The characterisation of mercury pollution in the faeces of Baltic region breeding black storks

• Fish-eating birds, such as black storks, are at risk for higher mercury intake; • Bird faeces can be used to determine mercury pollution in the environment; • Faeces are non-invasive sampling method for (local) environmental pollution determination; • Based on storks’ behaviour juvenile and adult storks’ faeces are distinguishable; • There are temporal, spatial and age-based differences in mercury concentration levels. Mercury is a pollutant that negatively impacts wildlife's physical fitness, reproduction success, as well as foraging and migrating abilities; especially high risk is posed to the animals that consume fish as their main food source, such as black storks Ciconia nigra . The black stork is a sensitive and solitary bird species, whose populations in north-eastern Europe have declined in recent years for various reasons. In this study, we present the first-ever multi-year assessment of mercury pollution in black stork faeces, analysing samples collected over a six-year period (2018 – 2023) from 121 nests across Latvia. We found a temporal and spatial total mercury pollution pattern and a distinction between total mercury pollution in juvenile (range: 4.75 – 521.78 ng/g) and adult (range: 7.32 – 365.60 ng/g) black stork faeces. Total mercury concentrations also differed based on faeces colour – light faeces had a statistically significant lower mercury concentration than dark faeces. In addition, we tested juvenile black stork food samples gathered from the nests and found up to five times higher total mercury concentrations than the government-permitted limit. Causes for the variability of mercury pollution across different nest sites as well as between adult and juvenile black storks are still open for future studies.