A number of researchers have investigated avian eggshell as a dating material and found it to be a reproducible and accurate dating material.
Berger et al. (1975) have radiocarbon dated egg samples of the extinct giant elephant bird Aepyornis sp., from sites in the southern Malagasi Republic. They removed the exterior layer of eggshell with dilute acid as a precaution against contamination and suggested that the material was useful because the five results matched those obtained from other materials in association (Berger et al. 1975:709).
Horowitz et al. (1978) have dated ostrich eggshell fragments excavated from the prehistoric site of Voigtspost, in Orange Free State, R.S.A. Sample Pta-1483 was dated at 1220±50 BP (d13C=-6.7 per mille), while Pta-1520 gave a date of 6350±75 BP (d13C=-8.3 per mille) (Horowitz et al., 1978:154). Two modern samples of eggshell were also dated. After correction for isotopic fractionation, they gave an apparent age of 200 years in comparison to modern atmospheric carbon. Horowitz et al. (1978) concluded ostrich eggshell dates may be too old by this amount and advised caution in interpretion. There are a number of possible explanations for the old date of modern eggshell. It may have been caused by the bird drinking water from sources such as inland freshwater lake systems, which can contain dissolved bicarbonate or CO2, depleted in 14C.
Williams (1981) has reported a date from eggshell likely to be Genyornis newtoni, an extinct Pleistocene ratite. This was given a minimum age of 40680 BP (SUA-1337) by the laboratory. He also refers to SUA-885 which gave ages of 35900±2980 and 35900±2170 BP, concluding that the eggshell was probably from a species of duck. Williams (1981:138) also dated a modern emu eggshell which gave a date of 560±70 BP (SUA-1069). This was older than expected. According to Barbetti (pers. comm.), d13C was estimated at -5±2 per mille. A 1981 revision of dates from the Sydney laboratory gave a new value for the sample of 440±80 BP.
Long et al., (1983) dated fossil ostrich eggshell (Dromaius novaehollandiae) from the Sahara and elephant bird (Aepyornis sp.) from the southern Malagasi Republic. They considered the effect of old carbon uptake on the 14C label of the material by spiking the feed of a group of domestic chickens with limestone, over 25 days. Finding no measureable difference in the 14C activity between the group fed low-limestone feed and the group fed high-limestone feed, they concluded that eggshell should be reliable for dating even when carbonates depleted in 14C were ingested. No paired dates from other samples were included.
Freundlich et al.., (1989) used ostrich eggshell to date human occupation in the Eastern Sahara. They compared uncorrected eggshell dates with those from a variety of other materials and found an average difference of 350±60 radiocarbon years. The authors' concluded that d13C measurement and normalisation to -25 per mille wrt PDB should result in good agreement with other materials from identical contexts. They refer to the work of Haas and Haynes (1980), and Wendorf and Schild (1984) in the same region. Both used ostrich eggshell for dating and found no significant age differences with dates from other materials in association (Haas and Haynes, 1980; Wendorf and Schild, 1984, in Freundlich et al., 1989).
Higham (1994) has recently dated a series of moa eggshell samples from prehistoric New Zealand sites. He suggested that eggshell is a reliable material which survives well in archaeological contexts, possesses a minimal inbuilt age and usually dates the event in question most accurately. He compared radiocarbon dates of eggshell with those of other materials in identical contexts and found the differences to be statistically insignificant.