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 sixyear 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.

Human-induced direct mortality affects huge numbers of birds each year, threatening hundreds of species worldwide. Tracking technologies can be an important tool to investigate temporal and spatial patterns of bird mortality as well as their drivers. We compiled 1704 mortality records from tracking studies across the African-Eurasian flyway for 45 species, including raptors, storks, and cranes, covering the period from 2003 to 2021. Our results show a higher frequency of human-induced causes of mortality than natural causes across taxonomic groups, geographical areas, and age classes. Moreover, we found that the frequency of human-induced mortality remained stable over the study period. From the human-induced mortality events with a known cause (n = 637), three main causes were identified: electrocution (40.5 %), illegal killing (21.7 %), and poisoning (16.3 %). Additionally, combined energy infrastructure-related mortality (i.e., electrocution, power line collision, and wind-farm collision) represented 49 % of all human-induced mortality events. Using a random forest model, the main predictors of human-induced mortality were found to be taxonomic group, geographic location (latitude and longitude), and human footprint index value at the location of mortality. Despite conservation efforts, human drivers of bird mortality in the African-Eurasian flyway do not appear to have declined over the last 15 years for the studied group of species. Results suggest that stronger conservation actions to address these threats across the flyway can reduce their impacts on species. In particular, projected future development of energy infrastructure is a representative example where application of planning, operation, and mitigation measures can enhance bird conservation.

Summary Detecting factors causing the decline of wildlife populations provides essential knowledge for their effective conservation. Populations of Black Stork Ciconia nigra are decreasing in northern Europe; however, there are no detailed analyses of its survival, which frequently is a key demographic parameter affecting population dynamics in long-lived species. We used long-term data from re-sighted colour-ringed birds and satellite-tracked birds to estimate age- and sex-specific survival in a rapidly declining Black Stork population in the Baltic region at the northern end of the European range. Apparent survival (0.89) among colour-ringed birds older than one year was not significantly different from the previously reported estimates in Central Europe and the estimated real survival of GPS-tracked birds (0.77). However, the apparent survival of first-year (1y) birds was only 0.04, which is remarkably lower than earlier estimates in Central Europe. The real survival of GPS-tracked 1y birds was somewhat higher (0.11), but still much lower than estimates in other long-lived species. Apparent survival was three times lower in 1y females (0.013) than 1y males (0.045); this could be explained in part by a higher mean natal dispersal of females (189.1 km), compared with that of males (72.0 km), as well as by sex-specific mortality due to poor foraging conditions. There were no significant differences in apparent survival between the male and female storks older than one year. To better address the population decline, further research is needed to determine the factors causing low survival in young Black Storks, including the roles of food availability and climate change.

Bird–forestry relationships have been the subject of research and conservation initiatives for decades, but there are few reviews of resulting recommendations for use by forest managers. We define “bird-friendly forestry” as forest management that applies recommendations from research seeking to reconcile logging with bird conservation in natural forests used for timber production. We reviewed relevant studies to synthesize 10 principles of bird-friendly forestry: (1) protect and enhance vertical structure through uneven-aged silviculture; (2) leave abundant dead wood in different decay stages; (3) maintain residual large green trees; (4) create and maintain sufficient amounts of uncut reserves and corridors; (5) maximize forest interior by retaining large contiguous forest tracts in landscapes with sufficient functional connectivity; (6) maintain buffers along streams, rivers, and wetlands cultural and urban landscapes; (7) maintain horizontal stand structure and enhance vegetation diversity by creating canopy gaps; (8) extend the temporal scale of logging cycles; (9) minimize post-logging disturbance to forests, particularly during the bird breeding season; and (10) manage for focal species and guilds. These principles may serve as guidelines in developing bird-friendly management plans customized for regional priority species, with a clearly articulated vision and quantitative objectives through which success can be measured.

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 PYRO-915+ 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.

Female birds whose bodies contain environmental contaminants produce eggs with shells that are likewise contaminated, making bird eggshells appropriate indicators for monitoring environmental toxins. Common contaminants include organic mercury compounds, especially methylmercury, which are known to bioaccumulate and biomagnify in the food chain. Black storks (Ciconia nigra) predominantly consume fish and are thus at risk for high mercury intake. In this study, we used eggshells of black storks as a proxy to reconstruct the concentration levels and distribution of mercury, a wellknown toxic element, in various parts of Latvia. Preliminary analyses have shown that deposition levels of mercury vary in different parts of the eggshell. Specifically, the shell and shell membrane differ in their level of mercury contamination by an average factor of nine; therefore, we measured the mercury content in these components separately whenever possible. We analysed 34 eggshell samples from nesting sites in Latvia using an atomic absorption spectrometer with Zeeman correction Lumex RA-915M and its attachment for pyrolytic combustion. We found that mercury concentrations varied from 5 to 22 ng/g in eggshells and from 42 to 293 ng/g in shell membranes. We discuss possible contamination sources and reasons behind this disparity.

Sex ratio is an indicator of population health as unexpected biases may indicate potential threats. We studied nestling sex ratio in Black Stork Ciconia nigra populations in order to check potential biases and differences along east–west and north–south gradient across its distribution range in Europe. We also studied variation between years, and checked potential correlations with weather variables. The overall sex ratio of nestlings in Europe was nearly equal with a non-significant deficiency (47.1 %) of males, the larger sex. Although yearly fluctuations in sex ratio were detected, no significant effect of the year alone was found, only simultaneously with population and brood size. There was a tendency to have a higher proportion of female nestlings in larger broods, but the pattern was probably scattered by the effect of reduction of largest broods. Compared to Western and Eastern Europe, a significant deficiency of male nestlings was found in Central Europe (Poland), whereas no differences were found along the north–south gradient. We did not find any effect of temperature, but rainfall during the incubation period was negatively correlated with the proportion of male nestlings in Central (Poland) and Western Europe (France) whereas in North-Eastern Europe (Latvia) the same effect of the precipitation in pre-breeding period was found.

The contamination with DDT (1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane) and its breakdown products as a potential cause of population decline of Black Stork has been studied in Latvia since 2008. In order to test if contamination is local or widely distributed we collected failed eggs from nests also in Estonia, Belarus, Poland, Czech Republic, Germany and Belgium. DDT or its breakdown products were found in all analysed eggs. The ratio of p,p′-DDT as a proportion of all DDT products increased during the years of our research. Since the Black Stork has so far not been mentioned as a species affected by DDT we do not have information about its critical contamination effect levels. However, we also present data suggesting that DDT contamination might have affected Black Stork historically. Current data suggest that the reduction of eggshell thickness has not (yet) reached the levels described as critical for other affected species. However, we found a significant reduction of egg volumes, which in turn negatively affects hatching, as eggs below a threshold size fail to hatch. The ratio of small eggs versus normal eggs has increased by almost 10 % in recent years and is strikingly higher as compared to the pre-DDT period. There is also a significant correlation between eggs of reduced size and their respective DDE (1,1-bis(4-chlorophenyl)-2,2-dichloroethene) values.