Investment in offspring sex ratio of the white stork Ciconia ciconia in various environmental conditions

The main objective of this project was to test how the white stork invests in the sex of offspring under different environmental conditions. The main factors we considered were the weather conditions and agricultural land cover in the white stork territories. We predicted that under unfavorable conditions, white stork females would lay more eggs with female embryos, and in favorable conditions, with males. Another aim was to test whether the sex ratio changes during brood rearing and if nestlings are removed due to their sex. The study was conducted in the Wielkopolska region (Western Poland) in several study areas under different environmental conditions and different stages of agricultural intensification. Data on the basic parameters of the broods were collected for about 50 pairs per year. Arrival dates of birds at several nests were collected via questionnaires. Clutch size, number of hatched chicks, and their growth rate were established during direct inspections in nests. Blood samples were also taken. Due to the lack of sex determination of chicks directly in the field based on morphological characteristics, molecular analysis was needed.

In Quantum GIS based on habitat grids – Corine Land Cover database, in buffers with a radius of 2km around the nest, each major habitat type and degree of fragmentation of particular habitat types were calculated. Based on the collected material, we concluded that white storks invested equally in both males and females. We did not observe a significant deviation in sex ratio (proportion 1:1) between study years. Both adults' arrival dates and territory quality did not have a significant influence on chick sex. Males and females hatched with the same probability in nests where adults arrived earlier (in good fitness) and later (weaker). Nestling sex did not reveal a relationship with hatching order. However, we found that white stork males at the age of 15-20 days start to grow faster than their female siblings. Therefore, by the end of the nestling period, they were heavier and had longer bills. This growth difference is probably related to testosterone production. Hence, the earlier suggested male dominance in white stork broods is an effect of a faster growth rate, not earlier hatching. We did not find significant differences in the survival rate between male and female white stork chicks, but in good territories, males survived better.