Datasets of egg parameters of long-lived bird species, the white stork Ciconia ciconia

Abstract Egg traits are central to avian reproductive investment and may reflect environmental conditions experienced during breeding, migration, and wintering. However, standardized long-term datasets of individual egg measurements remain scarce. Here, we compile and describe two datasets for the white stork Ciconia ciconia , integrating egg morphology, i.e. clutch size, eggshell thickness, and chemical composition across broad spatial and temporal scales. The first dataset contains 8,785 individual egg measurements from 2,137 clutches collected at 11 study sites in Algeria, Germany, the historical region of Macedonia, Poland, and Spain. It includes egg length, breadth, estimated volume, clutch identity, breeding year, nest identity and site metadata. Clutch size and egg dimensions differed among study sites: Polish populations generally had larger clutches and broader, higher-volume eggs than populations in Spain and Algeria, although egg length showed the opposite pattern. The second dataset comprises eggshell-thickness measurements and concentrations of 17 macro- and trace elements in eggshells and internal egg contents from 190 unhatched or post-hatch eggs collected at four Polish study sites. Eggshell thickness was measured in four longitudinal regions and was greatest at the equator and lowest at the blunt pole. Element concentrations varied among regions of eggshells, and compartments of egg contents (albumen, yolk, mixed contents and embryos). Together, these datasets provide a harmonized resource for investigating spatial and temporal variation in white stork reproductive traits, environmental exposure, and population ecology, and can be combined with climatic, land-use, migration and population-trend data for ecological, evolutionary, and conservation research.