Counting chicks before they hatch: Extending the observed lifetime to better characterise evolutionary processes in the wild

Evolutionary theorists have emphasised 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 recognised 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 fertilisation status of unhatched bird eggs—hatching failure can arise through fertilisation failure or prenatal mortality—two recent studies show fertilisation failure to be extremely rare within two 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 characterisation of the evolutionary dynamics of wild populations.

Methods
Nestboxes in Wytham Woods (Oxfordshire, UK) were visited approximately weekly to observe progress of all breeding attempts, more frequently around the time of expected hatching. The archived file includes all recorded great tit nesting attempts from 2008-2009.