Unhatched eggs: methods for discriminating between infertility and early embryo mortality

Unhatched eggs are a common phenomenon in birds and are often referred to as being 'infertile', which (confusingly) can mean at least two things: (1) that the ovum has not been fertilized or (2) that the embryo has died during development. These two broad categories of hatching failure can be difficult to distinguish, particularly in the early stages of embryo development. We describe methods to distinguish between infertility (due to insufficient sperm) and early embryo mortality in passerine eggs using the Zebra Finch Taeniopygia guttata as a model. We also describe how we successfully adapted these methods for use on eggs from a wild species, the Tree Sparrow Passer montanus, collected after the incubation period, and show that sperm can be visualized on the perivitelline layer of unhatched eggs even several weeks after laying.