Non-destructive sampling of maternal DNA from the external shell of bird eggs

The use of non-destructive sampling methods to collect genetic material from wildlife allows researchers to minimize disturbance. Most avian studies employ capturing and handling of young and parents to draw blood for DNA analysis. In some cases adult female birds are difficult to catch, so maternal genotyping has required collection of contour feathers from nests, or destructive sampling of eggs. Many species do not leave contour feathers in the nest, and destructive sampling has been unreliable due to contamination with embryonic DNA. Alternative field sampling techniques for collection of maternal DNA from birds are therefore desirable. Here we demonstrate that avian maternal DNA can be isolated in a non-invasive and non-destructive way from the external surface of eggs. We used cotton swabs to collect maternal DNA from the external shells of herring gull (Larus argentatus) and Caspian tern (Sterna caspia) eggs. DNA was then amplified by the polymerase chain reaction (PCR) for microsatellite genotyping. We verified that the DNA samples were maternal by comparing microsatellite profiles to those obtained from adults and chicks from the same nests. In 100% of Caspian tern (n=16) and herring gull families (n=12), the egg swabs that amplified matched the maternal microsatellite genotype. In a screening of many nests of both species, we successfully amplified microsatellite markers from 101/115 (88%) egg swabs. Swabs from eggs with blood stains on the shell were more likely to amplify successfully than those from clean eggs. The advantages of this new method include increased parentage assignment/exclusion power, and increased availability of maternal DNA for genotyping of species that do not deposit contour feathers in nests.