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Mallard Recruitment in the Agricultural Environment of North Dakota

Biases in Analysis Techniques


The simulation model used in the previous section is valuable for illustrating problems that arise in dealing with nesting data. The basic problem in nesting studies where nests are found by presence of the hen is the investigator's inability to find all nests. The use of radio-marked birds does not overcome the problem. Figure 12 illustrates that 7 of 16 simulated nests were not found. In 1 instance (Fig. 12, bird 10) 4 nests were undiscovered. The pattern of missed nests is similar to that presented by Miller and Johnson (1978) for nests found by cable-chain drag (Higgins et al. 1969). The bias caused by missed nests has been intensively studied (Mayfield 1961, 1975; Miller and Johnson 1978; Johnson 1979; Klett and Johnson 1982). In our simulated population of nests, 9.9% were successful. The apparent hatch rate in our simulated sample was 21.4% and the estimate based on the Mayfield method was 11.8%. Use of the Mayfield method apparently eliminates most of the bias, but there is some question about the validity of the assumption of constant daily survival rate (Klett and Johnson 1982).

Miller and Johnson (1978) showed that the number of initiations can be estimated by dividing the number of successful nests by the hatch rate. We used a similar technique to estimate the number of nests initiated in various types of habitat by dividing the number of nests found by the survival rate to the date when found (Table 20). The difference between the number found and the estimated initiations was great (142 vs. 285), but the relative importance of the habitats was very similar. The results do not change our original conclusions about grassland being most used and cropland least used. There was some minor shifting in rank among wetland, right-of-way, and odd area that have very similar use.

Table 20. Comparison of nests found in 6 habitat types and number of estimated initiations.a
Habitat N Percent of total nests N Percent of estimated initiations
Grassland 57 40.1 105 36.6
Hayland 17 12.0 35 12.2
Cropland 3 2.1 7 12.2
Wetland 23 16.2 46 16.2
Right-of-way 20 14.1 48 16.6
Odd area 22 15.5 45 15.8
aN = n ÷ sa where N = estimated initiations, n = number found,   s = daily survival rate, and a = mean age of nests when found.

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