Great tits lay increasingly smaller clutches than selected for: a study of climate- and density-related changes in reproductive traits.

1. The phenology of temperate environments and therefore timing of breeding has advanced in a number of bird species due to climate warming. Few studies, however, have examined the mechanisms behind the observed changes, the role of natural selection in them or the determinants of the selection. In other traits such as clutch size, even changes over years have been rarely studied. 2. We studied patterns and trends in timing of breeding, clutch size and fledgling production in the great tit Parus major in South-West Finland during 1953-2008, as well as natural selection on the timing and clutch size, based on fledgling production. We also examined connections between these parameters and a number of climatic and population intrinsic factors. 3. Laying date was earlier when the pre-breeding period was warm and tended to be earlier when breeding density was high, but it did not show any temporal change during the study period despite temporal increases in both explanatory factors. Number of fledglings decreased through declines in both mean clutch size and fledging success. Fledging success was better with higher breeding-time temperature and larger clutch size. Both the clutch size and fledging success were lower at higher breeding density. 4. Selection on laying date did not change through time, but there was a selection for early laying with high breeding-time temperature and high breeding density. Interestingly, in contrast to the decrease in reproductive output, the selection for larger than average clutch size strengthened with time, which was not explained by any tested factor. 5. We suggest that increasingly favourable conditions in winters have enhanced the survival and resulted in the observed increase in great tit breeding density. This may have most concerned young and otherwise low-quality individuals, which also most likely end up breeding in the increasingly occupied low-quality territories. This hypothesis was indicatively supported by increased within-year variation in both laying date and clutch size. The changes could also explain the lack of advancement in laying date as well as the increasing selection for large clutch sizes as the fittest individuals most likely occupy the best territories and lay largest clutches.