Editor’s Note: This is the second of our four-part series examining in detail the broad scope of 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. The remaining parts of this series will follow in weekly installments.

Many factors influence the reproductive output (recruitment) of waterfowl, both for an individual and a population. These include: 1) the number of potential breeding birds, 2) the proportion of individuals that actually attempt to breed, 3) location and timing of nesting, 4) clutch size and nest success (survival of the egg), and 5) survival of young produced. These components of reproduction are influenced by many things, some “nature” and some “nurture.” True to the theme of this series, it is critically important to understand and appreciate the tremendous variation (heterogeneity) that exists in each of these components.

Number of Potential Breeding Birds

The primary determinant of the potential number of birds that can breed in a given year is the survival of females in a population. Much of the discussion of survival will be covered in Part III of this series, but we must identify a few survival issues now to fully understand recruitment. First, survival of all waterfowl species varies among species, sex, age, condition of bird, geographical location, climate, and habitat condition. A conclusion of Part II (see below) is that variation in survival is the most important factor determining “lifetime reproductive success” (the total number of breeding-age offspring produced by a female during her lifetime). Second, the number of waterfowl breeding in an area is determined by “homing” or philopatry of females to natal (the area where they were hatched) or previously successful breeding sites; “pioneering” or “immigration” of birds into non-natal areas; and “emigration” of adults that may have bred in the area in the previous year but chose a different area to settle in subsequent years.

“Homing” is measured as a “return rate” to a location, whether it be a breeding, migration, or wintering site. Return to an area is a reflection of: 1) survival of the bird that allows it to return to that site, and 2) the relative success of the bird in that site that encourages it to return. In a simple analogy, if we visit a restaurant and the food is poor, in short supply, or costly we are unlikely to return. No wonder we all have our favorite “diners.” Ducks are no different. Return rates to habitats, especially breeding locations, are higher for adults than for juveniles. This is no surprise, in part because juveniles have lower survival rates than adults. Further, adults have the benefit of previous breeding and nesting “experience.” The experience of the breeder is affected by habitat conditions, density of co-breeders, and predation or other disturbances. If too many conspecifics occur in a site, resources may be limited both for the adult and the forthcoming brood. Also, too big a “crowd” may attract more predators. Competition and the risk of being eaten (or the eggs destroyed) are not factors likely to encourage return to an area. But what if that area is the only “diner” in town? If this is the case, then the population either produces poorly, or birds seek other locales. If they find suitable habitat elsewhere, they may move or “immigrate” into that area. Long term studies of snow geese, Canada geese, and wood ducks indicate that the most successful females (the “super hens”) have genetic predispositions in their broods for some ducklings/goslings to be “pioneers,” i.e. these young will not return to natal sites regardless of their quality and instead will seek different breeding sites. As in entrepreneurship, it takes only an occasional “hit” to make this strategy work, often in a big way. This is the name of the game in speciation and genetics - to maximize your distribution and contribution - conquer the world if you will.

Proportion of Birds that Attempt to Breed

The percentage of females in waterfowl populations that attempt to breed in any given year is different among species (e.g., some species such as geese, eiders, etc. do not reach maturity until ages two-to-four) and is influenced by weather, age (after maturity), breeding experience(s), timing, body condition, wetland and habitat condition, and maybe population density. Certain of these factors are innate, others are directly influenced by abiotic factors on breeding grounds, and still others are the product of “cross-seasonal” effects (factors occurring on migration and wintering grounds that influence reproductive potential many months later). Most, perhaps all, of these factors are interrelated. None are completely independent from each other. Also, most events that lead to eventual breeding attempts depend on the timing and success of preceding events. For example, body condition of a female is influenced by age, prior experience, and habitat conditions on migration and wintering areas. This affects the timing of pairing, quality of mate obtained, progression of prebasic molt of females, spring migration departure time, storage of nutrient reserves used in migration, time of arrival on breeding grounds, egg production and incubation, probability of encountering and surviving inclement weather, predation, competition with others, capability to renest if needed, and so on. Understandably, ducks, geese, and swans are impacted by a succession of habitat conditions (including anthropogenic or “man-related” influences) throughout the year. Success or failure in each area has consequences on the next event and area used, from the simple factor of surviving to the next event to the condition of the bird when the next event begins. For these reasons alone, management of waterfowl in North America must address habitats used throughout the annual cycle. Some disproportionate conservation effort may be deserved for specific times and areas, but not at the expense (or exclusion) of the integrity of the entire range of a species.

Studying the percentage of “nonbreeders” in waterfowl species is difficult because of their mobility, extensive areas used during the annual cycle, and limited number of marked birds in most populations. The best data are from long term studies of geese, swans, and brant although some long-term data also exist for wood duck, canvasback, and mallard. Also, many techniques used to track individual birds over long periods have potentially severe biases (e.g., radio transmitters placed on birds may impede their ability, or willingness, to attempt breeding because of constraints on energetics, mating, egg production, etc). Despite limitations of current technology and methods, data suggest the incidence of nonbreeding may approach 20-30 percent for geese, brant, and swans; 10-20 percent for diving ducks, and 5-15 percent for dabbling ducks. If any of the above factors increase the incidence of nonbreeding, the impacts on size of fall flights could be substantial. And, if some females ( a “super hen,” for example) almost always attempt to nest (and renest multiple times if necessary) while others seldom or never nest, then impacts of factors affecting the “super hen” are much more important than those affecting nonbreeding hens.

Location and timing of Nesting and Clutch Size

It should be apparent by now that both environmental (nurture) and intrinsic (nature) factors influence all aspects of waterfowl life histories, including reproduction. Location and timing of nesting, renesting (if it occurs in a species), and clutch size are not exceptions. The usual factors apply - age, experience, body condition, mate quality, habitat condition, and climate. Certain individuals seem to be able to nest at opportune times and in good locations and some do not. Here, I submit that success begets success and failure usually begets failure. Studies of geese have been most instructive (because of the ability to mark individual birds and follow them through successive years). The cycle begins with a successful nesting attempt - success being defined as producing a breeding-age offspring. Likely, this success was attained because the breeding female was older, had a good quality mate, and was in good body condition. This combination of traits allowed the female to arrive on breeding grounds earlier, enabling them to select better habitats and territories, have dominance over resources and reduce competition, nest in a more secure location (general habitat or within a colony), produce a larger clutch (up to some species-specific pre-programmed point), be more vigilant in incubation, hatch young at times that coincided with optimum food availability for broods, guide broods to better and safer foraging areas, etc. This sequence of success ultimately creates higher recruitment for that female. Furthermore, having experienced this success, the female is more likely to repeat this pattern in subsequent years if she survives, and the young are given the advantage of earlier maturation and positive learning experiences. These female offspring of the “super hens” then become the next generation of “super hens.” Recent long term studies of geese, brant, wood ducks, hooded mergansers, shovelers, tufted ducks, common pochards, and goldeneye confirm the long suspected phenomena that some familial lineages are more successful than others.

Nest Success

Recently, much contemporary waterfowl management in North America, at least for prairie-nesting ducks, has advocated a need to increase “nest success.” Obviously, a nest must hatch before young can be produced. Data suggest that nest success in at least part of the traditional breeding area in the prairie pothole region is lower than in past times because of higher predation rates and poor quality upland and wetland habitats. However, one must understand that predation on nesting females and their eggs always has been a dominant force in the evolution of waterfowl traits and life history strategies, and that high predation rates typically occurred in many breeding areas and for many (perhaps most) species, perhaps especially for the prairie-nesters. So, low nest success historically was the norm, not the exception. The key to successful management is to understand “how much” nest success is needed to sustain populations given contemporary changes in land uses, disturbance, predator populations, harvest, etc.

Some studies in the prairies during the 1970s and 1980s suggested 15 percent nest success was the magic point needed to obtain stability in duck populations. This level is an over-generalization for species/populations and locations, and has been misused by many biologists, agencies, non-governmental groups, and the popular press seeking to promote certain agendas. This number might be an average annual “rate” that is needed for stability of some populations, but it is doubtful this figure represents the evolutionary norm. Moreover, it does not reflect the average “life-time” nest success need of an individual. For true population stability, a female only has to produce one breeding age female offspring during her lifetime to replace herself. Theoretically, if all females eventually produce one female recruit, it doesn’t matter how good or poor nest success is in an individual year. Likewise, theoretically it wouldn’t matter whether a female dies or not after she produces a breeding age female offspring. Unfortunately, not all females ultimately produce one breeding age female. The population must rely on the “super hen” to make up the difference for those that do not live or do not produce recruits. Here, we begin to appreciate the problems of averaged “size-of-the-pile” statistics used in analyses of “population dynamics.” They fail to appreciate and incorporate individual heterogeneity.

Realistically, the critical questions about nest success become: 1) are contemporary landscapes capable of supporting population sizes that ultimately can replace themselves given current nest success rates, 2) can landscapes be improved (via whatever means) to increase recruitment, and/or 3) can individuals (birds and species) learn (ala Canada geese and mallards nesting on buildings in cities) new strategies to nest successfully? These questions are not confined solely to breeding ground habitat condition either. As already stated, the succession of events leading up to breeding are not independent. Nest success is no exception because it depends on timing and location of nesting, experience and condition of female, mate quality, vigilance, and many other factors that are influenced by habitat conditions, disturbance, energetics, etc. on migration and wintering areas.

Brood and Subadult Survival

At this point, perhaps little more needs to be said about the factors that cause variation in waterfowl recruitment, including the need for young to survive to breeding age. The importance of survival of young after hatch has been overshadowed by the “press” surrounding nest success issues. However, recruitment is not achieved upon hatching of a nest or when the young reach flight stage. True recruitment is when offspring reach breeding age. The importance of first-year (and subadult) survival will be expanded in Part III of this series. It varies among species, locations, and parentage. It may be especially critical for geese and swans that have extended parental care and for the “super hen” that gives her offspring the advantage in time, space, and resources.

Lifetime Reproductive Success

In a generalized form, what matters evolutionarily is not so much how the “population” does, but rather how the “individual” does. Even concepts of altruistic “group-“ and “kin-selection” have the ultimate common currency of passing genes to future generations. Perhaps hunters (or others with vested interests in the number of hunters) care only about total numbers (the “size of the pile”) and not the individual (“who is in the pile”). But, they should. The ultimate future of waterfowl (and hunting opportunity) depends on sustaining the genetic integrity of populations - which means sustaining the “super hen” component.

“Super hens” are the females that produce the most female recruits over their lifetimes, not necessarily in an individual year. The few long-term studies of waterfowl that have documented lifetime reproductive success (LRS) conclude that the number of breeding attempts in a female’s lifetime (i.e. longevity and high annual survival) is the strongest correlate of LRS. Also, early nesting females generally fare better than late nesters and those with greater body mass (fatter hens) ultimately produce more young than leaner birds. These two attributes are related. This tells us that annual nest success is important for LRS, but much less so than survival, age, and physical condition of the hen. The study with the largest, most comprehensive data set for ducks (see Blums et al. in the Selected References) exemplifies this conclusion. This study for the first time simultaneously considered both direct (i.e., factors on breeding areas) and indirect (body condition and survival on nonbreeding areas) effects of potentially interacting variables on estimates of LRS. A remarkable, yet disturbing, result of this study was that 59-74 percent of all females in the populations studied never produced breeding age offspring. Maybe the 80:20 (or 60:40) rule applies for ducks. Other long-term data from wood ducks, hooded mergansers, geese, and swans suggest it does. If indeed 20 to 40 percent of the females in a population produce 60 to 80 percent of future breeders, then the implications for management are staggering.

If only 20 to 40 percent of female waterfowl produce most recruits, then these females must average three-to-four females recruited to the next generation to achieve population stability. Current waterfowl management is based on averages. It does not recognize variation in individual performance. Obviously, the death of one of these “super hens” is three-to-four times more costly than the death of a dud (unless the super hen has already done her job at that point). Hunters should ask themselves: Is that hen mallard I just shot a “super hen,” a member of the valuable 20 percent? Or is it the dud, the compensatory duck, the remaining 80 percent? In Part III of this series, we will delve into the issues affecting survival of waterfowl and discuss the potential problems and biases of survival and life-model analyses that do not incorporate heterogeneity.

Selected References

Blums, P., and R.G. Clark. 2004. Correlates of lifetime reproductive success in three species of European ducks. Oecologia 140:61-67.

Cooke, F., R.F. Rockwell, and D.B. Lank. 1995. The snow geese of La Perouse Bay: natural selection in the wild. Oxford University Press, New York. 297pp.

Heitmeyer, M.E. 1988. Body composition of female mallards in winter in relation to annual cycle events. Condor 90:669-680.

Heitmeyer, M.E., and L.H. Fredrickson. 1981. Do wetland conditions in the Mississippi Alluvial

Valley influence mallard recruitment? Transactions of the North American Wildlife and

Natural Resources Conference 46:44-57.

Hoekman, S.T., L.S. Mills, D.W. Howerter, J.H. DeVries, and I.J. Ball. 2002. Sensitivity

analyses of the life cycle of midcontinent mallards. Journal of Wildlife Management

66:883-900.

Johnson, D.H., J.D. Nichols, and M.D. Schwartz. 1992. Population dynamics of breeding waterfowl. Pages 446-485 in B.D.J. Batt, A.D. Afton, M.G. Anderson, C.D. Ankney,

D.H. Johnson, J.A. Kadlec, and G.L. Krapu, editors, Ecology and management of breeding waterfowl. University of Minnesota Press, Minneapolis.

Newton, I. 1989. Lifetime reproduction in birds. Academic Press, New York. 479pp.

Raveling, D.G. 1981. Survival, experience, and age in relation to breeding success of Canada geese. Journal of Wildlife Management 45:817-829.

Stearns, S.C. 1992. The evolution of life-histories. Oxford University Press, Oxford.

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 organizations.