Genetic evidence of female specific eggshell colouration in the Common Crane (Grus grus)

The large variation in colouration and patterning of bird eggs suggests a variety of functions. For instance, in cases of intra- and inter-specific brood parasitism, the recognition of own eggs by the parents could be essential for their reproductive success. However, individual specific signatures may also be of interest from an applied point of view, as it would be possible to monitor individual females across breeding seasons by identifying their eggs. This would be of particular importance for species that are difficult to catch and ring such as the Common Crane (Grus grus). Since 2004, nest monitoring of this species has been conducted by one of us (W.M.) in north-east Germany, which led to the development of a semi-quantitative method to identify female cranes by diagnostic egg features including ground colour and spots of eggshells. In order to verify this approach, we quantitatively determined the spot patterns on eggshells from eggs of 19 females identified by this method. We used standardised photographs of the eggs laid across three seasons and the computer program “Egg Shell Pattern ANAlysis” (ESPANA). The resulting data were statistically analysed by conducting principal coordinate analyses and analyses of similarity. To prove the identity of the putative females, we extracted DNA for microsatellite analyses from eggshell pieces collected after hatching from up to seven breeding seasons. Our analyses confirmed that Common Cranes lay eggs with individual specific patterns and confirmed the reliability of the semi-quantitative method of identification. Microsatellite genotypes based on nine loci were identical for all samples from each particular, putative female. Therefore, the semi-quantitative approach of identifying females based on their clutches is indeed an innovative monitoring tool that will make many species accessible for addressing important issues in population biology, ecology and conservation.