing (mortality) agents take two forms: 1) natural and 2) human-caused (this also is natural, but for sake of this discussion and the changing and controlling nature of it, waterfowl managers treat it separately).

Natural Mortality

The big three natural mortality agents for waterfowl are 1) predation, 2) weather, and 3) disease. A fourth agent, starvation, generally is not considered a major factor causing mortality in waterfowl (however, the recent overpopulation of snow geese has depleted certain “goose pastures” in the Arctic and essentially starved young goslings). In many cases, the natural mortality agents are related. For example, extended cold and ice may increase incidence of certain diseases or predation during winter.

Most natural mortality agents have at least some “selectivity” in who gets killed, and when they are killed, especially if the agent is predation. Television nature shows and many stories of natural selection usually imply “survival of the fittest” being manifested as predators catching the “weak” individuals in a population. Predation of waterfowl (or any living organism) can occur at any time and place, and typically happens when an individual is placed in a compromised position. The compromise may be caused by weakness or poor physical condition (e.g., an inability to escape), poor visibility or inattention (e.g., bad weather, dense cover, distraction), exposure (e.g., poor habitat that causes a female and her brood to forage in more risky areas or on degraded sites with inadequate food or cover), and/or a physiological event (e.g., confinement of a female on the nest during incubation).

Enter heterogeneity (the differences between individuals) again! Age, physical condition, accumulated experience, and so forth all affect whether a bird is in a compromised position, and for how long. This may be especially true during breeding. By far the greatest mortality agent on breeding grounds is predation on nesting females and pre-fledged young. Many studies of mammal (e.g., red and arctic fox, raccoons, mink), avian (e.g., raptors, gulls, crows, ravens), and even fish and reptile (e.g., bass, bullfrogs) predation indicate that waterfowl mortality is greatest in areas of poor habitat, crowded conditions, or simply within the range of a skilled predator. To no surprise, a female’s (and her broods) chances of survival are better if she is older; has a good quality, vigilant mate (or attentive parent); good body condition; and experience (including, perhaps, experienced parents). These individual qualities generally increase the probability that a female will nest early and hatche young successfully (at least historically). Further, if an early nest hatches it means the female will not have to renest (for those species that are inclined to do so, like most dabbling ducks), which would increase the time she is exposed to predators while sitting on a nest. The greatest mortality on ducklings/goslings occurs in the first few days following hatching. Consequently if a female hatches a brood early in the spring when wetlands are coming alive following ice-out, many alternate foods are available for predators to seek and eat besides ducklings. Moreover, food, cover, and water are plentiful because wetlands haven’t started drying.

Less is known about natural predation on migration and wintering areas, and it appears to be less common or important than on breeding areas. Nonetheless, the few studies that have documented predation (primarily raptors and falcons) on nonbreeding areas indicate mortality is greatest for individuals in poor body condition, and that are isolated from or on the edges of flocks, unpaired, and in open habitats. Further, while actual death from predation may be infrequent on migration or wintering areas, its potential presence has been an important evolutionary force dictating behavior and habitat use of birds during migration and winter. This also holds true for such factors as pair formation and nutrient reserve storage.

Disease, weather, and starvation are lesser mortality agents than predation, especially on breeding grounds. However, they can be significant at certain times and locations. To a degree some disease outbreaks and weather events are non-selective and essentially kill everything in their path. However, limited data again suggests older (up to a certain point of senescence) and better condition birds may be less susceptible.

Human-Caused Mortality

The two primary human-caused mortality agents for waterfowl are: 1) harvest, both sport and subsistence; and 2) contaminants. Other human activities also cause mortality in waterfowl such as collisions (wires, roads, farm machinery such as hay mowers, etc.), but they are relatively unimportant events in most areas. Clearly, the largest human-caused mortality agent is sport hunting and it is confined to fall and winter. In contrast, predation, the major natural mortality agent, operates mostly in spring and summer. I do not discount subsistence harvest as being inconsequential, because it is substantial for some species that nest in far northern latitudes such as goose, brant, and eider colonies on the Yukon-Kuskokwim Delta in Alaska, along Hudson Bay, and in other Arctic sites. Subsistence harvest occurs mostly in spring, but some substantial harvest also occurs in key staging areas in fall.

Sport hunting (and probably subsistence harvest also) is not a random mortality agent. Every hunter knows that some birds respond better to calls and decoys than others. Why? The vulnerability of birds varies with time (during the day as well as over the season), location, climate and habitat conditions, species, sex, age, body condition, annual cycle event, social status, flock size, color phase, and hunting technique. Some discrimination in harvest is intentional and is related to regulations including shooting hours, season dates, zones, species and sex restrictions, etc. Other variation is caused by inherent differences in vulnerability of individual birds. It is easy to understand why some birds might be harvested at higher rates. Young birds and those in poor condition may be less wary in approaching decoys and responding to calling because they have less experience or they simply are hungry or seek company.

It is no surprise that the age-ratio (the number of juveniles per adult) of waterfowl shot by hunters is much higher in northern states and Canada in early fall compared to later seasons in the south. Hunters in northern areas enjoy the benefits of many recently hatched, inexperienced young birds, and harvest them at a high rate, as shown below.

Mallard Age Ratios

MIWI MNIAOH IN ILMOKY TN AL AR MSLA

State

Figure 1. The number of juveniles per adult in the hunter’s bag is highest in northern

states and lowest in southern states, as you can see from this graph of age-ratios of

mallards killed in the Mississippi Flyway in 2002. Source: USFWS.

By the time birds migrate to mid-latitude and southern states, many young are already dead, and hunters are dealing with more educated, older and wiser birds. In Louisiana and Arkansas, age-ratios of harvested mallards may be as low as 0.4-0.6:1. It stands to reason that the later and longer hunting seasons run in winter, the proportion of adults harvested will be higher. Not only is harvesting a high proportion of adults potentially harmful to populations, most adults shot in late winter are already paired. Adults form pair bonds earlier than juveniles and typically pair with another adult (the better mate selection). If a member of a pair dies (from hunting or another cause), the surviving pair member must find another mate. By late winter, most of the better quality adults are already paired and the unpaired bird is left with the dilemma – “Are there any good mates left?”

Very early hunting seasons also “prey” on a nonrandom segment of duck populations. If hunting seasons start early in the north, some late-hatched young can barely fly, if they can fly at all, and hunting on natal marshes is devastating to them. Further, many adult females that nest and hatch a brood late (perhaps because earlier nests were destroyed, forcing them to renest) remain on natal marshes well into fall because they are accompanying the late-hatched broods and undergoing their annual wing molt (when they cannot fly for six-to-eight weeks). Consequently, these “successful,” but late, females also are harvested at a high rate in natal marshes.

Understanding the types and degrees of these differences in sport harvest is essential to understanding the potential effects of various harvest regulations. The process of setting harvest regulations must seek to: 1) understand what proportions of “super hens” are killed each year, 2) document where and when “super hen” mortality is greatest, and 3) attempt to minimize deaths within this group.

Much remains unknown about contaminants, a second human-caused mortality agent. Some contaminants are differentially lethal to certain population segments, (e.g., agricultural chemicals used near prairie potholes affect nesting females and young broods). Since Rachel Carson wrote the book “Silent Spring,” we now appreciate the potentially devastating consequences of certain chemicals in our environment. Fortunately, the most potent organochlorines now are banned from use. However, many other highly toxic chemicals and naturally occurring heavy metals remain in use (or are present in the environment), including herbicides, pesticides, petroleum products, solvents, etc. Some of these chemicals may not directly kill a duck, but they can become “biomagnified” in body tissues of birds and reduce their reproductive capacity, or immunity to disease, or less favorable environmental conditions. Lead was one natural element that was widely distributed in hunted wetlands and exacerbated exposure to waterfowl. Lead shot is now banned from use in waterfowl hunting because of its lethal effects. It is our obligation to reduce or eliminate exposure of the other contaminants as well.

Life-cycle Models

Biologists have attempted to model the life cycle, along with annual and long-term changes, of waterfowl populations using a combination of recruitment (Part II of this series) and survival (Part III) estimates. Unfortunately, most models use average rates for the population in question rather than subdividing the population into segments based on heterogeneous recruitment and survival rates. If heterogeneity is not identified or accounted for, the estimates of survival and recruitment of the population that are based on “averages” can be highly biased and misleading. Doug Johnson and colleagues (see Selected References) provided an excellent example of the potential impacts of ignoring heterogeneity in such models.

Using a modification of Johnson et al’s example, consider a hypothetical mallard population of 1000 females with two groups – the “super hens” and the “duds.” Group 1, the super hens, comprises 20 percent of the population (200 birds), has an average annual survival of 80 percent (160 of the 200 survive in a given year), and an average annual recruitment of two young per breeding female. Consequently, the 160 surviving females produce 320 young for a total of 480 females the next year. Group 2, the duds, comprises 80 percent of the population (800 birds), has 50 percent survival (400 of the 800 survive), and an annual recruitment rate of 0.5 young per female. This group of 400 surviving females produces 200 young the following year for a total of 600 females. Combined (480 super hens + 600 duds), the population now has 1080 females, or an 8 percent growth rate. Further, the proportion of super hens in the population now has increased from 20 percent to 44 percent of the population. Waterfowl managers like to be in this position (as would an investor in a stock portfolio) because contributions of the better quality females should be compounded (like interest) in future years.

Conversely, if averages are used for the above population, instead of separating the groups, then this population has 56 percent survival and 0.8 young/female recruitment and a total population size of 1008 females, an 0.08 percent increase. In this example, the super hen dominated population is actually growing nicely (8 percent), but the averaged statistics do not recognize the differential productivity of the super hens, and suggests the population essentially is stable (0.08 percent) with little growth. It also demonstrates why duck management should not overemphasize average nest success rates. In this example, 80 percent of breeding females had very poor (0.5 young/female) recruitment (only partly affected by nest success), yet the population grew well (8 percent) when it contained a modest (20 percent) amount of super hens that survived well.

Now, let’s say some factor such as changed harvest regulations are capable of increasing by two to three times the annual harvest rate (such as liberal vs. restrictive AHM regulations or spinning-winged decoys that will be discussed in Part Four of this series). This alters the survival rate of the super hen group from 80 percent to 50 percent. In this case the duds still produce 200 young and have a total size of 600, but the super hen group has only 100 survivors that produce 2 young/each for a total size of 300. Combined the population now has only 900 individuals, a 10 percent annual decrease in population size, and a 17 percent reduction in recruitment compared to the prior year. Here, the decline in recruitment was solely caused by increasing mortality rate of super hens, not from any change in nest success, brood survival, etc. Stated another way, only 60 more dead ducks (from an initial population of 1000) caused the population to go from growing eight percent annually to declining 10 percent annually. A potential irony exists here: greater survival of a certain population segment may be more important to recruiting young than actual breeding success of the entire population. This begs the question: which is easier and less risky to achieve - changing hunting regulations to increase survival or changing habitat conditions across extensive breeding areas?

Johnson and colleagues further noted that “heterogeneity also can lend the appearance of compensation between hunting and natural mortality.” Consider, for example, two subpopulations in equal numbers. In year one, birds in group A survive natural mortality at a rate of 80 percent and hunting mortality at a rate of 90 percent. Without compensation, annual survival is 72 percent (0.8 x 0.9). Birds in group B have corresponding survival rates of 60 percent and 80 percent, for an annual rate of 48 percent. The annual survival for the combined groups is 60 percent. If rates of hunting and annual survival were known precisely, survival from natural mortality would be estimated as annual survival divided by hunting survival (.6/.85) = 70.6 percent. Suppose now that hunting mortality rates are doubled for each group the following year. Hunting survival rates then drop to 80 percent and 60 percent, and annual survival rates decrease to 64 percent and 36 percent; the averaged annual survival rate is 50 percent. Survival from natural mortality is now 71.4 percent. Thus, the natural mortality rate appears to decline as hunting rate goes up. Consequently, without considering heterogeneity, the natural and hunting mortality forces appear compensatory, when in fact they are not.

I apologize for the above math exercise, but mathematics are a necessary part of understanding how mallards are doing these days, and to comprehending the effects of current management activities, especially hunting regulations. Further, it is essential to recognize all of the factors in the equation to achieve the correct answer. In the above simplified examples, it should be clear that potentially huge mistakes could be made if heterogeneity is ignored. Part IV will take a detailed look at some “big” issues in the regulations debates and offer thoughts on how to avoid analytical mistakes and protect the “super hen.”

Selected References

Heitmeyer, M.E., and L.H. Fredrickson. 1988. Heterogeneity in mallard populations - winter

perspectives. Pages 37- 42 in M.A. Johnson, editor. Mallard Symposium, Bismarck,

ND.

Heitmeyer, M.E., L.H. Fredrickson, and D.D. Humburg. 1993. Further evidence of biases associated with hunter-killed mallards. Journal of Wildlife Management 57:733-740.

Johnson, D.H., J.D. Nichols, M.J. Conroy, and L.M. Cowardin. 1988. Some considerations in

modeling the mallard life cycle. Pages 9-22 in M.W. Weller, editor. Waterfowl in

winter. University of Minnesota Press, Minneapolis.

Krementz, D.G., R.J. Barker, and J.D. Nichols. 1997. Sources of variation in waterfowl survival rates. Auk 114:93-102

Pollock, K.H., and D.G. Raveling. 1982. Assumptions of modern band-recovery models, with emphasis on heterogeneous survival rates. Journal of Wildlife Management 46:88-98.

Powell, L.A., W.R. Clark, and E.E. Klaas. 1995. Using post-release stratification to detect heterogeneity in mallard survival. Journal of Wildlife Management 59:683-690.

Williams, B.K., J.D. Nichols, and M.J. Conroy. 2002. Analysis and management of animal populations. Academic Press, New York. 817pp.

Dr. Mickey Heitmeyer has investigated waterfowl and wetlands for nearly 30 years, and is widely recognized as one of the world’s leading experts on the biology of mallards. His professional background includes: technician and wetland manager for the Missouri Department of Conservation, research biologist for the University of California-Davis, Director of Research and Outreach for the California Waterfowl Association; Group Manager of Conservation Programs and Director of the Institute for Wetland and Waterfowl Research for Ducks Unlimited, Inc. and Ducks Unlimited, Canada. He is currently a Research Associate with the Gaylord Memorial Laboratory at The University of Missouri. He is an avid duck hunter and is owner of Dark Cypress Farms and co-owner of Cato Slough LLC. Both are conservation properties in southeast Missouri that offer wetland management services and limited high-quality sporting opportunities. He and several other wetland scientists and managers currently are forming a nonprofit entity and a consulting LLC to provide wetland ecosystem services (including waterfowl research, continuing education and habitat management ) rarely or never provided by traditional agency, academic and nongovernmental