The occurrence of runt eggs in waterfowl clutches

Eggs that are abnormally small are called runt eggs, and they occur in clutches across a wide variety of bird species. We surveyed waterfowl researchers to determine the natural frequency of occurrence of runt eggs in wild nesting ducks, geese, and swans. Of 551,632 eggs examined, 215 were runts, yielding a frequency of 0.039%. They occurred at about four times this frequency (0.156% in 21,832 eggs) in a museum oological collection. Runt eggs were not significantly more common within any taxonomic group among wild waterfowl, and their occurrence was not related to the mean clutch size of a species or to whether waterfowl exhibited regular conspecific nest parasitism. Cavity-nesting waterfowl had lower incidences of runt eggs than ground-nesting species. Our results represent the largest examination of the occurrence of this size anomaly in eggs of wild birds, and are consistent with the hypothesis that runt eggs result from a temporary impairment of the reproductive tract. This may explain why runt eggs occur in all bird species studied.