Many species of birds lay clutches that include eggs of varying sizes. Smaller eggs laid late in a clutch may facilitate brood reduction when there is insufficient food, whereas larger eggs may offset the disadvantage of late-hatching chicks when hatching is asynchronous. Alternatively, egg size may not be adaptive but may reflect energetic constraints on females. Magellanic Penguins (Spheniscus magellanicus) usually lay clutches of two eggs that are similar in size. In 5.1% of clutches (n = 7,085; 1983–2006), the volume of the smaller egg was <88.8% of the volume of the larger egg. We wondered whether this was possibly an adaptive trait. We found that laying a dimorphic clutch was costly and not adaptive. Eggs in dimorphic clutches failed to hatch 30% of the time, even if the eggs were larger than average. Eggs in normal clutches failed to hatch only 8% of the time. When both eggs hatched, no chicks fledged from 68% of dimorphic clutches, whereas no chicks fledged from 62% of normal clutches (G = 8.2, P = 0.02). Laying a dimorphic clutch is not a trait; females that laid a dimorphic clutch did not always lay dimorphic clutches and commonly laid eggs of similar size in other years. We did not find evidence of a constraint on females, in terms of body condition, age, laying date, persistence of the mate, or environmental conditions inferred from mean annual reproductive success. Dimorphic clutches are not adaptive and appear to be noise in the system.