Northern Prairie Wildlife Research Center
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
||Percent of total nests
||Percent of estimated initiations
= 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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