So don't think mammals here guys, and by that I mean where the male is XY and the female is XX. These guys' sex chromosomes are Z and W, and in their case the females are the heterogametic species. Put simply, ZW=Female, ZZ=Male. Because of the way that this occurs the females can ONLY produce male offspring. If any of the hatchlings were female they woudn't be candidtates for a virgin clutch. It says in the article that the two clutches that were tested were already candidates because they were male and had some slugs/stillbirths in the clutch.

Mike - without re-reading the article, and if I read it right, they conducted DNA analysis to determine whether there was genetic material from another animal.