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Mallard Recruitment in the Agricultural Environment of North Dakota

Implication for Management and Research


Cowardin and Johnson (1979) previously demonstrated that
       R = HZB ÷ 2                                                 

where                                                              

   R = young females fledged per female in the breeding population,
   H = hen success,                                                
     Z = survival of at least 1 brood member to age IIc, and	       
   B = average brood size at fledging.                             

If we enter empirical estimates for H and Z obtained in this study and 4.9 for B from Cowardin and Johnson (1979) we obtain

R = 0.15 × 0.74 × 4.9 ÷ 2 = 0.27.

To assess the implications of this statistic we require a model relating recruitment and survival to annual rate of population change. We will use the model presented by Cowardin and Johnson (1979:24). This model is for a closed population and does not incorporate compensatory mortality. We suspect that a compensatory mechanism may operate to an unknown degree (Anderson and Burnham 1976), but there are insufficient data to construct an appropriate model.

If, in fact, recruitment is averaging 0.27, the model predicts a population declining at 20%/year (Table 21), and hen success would have to reach 31% to obtain a stable population. Obviously, if an annual 20% decline were taking place throughout the breeding range there would soon be no mallards. Such a generally severe decline in mallard populations has not been demonstrated.

Table 21. Predicted rates of population change based on model presented by Cowardin and Johnson (1979), recruitment data from central North Dakota (this study), and unpublished data from Lostwood National Wildlife Refuge in northwest North Dakota.
Source Nest success Hen success Predicted annual change (%)
Central North Dakota 8.3a 15.2a -20
Needed for stability 15.2 31.0 0
Lostwood NWR 42.0a 59.8 +38
aReal measurements; other numbers were derived from the   model.

We believe that 2 factors temper this result:

  1. There is considerable geographic variation in nest success. Preliminary nest success estimates by A. D. Kruse (pers. commun.) demonstrated nest success of 42% at the Lostwood National Wildlife Refuge in western North Dakota. This success rate would result in a 38% annual rate of increase (Table 21). We suspect that nest success on our study area represents some of the worst conditions in the mallard breeding range. Hochbaum (1945), stated that the ability to pioneer is highly developed in the mallard. Dzubin (l969b) pointed out that yearling hens may pioneer into habitats that are unsuitable for brood production. Krapu et al. (1983) showed a correlation between pond availability and breeding population density, suggesting the importance of pioneering, but they also suggested that strong breeding-site philopatry accounts for some of the variation in habitat occupancy. Patterson (1979) classed the mallard as an r-selected species, although he did not show a clear relationship between pond and pair densities. If mallards pioneer readily to areas where they produce few young, it is probable that the population on our study area is being supplemented by production from other areas characterized by higher recruitment.

  2. Our estimates of recruitment may be too low. We have shown that the potential biases in our methods would underestimate rather than overestimate recruitment. Furthermore, 2 years of our study, 1977 and 1980, were extremely dry and probably below average in recruitment.

The Management Problem

Success of mallard management must be judged by comparing results to goals. The goals may be different on the continental, regional, and local level. Numerical population goals for mallards may have some meaning on a continental scale, but at the regional and local level they can have little meaning if the population readily shifts among regions in response to changing habitat conditions. Boyd (1981a:92) pointed out the futility of such goals. Even continental numbers fluctuate in response to climatic changes (Boyd 1981b). These problems make it difficult to assess long-term trends, let alone determine which factors affect population size. This study demonstrates how measurement of recruitment parameters on a local study area can be used in conjunction with population models to furnish a prediction of population change that is independent of the breeding population size on that area. Such a prediction also requires an estimate of annual survival rate. Obtaining such an estimate was not an objective of our study, and we used previously published estimates. The prediction of change can be used as a means of setting goals. For example, if our goal for central North Dakota was to manage for a stable or increasing population, our prediction of 20% rate of decline means that the goal is not being achieved.

Management to correct the predicted decline would have to improve either the annual survival rate or the recruitment rate. Usually, harvest management is the tool by which managers attempt to change annual survival rate, but there is considerable argument as to whether survival rates can be altered through harvest management (Patterson 1979). We will not enter this argument here, but will discuss importance of recruitment rate and the implications of the low rates derived for our study area.

Recruitment rate may be the most important factor affecting population size. Hochbaum and Caswell (1978) used a simulation model to predict changes in mallard populations of the Canadian prairies. From a local perspective, they concluded that harvest management within their region would have minor impact on population, but that substantial gains might be achieved through increased recruitment. Their result is not in agreement with the findings of Couch and Boyd (1984) who concluded that harvest on the breeding grounds is an important factor explaining a serious decline in the export of ducks from the breeding grounds of Canada and the northern United States. Martin et al. (1979) used continental estimates of recruitment and survival to predict populations the following spring. To have their modeled predictions track actual population counts, they had to either reduce recruitment by 42% or the survival by 21%.

Our data and predictions of a declining mallard population can be explained by low recruitment on our study area and high mortality during the summer. These predictions cannot be verified by examining population counts on the same area. In fact, the situation there is similar to what Dzubin (1969b) called lethal brood areas where pairs were attracted to an area but failed to recruit young because of lack of brood habitat, except that in our area failure of nests was probably the most important factor causing low recruitment.

This low recruitment rate is probably the result of degradation of nesting habitat through intensification of agriculture. This situation now prevails over much of the most important breeding range of the mallard. The problem manifests itself in low nest success resulting from predation on eggs and ducks. Sargeant et al. (1984) demonstrated that predation not only lowers nest survival rate but also has a major impact on survival through loss of nesting hens. Our data on loss of hens corroborate his findings. Bailey (1981:67) used a theoretical approach to problems in waterfowl management and concluded that ". . . the results of my theoretical concoctions suggest that for the mallard, it would be very optimistic to expect that population sizes will continue unaffected by degradation of habitat much into the future." We concur with his conclusion.

The telemetry study area, though large in area compared to other study areas, is small in comparison to the entire breeding range of the mallard. We cannot assess the geographic extent of the problem, but there is some evidence that it may be widespread and growing. During the summer of 1982, the Central Flyway Technical Committee conducted a nesting study on 3 transects each in North Dakota, South Dakota, and Montana. Nest success for the mallard was only 6.6% (M. Johnson, unpubl. rep. presented to the Central Flyway Technical Committee, March 7, 1984) compared to 8.3% estimated in this study. In addition, the area where grassland habitat is being lost to agriculture is large and extends into much of the Canadian prairie as well as the United States.

Research Needs

The long-term prospects for mallard populations have not been fully evaluated on the basis of existing data. Martin et al. (1979) presented information on basic statistics of population size, annual recruitment, and annual survival rate as derived from extensive surveys. They also point to inconsistencies in the data. Identification of the reasons for these inconsistencies and resolution of problems should receive high priority. Boyd (1981a,b) has presented new methodology for relating weather to population fluctuations and made some tentative predictions as to future trends in mallard populations. Predictions as to future trends should be improved by development of population models that span the entire year. Such models would also aid in identifying weakness in data and guiding future research. Close scrutiny of mallard population dynamics from a continental viewpoint is required to put our results in perspective.

Additional field studies will be required to determine the extent and magnitude of the problem identified in our work. Such studies must further clarify the complex interrelations among habitat, land use, mallard populations, and predator populations. Understanding of these long-term problems can only be gained through carefully conducted long-term research. Even though there are some data available on nest survival rates in parts of the mallard breeding range, there are important areas and habitats for which no data are available. In addition, methodology for determining the cause of nest loss is often inadequate. Data on the survival of young from hatching to fledging is a key component of recruitment for which data are extremely limited.

The Future

Because of the fertility of the northern prairies of the United States and south central Canada and a growing world market that provides an outlet for grain, it is apparent that intensive use of these lands for agriculture will continue. Waterfowl managers are faced with the problem of maintaining populations that will provide for both consumptive and nonconsumptive uses by current and future generations despite a habitat base that is declining in quantity and quality. We believe that the data presented here and in related studies (e.g., Sargeant 1984) demonstrate that the loss of nests and hens occurring in the heart of the mallard breeding range is a critical problem. Solution of this problem will be costly and perhaps controversial, but it cannot be ignored. The mallard is a highly adaptable species, but maintenance of huntable populations will require management actions on 3 fronts—preservation and management of essential habitat, regulation of harvest, and maintenance of a recruitment rate that will compensate for annual mortality. Management that does not address all 3 areas is destined to fail.
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