Mallard Recruitment in the Agricultural Environment of North Dakota
Relation of Reproductive Effort to Condition of the Hen
Krapu (1981) demonstrated that nutrient reserves are critical for reproduction by hen mallards and that weight is a good index to nutrient reserves and can be compared to reproductive effort. Our knowledge of reproductive effort for each individual is not complete because some hens had expended an unknown amount of reproductive effort before capture. Furthermore, some nests went undetected after capture.
We compared the condition index at the time of capture to the known production of eggs after capture. To make these comparisons, we used a least squares means procedure (SAS Institute, Inc. 1982:177-178) that aids in analysis of data from an unbalanced design. Both the probability of producing 1 or more eggs and the known number of eggs produced after capture were used as measures of reproductive effort. Birds were placed in 1 of 5 condition classes by standardizing the condition index (CI) by (CI - mean CI) ÷ square root of VCI and rounding to the nearest integer. Mean CI is the mean condition index of the sample and VCI is the variance. The procedure yields class intervals -2, -1, 0, 1, 2 of 1 standard deviation. Least squares means for probability of production and eggs produced were then calculated for each condition class. These measures of production must be considered indices because the means contain true zero values and an unknown number of zero values for undetected nests. The indices were tested for effects due to condition class, age of hen, time of capture, and interactions.
Both probability of production and eggs produced increased with increasing condition class (Fig. 8) as would be suspected from Krapu's (1981) data. Both reproductive indices also were significantly higher for ASY birds than SY birds. There was a significant decline in eggs produced with time but not for probability of production. There were no significant interactions.
The present study, though not designed to investigate the importance of condition to reproductive effort, did add evidence supporting the conclusions of other workers (Harris 1970, Ankney and MacInnes 1978, Raveling 1979, Krapu 1981). The results, though based on indices to condition, reinforce Krapu's conclusion that food resources in the breeding habitat contribute to the ability of mallard hens to renest. The greater probability of production and larger number of eggs laid by older birds demonstrate the higher productivity of older hens.
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