Many bird species use specific vocalizations in the context of courtship or (attempted) copulations. These vocalizations can be used to gain information about the courtship behaviour of individuals, even when courtship or copulation behaviour itself is difficult to observe. We recorded courtship vocalizations of Blue Tits Cyanistes caeruleus during three breeding seasons. In 2014, we recorded courtship calls at 14 locations spread out over a 40 ha forest containing a nestbox-breeding population of Blue Tits. Courtship calls occurred throughout the breeding season, but were most common in the period just before and during egg-laying. The rate of courtship calling showed a clear daily peak around dawn. In 2012 and 2013, we recorded courtship calls during the fertile period of the male's social mate. We monitored the presence of the female inside the nestbox and describe the occurrence of courtship calls that are apparently not directed to the social mate. The rate of these potential extrapair courtships correlated positively with a male's extra-pair paternity gain in 2012, but not in 2013. Extra-pair courtship call rate did not relate to paternity loss in the male's own brood.