Great Tits (Parus major) are adapting to a warming world by changing their timing of reproduction. To investigate the selection pressure of climate change on these birds, the Netherlands Institute of Ecology (NIOO-KNAW), set up an experiment. A four-generation selection program, selecting on laying date was used create two extreme genotypes: an early laying and a late laying selection line. In 2017, the fourth generation (F4) eggs were transported from the aviaries into the wild, to study the breeding timing and behaviour of the two selection lines in 2018. To reliably compare the two selection lines, they must have an equal fitness and only differ in their laying date. However, accidentally fixed trait-irrelevant genes may have led to the appearance of other traits, perhaps influencing fledging success. Thus, to test whether the fitness of the two selection lines differs, I studied the fitness of the F4 hatchlings, comparing the selection lines in four components chosen to represent fitness: weight, tarsus, fledging success and wing (length of the eighth feather of the primary remiges). Results showed no differences in those four components, indicating an equal fitness in the selection lines. This equality makes them reliable study subjects in their breeding season in 2018.