This study consists of two parts. In the first part, a method of measuring cranium size of great tits was developed, and in the second part cranium size was compared with measures of fitness, namely winter survival and reproduction. In autumn of 2010, tits were captured and cranium width, length, and height, were measured in addition to tarsus length, body mass and wing length. In spring 2011, the birds that had survived winter were recorded, along with measures of reproductive performance. Cranium size was estimated from external skull measures on the birds caught in the autumn. The data were used to test the following predictions: 1) Great tits with larger cranium, have a higher survival during winter than those with smaller cranium, and 2) birds with larger cranium have higher breeding success than birds with smaller cranium, as reflected in time of 1. egg, number of eggs laid, day hatched, number of young hatched, number of young at day 15, number of young fledged, and the mean body mass at day 15 of young fledged.
There was no significant correlation between survival and cranium size. Significant correlation was only found for one measure of reproduction, namely the mean body mass at day 15 of young fledged, which was higher for females with larger cranium. For males there were no significant correlations.For cranium size I used a product of cranium width and length. I recognized great difficulties of measuring cranium size in a repeatable way, and not all cranium measures were repeatable between and within observers. This is important to take into account to make the measuring of cranium size more accurate.