Experience reduces route selection for conspecifics by the collectively migrating white stork

Migration can be an energetically costly behavior with strong fitness consequences in terms of mortality and reproduction.1,2,3,4,5,6,7,8,9,10,11 Migrants should select migratory routes to minimize their costs, but both costs and benefits may change with experience.12,13,14 This raises the question of whether experience changes how individuals select their migratory routes. Here, we investigate the effect of age on route selection criteria in a collectively migrating soaring bird, the white stork (Ciconia ciconia). We perform step-selection analysis on a longitudinal dataset tracking 158 white storks over up to 9 years to quantify how they select their routes based on the social and atmospheric environments and to examine how this selection changes with age. We find clear ontogenetic shifts in route selection criteria. Juveniles choose routes that have good atmospheric conditions and high conspecific densities. Yet, as they gain experience, storks' selection on the availability of social information reduces-after their fifth migration, experienced birds also choose routes with low conspecific densities. Thus, our results suggest that as individuals age, they gradually replace information gleaned from other individuals with information gained from experience, allowing them to shift their migration timing and increasing the timescale at which they select their routes.

Winged resistance: Storks and gulls increase carriage of antibiotic resistance by shifting from paddy fields to landfills

Waterbirds are vectors for the dissemination of antimicrobial resistance across environments, with some species increasingly reliant on highly anthropized habitats for feeding. However, data on the impact of their feeding habits on the carriage of antibiotic resistance genes (ARGs) are still scarce. To fill this gap, we examined the microbiota (16S rRNA amplicon gene sequencing) and the prevalence of ARG (high-throughput qPCR of 47 genes) in faeces from white storks (Ciconia ciconia) and lesser black-backed gulls (Larus fuscus) feeding in highly (landfill) and less (paddy fields) polluted habitats. Faecal bacterial richness and diversity were higher in gulls feeding upon landfills and showed a greater abundance of potential pathogens, such as Staphylococcus. In contrast, faecal bacterial communities from storks were similar regardless of habitat preferences, maybe due to a less intense habitat use compared to gulls. In addition, birds feeding in the landfill carried a higher burden of ARGs compared to the surrounding soil and surface waters. Network analysis revealed strong correlations between ARGs and potential pathogens, particularly between tetM (resistance to tetracyclines), blaCMY (beta-lactam resistance), sul1 (sulfonamide resistance) and members of the genera Streptococcus, Peptostreptococcus, and Peptoclostridium. Our work demonstrates how transitioning from paddy fields to landfills fosters the carriage of ARGs and potential pathogens in the bird gut, shedding light on the ecological role of these avian vectors in antimicrobial resistance dissemination.

Biovectoring of plastic by white storks from a landfill to a complex of salt ponds and marshes

Research into plastic pollution has extensively focused on abiotic vectors, overlooking transport by animals. Opportunistic birds, such as white storks (Ciconia ciconia) often forage on landfills, where plastic abounds. We assess plastic loading by ingestion and regurgitation of landfill plastic in Cadiz Bay, a major stopover area for migratory white storks in south-west Spain. On average, we counted 599 storks per day moving between a landfill and a complex of salt ponds and marshes, where they regurgitated pellets that each contained a mean of 0.47 g of plastic debris, dominated by polyethylene. Modelling reliant on GPS tracking estimated that 99 kg and >2 million particles of plastic were biovectored into the wetland during 2022, with seasonal peaks that followed migration patterns. GPS data enabled the correction of field censuses and the identification of plastic deposition hotspots. This study highlights the important role that biovectoring plays in plastic transport into coastal wetlands.

Influence of Freezing Temperature, Freezing Duration, and Repeated Freeze/Thaw Cycles on Electrophoretic Profiles in the White Stork (Ciconia ciconia)

Plasma electrophoresis is an ancillary diagnostic tool in avian medicine, with agarose gel electrophoresis (AGE) and capillary zone electrophoresis (CZE) being the most common techniques. Frozen samples can be used for quantitative studies or comparative diagnostic purposes, but stability of avian plasma proteins under freezing is poorly described. To evaluate the influence of plasma freezing on electrophoretograms in white storks (Ciconia ciconia), heparin blood was sampled from 30 individuals during annual health examinations. Plasma samples were obtained after centrifugation of fresh samples and divided into aliquots. Both AGE and CZE were performed on fresh aliquots. The remaining aliquots were frozen at -20°C (-4°F) or -180°C (-292°F) and thawed following different protocols: 1 freeze/thaw cycle after 6 months at -20°C; 1, 2, 4, and 7 cycles over 12 months at -20°C; and 1 cycle after 18 months at -180°C. For both techniques, electrophoretic profiles obtained from these thawed aliquots were compared to fresh electrophoretograms. Quantitatively, significant differences (P < 0.05) in most fractions were seen from 6 months postfreezing at -20°C for both techniques. Fewer statistically significant differences were observed after 18 months under cryogenic preservation (-180°C). Qualitatively, AGE provided more repeatable and stable results than CZE over time on samples stored at -20°C, and electrophoretograms were stable after 18 months of cryogenic storage. An electromigration distortion associated with freezing was seen with CZE only. Plasma samples stored in a conventional freezer (-20°C) should not be compared to fresh plasma. For quantitative studies, cryogenic storage should be privileged.

Tissue distribution and correlations of heavy metals in wild birds from Southern Turkey: an ecologically important region on the west Palearctic migration route

Even if some are essential for biological functions, the accumulation of heavy metals above tolerable physiological limits is potentially toxic to also wild animals. The present study aimed to investigate concentrations of environmentally relevant heavy metals (As, Cd, Cu, Fe, Hg, Mn, Pb, and Zn) in feathers, muscle, heart, kidney, and liver tissues of wild birds (golden eagle [Aquila chrysaetos], sparrowhawk [Accipiter nisus], and white stork [Ciconia ciconia]) from Hatay province, southern Turkey. The metal concentrations of tissues were determined by a validated ICP-OES analysis method after microwave digestion. The concentration differences of metals in species/tissues and the correlations between essential/non-essential metals were determined by statistical analysis. According to the results, Fe (326.87±3.60 mg kg^-1) had the highest, and Hg (0.09±0.00 mg kg^-1) had the lowest mean concentration in all tissues. Compared to the literature; Cu, Hg, Pb, and Zn concentrations were lower; Cd, Fe, and Mn concentrations were higher. The correlations between As and all essentials; Cd and Cu, Fe; Hg and Cu, Fe, Zn; Pb and all essentials were significantly positive. In the conclusion, while essential Cu, Fe, and Zn are below the threshold value and do not pose a risk, Mn is close to the threshold value. Therefore, periodically monitoring the pollutant concentrations in bioindicators is a key necessity for the early determination of biomagnification trends and prevention of potential toxic stress on wildlife ecology.

The prevalence of anthropogenic nest materials differs between two distinct populations of migratory birds in Europe

A major impact of human development is the transformation of natural habitats into farming lands and the expansion of built-up areas. Also, plastic pollution is affecting wildlife on a global scale. Discarded plastic is ubiquitous and accessible for birds, which can incorporate them into the nest structure. Here, we describe the differences in type, prevalence, and the amount of anthropogenic nest materials between two populations of terrestrial, mainly farmland bird, the white stork Ciconia ciconia, on a broad geographical scale, from two migratory divides-eastern in Poland and western in Spain (in total 303 nests). In the two populations, we detected significant differences in the incorporation of anthropogenic nest material, as measured by the Human Footprint Index (HFI) and the Impervious Surface Areas (ISA). We found that ISA was positively related to anthropogenic nest material incorporation in the Spanish population, and HFI was positively related to anthropogenic nest material, in contrast to the Polish population, in which the relationships were not significant. Moreover, we showed that the prevalence of nests with anthropogenic nest material was two times higher in Spanish than in the Polish white stork population. This study demonstrates that the behavior of incorporation of the anthropogenic nest material differs between two distinct populations of a single bird species.

White stork movements reveal the ecological connectivity between landfills and different habitats

BACKGROUND

Connections between habitats are key to a full understanding of anthropic impacts on ecosystems. Freshwater habitats are especially biodiverse, yet depend on exchange with terrestrial habitats. White storks (Ciconia ciconia) are widespread opportunists that often forage in landfills and then visit wetlands, among other habitats. It is well known that white storks ingest contaminants at landfills (such as plastics and antibiotic resistant bacteria), which can be then deposited in other habitats through their faeces and regurgitated pellets.

METHODS

We characterized the role of white storks in habitat connectivity by analyzing GPS data from populations breeding in Germany and wintering from Spain to Morocco. We overlaid GPS tracks on a land-use surface to construct a spatially-explicit network in which nodes were sites, and links were direct flights. We then calculated centrality metrics, identified spatial modules, and quantified overall connections between habitat types. For regional networks in southern Spain and northern Morocco, we built Exponential Random Graph Models (ERGMs) to explain network topologies as a response to node habitat.

RESULTS

For Spain and Morocco combined, we built a directed spatial network with 114 nodes and 370 valued links. Landfills were the habitat type most connected to others, as measured by direct flights. The relevance of landfills was confirmed in both ERGMs, with significant positive effects of this habitat as a source of flights. In the ERGM for southern Spain, we found significant positive effects of rice fields and salines (solar saltworks) as sinks for flights. By contrast, in the ERGM for northern Morocco, we found a significant positive effect of marshes as a sink for flights.

CONCLUSIONS

These results illustrate how white storks connect landfills with terrestrial and aquatic habitats, some of which are managed for food production. We identified specific interconnected habitat patches across Spain and Morocco that could be used for further studies on biovectoring of pollutants, pathogens and other propagules.

Long-term management practices successfully reduce bird-related electrical faults in a transmission grid increasingly used by white storks for nesting

Bird nests on transmission lines can cause electrical faults which reduce service reliability. To address this problem, since the mid-90s, the Portuguese Transmission System Operator (TSO) has undertaken management actions to discourage white storks Ciconia ciconia from nesting in hazardous locations of the pylons. Here, we compiled and analyzed an 18-year series of data on electrical faults, TSO management actions to tackle these, and stork nests on transmission pylons in Portugal to: (a) determine the relative importance of bird-related faults over the total number of faults; (b) describe variations in bird-related faults across time (season of the year and time of the day); (c) describe spatial variations in bird-related faults and their association with the occurrence of white stork nests on pylons; and (d) analyze the trends, over the years, of the number of white stork nests on pylons, the TSO management actions and their effectiveness in reducing bird-related fault rates. Overall, birds accounted, on average, for 25.3% of the electrical faults in the transmission network, with the vast majority being attributed to white storks. The seasonal pattern of bird-related faults showed higher rates in April and in October-November. Faults occurred more often during the night period, when storks spend more time on the pylons. We found a positive spatial relationship between the electrical fault rate and the proportion of pylons with stork nests (and the correlated number of nests per 100 km of line). There was, however, considerable variation in the fault rates not explained by the stork nest variables, particularly during the non-breeding season. The TSO management actions (namely removal/translocation of nests in hazardous locations of the pylons, installation of anti-perching devices and provision of alternative nesting platforms) significantly reduced, as a whole, the annual number of bird-related faults between 2001 and 2018, despite the three-fold increase in the number of white stork nests on transmission pylons. A deeper understanding of how white storks use the transmission pylons outside the breeding season is needed, so that targeted management actions can be taken to reduce the remaining bird-related fault rates to residual levels.

Anthropogenic food subsidies reshape the migratory behaviour of a long-distance migrant

Bird migratory journeys are often long and hostile, requiring high energetic expenditure, and thus forcing birds to pause between migratory flights. Stopover sites allow migrants to replenish fuel reserves and rest, being crucial for the success of migration. Worldwide, the increasing accumulation of waste on landfills and rubbish dumps has been described to provide superabundant food resources for many bird species not only during the breeding and wintering seasons but also during migration, being used as stopover sites. Using GPS-tracking data of juvenile white storks (Ciconia ciconia) during their first migration from the Iberia Peninsula to the sub-Saharan wintering grounds, we uncover the effects of stopping en route on individual migratory performance. Particularly, we examine the benefits of stopping at artificial sites (landfills and rubbish dumps) when compared to natural stopover sites (wetlands, agricultural or desert areas) and explore the influence of anthropogenic food resources on storks' migratory strategies. Overall, white storks spent up to one-third of the migration in stopovers. We found that birds that stopped for longer periods made more detours, increasing migration duration by half a day for each stopover day. Stopping more often did not reflect on increasing in-flight energetic efficiency nor the likelihood of completing the migration. Juvenile storks used artificial sites in 80 % of the stopover days, spending 45 % less time and 10 % less energy foraging than when using natural stopovers. While stopping in landfills did not translate into differences in migratory performance, individuals in poor body condition possibly rely on these sites to improve body weight before proceeding, enabling them to successfully complete migration. Artificial stopover sites are attractive and likely increase the number and duration of stops for white storks. Even though the consequences of arriving late at the wintering grounds are unknown, it can lead to cascading consequences, influencing individual fitness and population dynamics.

Not so black, not so white: differences in microorganism load of contiguous feathers from white stork chicks

Many organisms are characterized by strikingly contrasting black and white coloration, but the function of such contrasts has been inadequately studied. In this article, we tested the function of black and white contrasting plumage in white stork Ciconia ciconia chicks. We found greater abundance and diversity of microorganisms on black compared with adjacent white feathers. In addition, nest size was positively correlated with the abundance and diversity of microorganisms on white feathers. Flight initiation distance (FID), defined as the distance at which adult white storks took flight when approached by a human, was negatively correlated with most measurements of microorganism abundance. Breeding success was generally positively correlated with the abundance and diversity of microorganisms on black feathers. The feather growth rate was positively correlated with some and negatively correlated with other measurements of microbial abundance and diversity. Finally, chick growth was negatively correlated with the number of microbial species on black feathers and positively with the abundance and diversity of microorganisms on white feathers. These findings are consistent not only with the role of microorganisms in the maintenance of a benign microbial environment which differs between black and white feathers, but also with the hypothesis that several taxa of microorganisms found in black and white plumage are virulent, with negative effects on the fitness of their hosts.

Abundance and spatial distribution of the structure supporting the nest of White Stork Ciconia ciconia in Guerbes-Sanhadja wetland eco-complex, northeastern of Algeria

In the present investigation, the spatial distribution of the nest of White Stork Ciconia ciconia was examined. Spearman's rank-order correlations test and the principal component analysis (PCA) were applied to a total of 227 nests recorded in the Guerbes-Sanhadja wetland eco-complex, northeastern of Algeria, over seven sites, for which the percentage of occupied nests reaches 89% (202 nest were occupied). Our goals are twofold: to explore the variation and distribution of the structure supporting the nest and to explain their spatial variability. The Spearman's rank-order correlation test show that steel electricity poles had non-significant correlations with tree, and only concrete electricity poles structure had statistically significant positive correlation with mobile phone antennas structure (R = 0.757; at p < .05), and the roofs of houses had statistically significant positive correlation with mobile phone antennas structure (R = 0.825; at p < .05). According to the PCA results, it was observed that the PC1, which explains 50.86% of the total inertia, further represents and synthesizes the dominant structure supporting the nest, i.e., tree, steel electricity poles, and concrete electricity poles, which were strongly correlated with PC1, having a component loading nearly equal to 0.766, 0.821, and - 0.929, respectively, while the PC2, which explains 30.30% of the total inertia, includes the structure rarely recorded in the studied region, i.e., wooden electricity poles and the roofs of houses.

A novel herpesvirus in a white stork associated with splenic and hepatic necrosis

We identified a novel herpesvirus in a captive juvenile white stork (Ciconia ciconia) that experienced progressive weight loss followed by death. Histologic findings included severe splenic and hepatic necrosis, and intranuclear inclusion bodies in hepatocytes and unidentified splenic cells. The virus was amplified, sequenced, and subsequently accessioned as Ciconiid alphaherpesvirus 1. Phylogenetic analysis was performed and revealed that this virus is more closely related to mammalian herpesviruses than those within the genus Mardivirus. Additional sequence of viruses in this area may elucidate the ancestral virus that jumped from reptilian to mammalian hosts.

Determination of nest occupation and breeding effect of the white stork by human-mediated landscape in Western Poland

Choosing an appropriate nest site is essential for successful breeding. Changes in land use cause populations of many species to decline although some species adapt to anthropogenic changes. The white stork Ciconia ciconia commonly uses artificial nest sites. Recently, white storks from Western Europe have been using landfills as feeding sites; the beginnings of this process are being observed in Central-Eastern Europe. The study aimed to determine factors influencing the probability of nest occupation and breeding effect in a Central-Eastern European population of white storks. We used long-term data from Western Poland on breeding effect, nest occupation, the structure supporting the nest, the proximity of the nearest landfills, landfill area, and land cover. The probability of nest occupation was significantly dependent on habitat quality (based on the share of the preferred type of land cover), the structure supporting the nest, and landfill proximity within a specific year. The breeding effect was influenced by habitat quality and nesting structure. We demonstrate that the type of nesting structure is an important factor influencing both the probability of nest reoccupation and breeding effect. However, the significance of landfills appears to be growing, and in recent years, storks prefer occupying nests closer to landfills, which may have significant consequences for the population of the white stork.

Daily energy expenditure in white storks is lower after fledging than in the nest

Many juvenile birds turn into long-distance migrants within weeks of fledging. This transition involves upheavals in their energy management as major changes in growth and activity occur. Understanding such ontogenetic transitions in energy allocation has been difficult because collecting continuous data on energy costs in wild developing birds was previously largely impossible. Here, we continuously measured heart rate and fine-scale movements of 20 free-living juvenile white storks (Ciconia ciconia) using on-board bio-loggers to explore individual and environmental factors relating to daily mean heart rate. In addition, we explored which specific energy management strategy storks use during these crucial early life stages. We found that daily mean heart rate increased with overall movement activity, and increasing body temperature, but that it decreased with age. Further, we found that during the nestling period, when growth costs are high, activity costs are low, and post-fledging that activity costs are increased while maintenance costs are low, indicating a constraint on overall energy use in both phases. Our observations are consistent with the hypothesis that individuals invested more energy per unit time while still in the nest than after fledging despite the high costs of flight.

Corticosterone implants produce stress-hyporesponsive birds

In birds, the use of corticosterone (Cort) implants is a frequent tool aimed at simulating systemic elevations of this hormone and studying effects on biological traits (e.g. physiology, morphology, behavior). This manipulation may alter adrenocortical function, potentially changing both baseline (Cort_BAS) and stress-induced (Cort_STRESS) plasma Cort levels. However, implant effects on the latter trait are rarely measured, disregarding downstream consequences of potentially altered stress responses. Here, we analyzed the effects of Cort implants on both Cort_BAS and Cort_STRESS in nestling and adult European white storks, Ciconia ciconia In addition, we performed a review of 50 studies using Cort implants in birds during the last two decades to contextualize stork results, assess researchers' patterns of use and infer current study biases. High and low doses of Cort implants resulted in a decrease of both Cort_BAS (31-71% below controls) and Cort_STRESS (63-79% below controls) in storks. Our literature review revealed that Cort_BAS generally increases (72% of experiments) whereas Cort_STRESS decreases (78% of experiments) following implant treatment in birds. Our results challenge and expand the prevailing assumption that Cort implants increase circulating Cort_BAS levels because: (i) Cort_BAS levels show a quadratic association with implant dose across bird species, and decreased levels may occur at both high and low implant doses, and (ii) Cort implants also decrease Cort_STRESS levels, thus producing stress-hyporesponsive phenotypes. It is time to work towards a better understanding of the effects of Cort implants on adrenocortical function, before addressing downstream links to variation in other biological traits.

How weather conditions in non-breeding and breeding grounds affect the phenology and breeding abilities of white storks

BACKGROUND

It has often been suggested that conditions in wintering grounds affect the breeding abilities of migratory birds. This is known as the carry-over effect. Heretofore, many studies have reported the relationship between conditions in wintering grounds, dates of departure from and arrival at breeding grounds, and breeding success. However, very few studies have shown how these conditions affect the capacity of females for egg production.

AIMS

To describe how conditions in the remote non-breeding areas in Africa affect reproductory abilities of migratory birds breeding in Europe.

METHODS

We recorded 863 arrival dates for 191 nests (in 2005-16), clutch sizes of 412 clutches, and egg dimensions of 1725 eggs (in 2003-16) of white storks in western Poland. We used generalised climatic indicators such as the Sahel precipitation index (SPI) and Indian Ocean Dipole (IOD) to assess conditions in wintering grounds prior to breeding and precipitation and temperature in breeding grounds during the breeding season.

RESULTS

The white stork arrived significantly earlier in seasons preceded by a warmer and rainier March, while conditions in Africa did not affect arrival dates. Females laid more (and larger) eggs in seasons preceded by rainier winters in Africa. Breeding success was not affected by conditions in wintering grounds, but only by the weather during breeding season.

CONCLUSIONS

Conditions in wintering grounds may affect migratory birds, not only in terms of their arrival dates, but rather with respect to females' egg production capacities. Studies on stable isotopes may yield more detailed findings as to how this process takes place.

Plastic ingestion in aquatic-associated bird species in southern Portugal

Excessive use of plastics in daily life and the inappropriate disposal of plastic products are severely affecting wildlife species in both coastal and aquatic environments. Birds are top-predators, exposed to all threats affecting their environments, making them ideal sentinel organisms for monitoring ecosystems change. We set a baseline assessment of the prevalence of marine plastic litter affecting multi-species populations of aquatic birds in southern Portugal. By examining 160 stomach contents from 8 species of aquatic birds, we show that 22.5% were affected by plastic debris. Plastic was found in Ciconia ciconia, Larus fuscus and L. michahellis. Ciconia ciconia ingested the highest amount (number of items and total mass) of plastic debris. Polydimethylsiloxane (PDMS, silicones) was the most abundant polymer and was recorded only in C. ciconia. Plastic ingestion baseline data are of crucial importance to evaluate changes through time and among regions and to define management and conservation strategies.

Hymenolepididae (Cestoda, Cyclophyllidea) of ciconiiform and pelecaniform birds: the genera Oschmarinolepis Spassky and Spasskaja, 1954 and Paraoschmarinolepis gen. n

The monotypic genus Oschmarinolepis Spassky and Spasskaja, 1954 (Cestoda, Hymenolepididae) is reviewed. Its validity is confirmed with an updated diagnosis, and its type-species, Oschmarinolepis microcephala (Rudolphi, 1819), is redescribed. The new monotypic genus Paraoschmarinolepis gen n. is erected with Paraoschmarinolepis multiformis (Creplin, 1829) comb. n., as its type-species. The morphology of both type-species, O. microcephala and P. multiformis comb. n. is reviewed on the basis of published data and examination of collection material from Plegadis falcinellus from Ukraine and Ciconia ciconia from Slovakia. We show that some material previously assigned to O. microcephala, belong in fact P. multiformis comb. n.

Intestinal helminths of the white stork (Ciconia ciconia Linnaeus 1758) from an inter-route site in Turkey

In Turkey, a study was conducted during the years 2009-2015 to detect the gastrointestinal helminth species of 18 white storks (Ciconia ciconia Linnaeus, 1758 (Aves: Ciconiiformes) sampled from aquatic or swamp areas of Bursa Province, one of the inter-route sites where storks intensely stay. The results of postmortem examination revealed that 17 (94.44%) white storks harboured one or more helminth species. Eight species of helminths were detected at the following prevalence rates: Dictymetra discoidea (38.88%), Chaunocephalus ferox (37.50%), Schistocephalus solidus (27.77%), Stephanoprora (Monilifer) spinulosa (18.75%), Echinoparyphium sp. (12.50%), Tylodelphys excavata (6.25%), T. clavata (6.25%), and Syncuaria ciconiae (6.25%). This study is the first report on the presence of all the above species except T. excavata from white storks in Turkey. Stephanoprora (M.) spinulosa was found in Ciconiiform birds for the first time in this study.

Is population structure in the European white stork determined by flyway permeability rather than translocation history?

European white stork are long considered to diverge to eastern and western migration pools as a result of independent overwintering flyways. In relatively recent times, the western and northern distribution has been subject to dramatic population declines and country-specific extirpations. A number of independent reintroduction programs were started in the mid 1950s to bring storks back to historical ranges. Founder individuals were sourced opportunistically from the Eastern and Western European distributions and Algeria, leading to significant artificial mixing between eastern and western flyways. Here we use mitochondrial and microsatellite DNA to test the contention that prior to translocation, eastern and western flyways were genetically distinct. The data show a surprising lack of structure at any spatial or temporal scale suggesting that even though birds were moved between flyways, there is evidence of natural mixing prior to the onset of translocation activities. Overall a high retention of genetic diversity, high N_ef, and an apparent absence of recent genetic bottleneck associated with early 20th century declines suggest that the species is well equipped to respond to future environmental pressures.

Characterization of plasma cholinesterase from the White stork (Ciconia ciconia) and its in vitro inhibition by anticholinesterase pesticides

Blood plasma cholinesterase (ChE) activity is a sensitive biomarker of exposure to organophosphorus (OP) and carbamate (CB) insecticides in vertebrates. Several studies indicate that more than one ChE form may be present in blood of birds. In this study the predominant ChE activity (acetylcholinesterase - AChE- or butyrylcholinesterase - BChE-), the range of ChE activity as well as ChE age-dependent changes in non-exposed individuals of White stork (Ciconia ciconia) have been established. The in vitro sensitivity of ChE to OP and CB insecticides such as paraoxon-methyl, carbofuran and carbaryl was also investigated. Plasma ChE was characterised using three substrates (acetylthiocholine iodide, propionylthiocholine iodide, and S-butyrylthiocholine iodide) and three ChE inhibitors (eserine sulphate, BW284C51 and iso-OMPA). The results indicated that propionylthiocholine was the preferred substrate by plasma cholinesterase followed by acetylcholine and butyrylcholine and the predominant enzymatic activity in plasma of White storks was BChE. Normal plasma BChE activity in White stork was 0.32±0.01μmol/min/ml for adults and 0.28±0.03μmol/min/ml for juveniles. So, the age had no significant effect on the range of BChE activity. The study on the in vitro inhibitory potential of tested anticholinesterase pesticides on plasma ChE activity revealed that paraoxon-methyl is the most potent inhibitor followed by carbofuran and finally by carbaryl. The percentage of in vitro plasma ChE inhibition was observed to be similar between adults and juveniles.