General methods
The study was carried out in the locality of Miraflores de la Sierra, Madrid, central Spain (40°48′N, 03°47′W) throughout the breeding seasons of 2017, 2018 and 2019. We studied a blue tit population breeding in nest-boxes in a deciduous forest, mainly dominated by Pyrenean oak (Quercus pyrenaica ). At the beginning of the breeding season, we started visiting nest-boxes every week to record the onset of nest construction. Then, we checked them every 2-3 days to record laying and hatching dates (hatching day = day 0). On days 11 (in 2019) or 12 (in 2017 and 2018), that is, once blue tit nestlings had developed yellow breast feathers (Peters et al. 2007), we measured feather colouration and body mass (see a detailed explanation in Colour measurementsbelow). Blue tit nestlings exhibit yellow breast feathers, which they moult about two months after fledging, during the post-juvenile moult (Cramp & Perrins 1993). This trait reflects light both in the longwave band of the reflectance spectrum (yellow to red wavelengths between 550 and 700 nm) and in the ultraviolet (UV) region (Shawkey & Hill 2005).