Adaptive final (last-laid) eggs in the Eurasian Blue Tit Cyanistes caeruleus

The study was carried out in 2021 using nestboxes. I examined the egg-laying pattern of the socially monogamous free-living Eurasian Blue Tit Cyanistes caeruleus that breeds only once a year in the northwestern Croatia. Nestboxes were visited daily to record the laying date of the first and subsequent eggs. A total of 132 eggs from 12 clutches with 11 eggs in each nest (11-egg clutch is the dominant clutch size in the study area) was measured. The mean egg volume was 1.19 cm3 (± 0.12), and the mean egg volume of all final eggs was 1.22 cm3 (± 0.09). In this study, the %D-value was 2.46. According to this value, the Eurasian Blue Tit population in north-western Croatia adopt a “brood survival strategy”, where females allocate greater resources in the final eggs.