A Blue Tit Cyanistes caeruleus population: its recent increase and breeding data

Before 1983, Blue Tits Cyanistes caeruleus never made up more than four pairs of the breeding population in a nestbox study carried out in South Central Sweden. From five breeding pairs in 1983, the population increased to 29 pairs in 2007 and remained high to the end of the study in 2011. Mean laying date was strongly correlated with mean April temperature, which increased during 1983–2011. During the same period mean laying date became nearly ten days earlier. Mean clutch size was 9.90 eggs but varied both within and between years and showed a negative relationship to population size. The mean number of fledglings varied strongly between years, partly because of predation but also due to nestling starvation.