DOI: 10.5061/dryad.70rxwdc7c
title: Data from: Counting chicks before they hatch: Extending the observed lifetime to better characterise evolutionary processes in the wild

description: 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.
description: 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.
description: # Data from: Counting chicks before they hatch: Extending the observed
 lifetime to better characterise evolutionary processes in the wild
 [https://doi.org/10.5061/dryad.70rxwdc7c](https://doi.org/10.5061/dryad.70rxwdc7c) ## Description of the data and file structure 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. ### Files and variables #### File: Nest\_data\_(2008-09\_Wytham\_GTs).csv **Description:** Data derived from observations of great tit breeding attempts in nestboxes in Wytham Woods (Oxfordshire, UK). Nestboxes were visited approximately weekly to observe progress of all breeding attempts, and more frequently around the time of expected hatching. The archived file includes all recorded great tit nesting attempts from 2008-2009. Missing values are represented by NAs. ##### Variables * Nest_ID: A unique identifier for each distinct breeding attempt * year: Year of breeding attempt (2008 or 2009) * clutch_size: Total number of eggs laid. If incubation was never started (such that the breeding attempt was potentially abandoned before laying was completed), the number of eggs in the nest is recorded, although it may not represent a completed clutch. * hatch_day: The hatching date for the earliest-hatching egg in the nest. Values represent 'Aprildays', which are the number of days since 31st March (i.e., 1st April represents an Aprilday of 1). Hatching date is estimated based on the appearance of chicks and/or the weight of the largest chick(s) in the brood (both calibrated using data from high frequency visits). * max_observed_brood_size: The maximum number of chicks in the nest that was recorded during post-hatching visits to the nestbox, including the ringing of chicks two weeks after the recorded hatch date (which can be the only occasion when the brood size was recorded. * number_fledged: The number of confirmed fledglings. Chicks are ringed fourteen days after the clutch's recorded hatch date, and the number of fledglings calculated as the difference between this and the number of dead chicks remaining in the nest after fledging has been completed.