Roseate Terns (Sterna dougallii) frequently lay single eggs and desert them without incubating them. In a multi-year study, single deserted eggs comprised 4.2% of all clutches and 2.7% of all eggs. Laying dates and masses of single deserted eggs resembled those of first-laid eggs of female-male pairs and multi-female associations, but their masses differed from those of second-laid eggs of female-male pairs. An egg-transfer experiment showed that most single deserted eggs were fertile and that chicks hatched from them were viable when raised by female-male pairs. I hypothesize that single deserted eggs result from females laying their first eggs before the pair has settled on the location of a nest site. Fostering of single deserted eggs could be used to increase chick production in small colonies.