Unhatched eggs represent the invisible fraction in two wild bird populations

Prenatal mortality is typically overlooked in population studies, which biases evolutionary inference by confounding selection and inheritance. Birds represent an opportunity to include this ‘invisible fraction’ if each egg contains a zygote but whether hatching failure is caused by fertilisation failure versus prenatal mortality is largely unknown. We quantified fertilisation failure rates in two bird species that are popular systems for studying evolutionary dynamics in the wild.  over three years, finding that overwhelming majorities (99.9%) of laid eggs were fertilised. These systems thus present opportunities to eliminate the invisible fraction from life-history data.