How does chemical pollution affect eggshell pigmentation?

    Eggs of gull-billed terns of the colony in Mesas de Asta (Cádiz, Spain) show a high exposure to the famous organochlorine pesticides known as DDTs. The study of the pigments responsible for its coloration reveals a potential negative effect on the reproductive health of the species.

    An investigation by the Research Group in Wildlife Toxicology of the Instituto de Investigación en Recursos Cinegéticos (IREC - CSIC, UCLM, JCCM), conducted in collaboration with the ZooBotánico de Jerez, has revealed that eggs of the gull-billed tern (Gelochelidon nilotica) colony in Mesas de Asta marshes, in the province of Cádiz (Spain), accumulate the highest levels of dichlorodiphenyldichlorethylene (DDE) in comparison with studies in other tern species from the Mediterranean basin..

    DDE is the result of the metabolism of dichlorodiphenyltrichloroethanes, better known as DDTs, a group of organochlorine pesticides sadly famous for their high toxicity and their massive use in agriculture during the second half of the 20th century. More than four decades after the ban on the use and commercialization of these insecticides, their high persistence in the environment motivates that they are still present in ecosystems at high concentrations, so they continue to biomagnify through the food chain until they reach birds. Thus the analysis of DDE in eggs of birds, especially those that feed on insects, serves as a non-invasive biomarker of exposure to DDTs and to assess the risk that this pollutant poses for their conservation.

    The gull-billed tern (Gelochelidon nilotica) is a long-distance migratory bird that travels to the Iberian Peninsula during spring and summer to breed. Its feeding habits are quite opportunistic, but its diet includes a significant proportion of insects. The population of the species in the Iberian Peninsula represents up to 85% of the population of the Western Palearctic flyway, which means that the data collected in this study is of great relevance for the conservation of the species (Photo: Imran Shah /CC BY-SA 2.0).

    The toxicity of DDTs and other highly persistent organochlorine compounds like polychlorinated biphenyls (PCBs) results in adverse effects on the reproductive system of birds, including a decreased eggshell thickness, which make eggs more brittle and easier to break. In this regard, the research shows that around 5% and 2% of the eggs of gull-billed terns from Mesas de Asta marshes had levels of DDE and PCBs, respectively, above the thresholds associated with a reduced reproductive success.

    However, results did not show an evident negative relationship between eggshell thickness and the levels of DDE or PCBs owing to a clear effect of the stage of embryo development on these parameters. This fact, which is probably associated with the utilization of calcium in the eggshell by the embryo during its development, highlights the importance of taking into account the stage of embryo development of the to avoid potential biases in the interpretation of results when the eggshell thickness is used as a biomarker of reproductive success.

    The study focused on the study of unhatched eggs abandoned during the incubation, something that can happen when nests are subjected to high predation rates or due to the temporary drying of the wetland during the summer. The rescued eggs were transported to the ZooBotánico de Jerez, where they were checked to determine their fertility. The fertile eggs were placed in an incubator to be monitored for embryo development.

    Scientists also studied the potential effect of DDTs and PCBs on protoporphyrin IX and biliverdin biosynthesis, which are the pigments responsible for the brown and greenish coloration of eggshells, with the aim of exploring their suitability for their future use as non-invasive biomarkers of the effect of these pesticides on the reproductive health of birds. Eggshell pigmentation fulfills important functions on egg thermoregulation, mechanical strength, and camouflage, and consequently on quality signalling and sexual selection, so that its alteration could have negative consequences on reproductive success.

    This is the first time that eggshell pigmentation and egg contaminant burden are studied in the same eggs, and results showed that, in fact, the accumulation of DDTs was negatively correlated with eggshell pigmentation by protoporphyrin IX and biliverdin. Therefore, despite eggshell pigmentation is an evolutionary trait whose variability is influenced by genetic or environmental factors, these results show that little changes in the pigment concentrations caused by pollutants may have negative consequences on the reproductive fitness of birds.

    In the field of Ecotoxicology, a biomarker is defined as any measurable change either biochemical, physiological or morphological that an organism develops when exposed to a toxic substance. The present investigation shows, once again, that non-invasive biomarkers are very useful tools for biomonitoring chemical contamination and evaluating the risk it poses for wildlife conservation..

    The scientific publication of this research is available at: