The carbon and oxygen isotopic ratios of carbonate samples from eggshells of seven species of seabirds were examined in relation to information on the birds' foraging habits and geographic distribution, in order to determine how the isotopic signatures of eggshell carbonate are influenced
by the feeding ecology of the females which laid the eggs. Results indicated a gradient of increasing relative abundance of the heavier isotopes with degree of participation in the marine ecosystem, from freshwater to estuarine to offshore to pelagic. For carbon, the isotopic gradient originates
from the different isotopic signatures of the base source material (particulate organic carbon, or POC) suspended in meteoric versus oceanic water, and the mixing of these waters in the coastal zone, compounded by trophic biomagnification of 13C relative abundance through the marine
food web. For oxygen, the isotopic gradient reflects the isotopic signatures of the source water ingested by the birds, itself reflecting the isotopic signatures and mixing of meteoric and oceanic water in the coastal zone. For consumers at similar levels of the food web, in this case high
end consumers such as piscivores, isotopic information can provide reliable inferences about relative participation in marine versus freshwater type ecosystems at the time of eggshell formation. Carbon isotopic information can also provide information on the trophic status of consumers occupying
similar foraging habitats. For freshwater and terrestrial feeders, δ18O values can provide information on the latitude of occurrence of the birds at the time of eggshell formation, due to the characteristic signature of local meteoric water. The stable isotope methodology
presented here can complement conventional dietary analyses, as well as provide useful information on its own, and adds an additional dimension to the study of avian foraging ecology. This approach will be particularly useful in places where frequent visits are impractical. An advantage of
this methodology is that it is non-destructive. No live specimens, nor remains thereof, need be taken.
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