Why females prefer to copulate with particular males is a contentious issue. Attention is currently focused on whether females choose males on the basis of their genetic quality, in order to produce more viable offspring(1). Support for this hypothesis in birds has come from studies showing that preferred males tend to father offspring of better condition or with increased survivorship(2-8). Before attributing greater offspring viability to a male's heritable genetic quality, however, it is important: to discount effects arising from confounding sources, including maternal effects. This has generally been addressed by comparing offspring viability from two different breeding attempts by the same female: one when offspring; are sired by a preferred male, and one when offspring are sired by a less preferred male, However, here we show that individual female mallard (Anas platyrhynchos) lay larger eggs after copulating: with preferred males and smaller eggs after copulating with less preferred males. As a result, females produced offspring of better body condition when paired with preferred males. After controlling: for these differences in maternal investment, we found no effect: of paternity on offspring condition. This shows that differences between half-sibs cannot always be attributed to paternal or maternal genetic effects.