Laying Dates, Clutch Size and Egg Weight of Captive Mallards

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Abstract
The relationships of clutch size, laying dates and egg weight were studied with pairs of captive mallards for 3 consecutive years. High repeatability estimates were calculated for all 3 traits. This indicates relatively low variability for individual performance from year to year even though there was a high degree of variation in the entire sample studied. Laying dates of individual females probably reflect phenotypic responses to the breeding environment. In the wild this may be modified by other factors such as the ability of the individual to find food. Clutch size declined between the 1st nest and each subsequent renest, as has been frequently recorded in the wild. The decline was also evident within a given nest sequence with earlier clutches being larger. A single regression equation describes clutch size decline within 1st nests and all renesting attempts. Even though laying date and clutch size appear to be under significant genetic control, earlier nesting by an individual results in a larger clutch being laid. This suggests that the main factor controlling clutch size is laying date. The analysis strongly points to an underlying genetic control, probably mediated through photoperiod. Eggs were heavier in larger clutches in 1st nests and all renesting attempts. Eggs were larger in successive renests suggesting that some of the loss of production of young from smaller clutches may be compensated for by production of larger ducklings later in the season. Duckling weight and egg weight were highly correlated. Further work is needed on the relationship of duckling weight and egg weight and on the possible physiological advantage inherent in duckling body size.