Does mallard clutch size vary with landscape composition: A different view

We report on the relationship between Mallard (Anas platyrhynchos) clutch size and cropland area in the landscape in western Minnesota during 1997-1999. We measured clutch size in two types of nest structures and fit a mixed-effects model to the data to examine the relationship. Our model also included covariates to control for the effects of year, nest initiation date, estimated pair numbers, and nest structure type. Unique landscapes associated with each nest (n = 134) ranged from 46.4-84.8% cropland. Clutch size was unrelated to cropland area, nest structure type, and estimated number of pairs with access to structures. Mean clutch size declined with nest initiation date early in the nesting season, but increased somewhat for nests initiated after 30 May. Clutch size also differed among years. Mean clutch size, adjusted for nest initiation date, was 11.0 ± 0.19 SE for 1997, 10.5 ± 0.19 SE for 1998, and 11.0 ± 0.19 SE for 1999. Conclusions regarding the significance of the year effect and the degree of nonlinearity due to nest initiation date were sensitive to potential clutch size outliers, but cropland area had no effect on clutch size regardless of the way we constrained clutch size. Nest parasitism by philopatric females laying in certain structures might explain the observed increase in clutch size in late nest initiations.