Mallard Recruitment in the Agricultural Environment of North Dakota

Models and Tests of Assumptions

Models have received increased use in waterfowl research. They are useful for making generalizations (Bailey 1981), setting priorities for research (Ringelman and Longcore 1980), and evaluating management options (Cowardin and Johnson 1979).

We found models useful for a variety of purposes in the conduct of this study and analysis of results. Cowardin and Johnson (1979) presented a model where size of population in year t + 1 was defined by the following equation:

            N_t + 1 = N_tS + N_tRS*                                       
     where                                                             
                                                                       
       N_t = the number of hens in the spring of year t,                
       S  = the annual survival rate of adult hens,                    
       S* = the survival rate of immature hens from fall to spring, and
       R  = the number of hens recruited to the fall population per hen
            in that  year's spring population.

The parameter recruitment (R) was separated into subcomponents, which were

      R = Pe^(1-P)²ZB ÷ 2,                                         
       P = nest success,                                           
       Z = survival of at least 1 brood member to approximately    
           age IIc, and                                            
       B = average brood size at fledging.

That model was the basis for selecting the attributes of recruitment to be studied, and a slightly modified model will be used for predicting population change. A stochastic model developed by statisticians and biologists at the Northern Prairie Wildlife Research Center, Jamestown, North Dakota (Cowardin et al. 1983), will be used to furnish a simulated population of nests for testing some of the basic assumptions discussed later and to evaluate potential biases that may have affected our study.

The basic structure of the model is illustrated in a flowchart (Fig. 11). Modifications and adaptations will be described in conjunction with some of our specific uses.

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