Robyn L. Bailey; Gerald E. Clark

“ Crowding in a double embryo egg is likely a significant barrier to hatching, since at least one chick must be able to bring the beak in contact with the shell for pipping ( [18] ) . Hatching may also prove fatal if the embryos are not positioned with access to the air cell or if the yolk sac becomes ruptured during hatching ( [1] ; [6] ) .
[5] reported that two of 152 fertile double-yolked chicken eggs (1.3%) survived long enough to pip the egg, but only one was able to hatch with assistance. ”

“ Few studies, however, have documented the rates at which twinning occurs within wild populations, likely because estimating rare events requires large sample sizes. Twinning in free-living birds has been reported for a small but diverse group of species ( [6] ) . As summarized in Table 1, twinning in wild birds is discovered either by (1) the dissection of unhatched eggs, or (2) by the presence of more nestlings than eggs. Given the rarity of twinning in birds, we document the successful hatching of twin eastern bluebirds (Sialia sialis) in a wild population in central Pennsylvania. ”

“ A presumed double-yolked egg of a free-living eastern bluebird pair hatched successfully, and twin nestlings lived for 11 days in a nest box shared by three siblings. Due to the rarity of twinning in wild birds, engaging the public to monitor large numbers of nests is the most likely approach to documenting twinning in wild populations, and citizen science provides the infrastructure for individuals to share observations. ”