Counting chicks before they hatch: extending the observed lifetime to better characterize evolutionary processes in the wild.

Evolutionary theorists have emphasized for over half a century that population sampling must be conducted at the intergenerational boundary if the distinct effects of selection and inheritance are to be reliably quantified, with individuals recognized at the point of conception and lifetime reproductive success (LRS) defined as the total number of zygotic offspring produced per zygote. However, in those species whose ecology is otherwise well-suited to individual-level population studies, the prenatal part of an individual's life is often difficult to observe. While uncertainty has long surrounded the fertilization status of unhatched bird eggs-hatching failure can arise through fertilization failure or prenatal mortality-2 recent studies show fertilization failure to be extremely rare within 2 of the most popular avian study species. As such, unhatched eggs are highly reliable indicators of prenatal mortality. Although the generality of these results remains unclear, they demonstrate that prenatality can be incorporated into the observable lifespan of free-living animals. This allows zygotic LRS to be retrospectively quantified using historical nest observations and facilitates a more complete characterization of the evolutionary dynamics of wild populations.