Like most other penguin species, Magellanic Penguins (Spheniscus magellanicus) are large-bodied birds that incubate their eggs for a prolonged period on hard substrates with little nesting material—all circumstances that could lead to high rates of egg breakage. However, Magellanic Penguin eggs at Punta Tombo, Argentina are seldom broken. From 1984 to 2001, only 2.6% of 10,023 eggs in our study areas broke or cracked. Most of those were broken in unusual or catastrophic events, mainly penguin fights and rainstorms. Low breakage rates appear to be attributable to thick eggshells. Shells of Magellanic Penguin eggs averaged 0.81 mm without the egg membranes—at least 56% thicker than expected for bird eggs of similar mass. The calcium required for those thick eggshells cannot be supplied by normal food intake because females lay eggs during a fasting period. It is also unlikely that sufficient skeletal calcium can be mobilized. An alternative potential calcium source is mollusk shells. To determine whether female penguins were selectively ingesting calcium to form thick eggshells, we examined stomach contents of birds during the egg period (settlement, egg laying, and early incubation) and the post-egg period (late incubation and chick rearing). Both females and males were more likely to have mollusk shells in their stomachs during the egg period than during the post-egg period. However, females were much more likely than males to have shells in their stomachs during the egg period, whereas the proportions of males and females with mollusk shells did not differ in the post-egg period. Selective ingestion of mollusk shells by Magellanic Penguins, resulting in thick eggshells, appears to be an adaptive response that reduces egg breakage.