Me and my colleagues are studying wild, hole-breeding bird populations using individual marking. The site fidelity of birds and the consistently high capture intensity allow the monitoring of the life, decisions and state of individuals, and the estimation of fitness and selection. There are three main themes in the focus of our research: reproductive decisions, nestling growth and development, and mate choice signals. We monitor the populations of resident tit species in winter as well. Classical field data are complemented by a broad array of technical tools, due to our own instruments and within- and extra-institutional cooperation: estimation of oxidative stress, estimation of the temporal dynamics of physiological condition, molecular sex determination, field spectrometry, feather macrophotography, electron microscopy and small angle X-ray scattering (SAXS). The field investigations have now been running for 45 years and they have been gradually extended to the character spectrum we cover today. Therefore, we now have the longest contiguous morphological, breeding and fitness data set of individually marked populations in Central Europe, and the longest sexual signal and field spectrometry data sets of the planet. These allow us to examine research questions that would not be possible to ask elsewhere, such as the evolutionary change of the individual optimization of reproductive investment, the fluctuating evolution of morphological traits, and the functional integration of sexual signals.