Wild bird eggs as bioindicators of environmental contamination: A decade of xenobiotic monitoring in Spain.

Between 2014 and 2024, unhatched eggs from several wild bird species were analyzed at the Center for Analysis and Diagnosis of Wildlife (CAD, Spain), including eggs from threatened species such as the bearded vulture (Gypaetus barbatus) and the Spanish imperial eagle (Aquila adalberti). The aim of this study was to characterize exposure of wild birds to environmental contaminants and discuss their potential implications for reproduction. To this end, eggs were subjected to detailed examination, including biometric measurements, evaluation of embryonic development, and investigation of possible causes of reproductive failure. In addition, chemical analyses were performed on eggshells and egg contents to detect residues of different groups of xenobiotics, including metals, pesticides, pharmaceutical compounds and polychlorinated biphenyls (PCBs). Several contaminants were detected in the analyzed samples, mainly persistent organochlorine compounds such as DDT degradation products (o,p'-DDE and p,p'-DDE) and PCBs. Residues of currently used pesticides, including pyrethroids and fipronil, were also identified in some samples, whereas pharmaceutical residues were not detected during the study period. Although most contaminant concentrations were relatively low, their persistence, potential sublethal effects, and the simultaneous presence of multiple compounds in the same sample suggest that chemical exposure could represent a potential risk factor for avian reproduction. The results highlight the value of wild bird eggs as bioindicators of environmental contamination and emphasize the importance of long-term toxicological monitoring, particularly for species of conservation concern characterized by long lifespans and low reproductive rates.