Data correlation of mercury in eggshells and eggshell membranes of wild birds

Wild birds are common study subjects as agents of mercury pollution, in particular predominantly fish-eating birds that are at risk for high mercury intake. The Black Stork is one such species in Latvia. Its conservation status is very unfavourable; by the IUCN criteria, it is considered to be critically endangered. Our data encompasses several species of wild birds; however, the bulk of data consists of black stork eggshells. If the target species is endangered, it is highly desirable to reduce the impact caused by the data collection itself. The collection of eggshell remains under nests is one such non-invasive method. Black storks often drop hatched eggshells and the remains of eggs out of their nests. The downside of this method is the fact that one cannot choose a preferable type of artefact to be collected, only what is there. Eggshell remains under nests vary greatly both in size and condition from almost complete eggs to tiny scratches of eggshell. We used an atomic absorption spectrometer with Zeeman correction LUMEX RA-915M paired with its attachment for pyrolytic analysis PYRO915+ to detect total mercury concentration in our samples. Pyrolytic combustion enables direct measurements without special pre-treatment steps, minimizing sample contamination risk and offering nearly immediate results. To assess our data, we did a statistical analysis using R version 4.2.1. We have previously observed that eggshell membranes contain higher levels of mercury than eggshells themselves. Here we check whether the amount of mercury found in various types of artefacts correlates, using 320 pairs of data for black storks for which both separate measurements were possible. Our data show that the correlation between the two is significant. In addition, we discuss the correlation between eggshells with attached membranes and membranes themselves, as well as between eggshells and eggshells with attached membranes.