BackgroundGenes of the Major Histocompatibility Complex (MHC) are essential for adaptive immune response in vertebrates, as they encode receptors that recognize peptides derived from the processing of intracellular (MHC class I) and extracellular (MHC class II) pathogens. High MHC diversity in natural populations is primarily generated and maintained by pathogen-mediated diversifying and balancing selection. It is, however, debated whether selection at the MHC can counterbalance the effects of drift in bottlenecked populations. The aim of this study was to assess allelic diversity of MHC genes in a recently bottlenecked bird of prey, the White-tailed Eagle Haliaeetus albicilla, as well as to compare mechanisms that shaped the evolution of MHC class I and class II in this species.ResultsWe showed that significant levels of MHC diversity were retained in the core Central European (Polish) population of White-tailed Eagles. Ten MHC class I and 17 MHC class II alleles were recovered in total and individual birds showed high average MHC diversity (3.80 and 6.48 MHC class I and class II alleles per individual, respectively). Distribution of alleles within individuals provided evidence for the presence of at least three class I and five class II loci the White-tailed Eagle, which suggests recent duplication events. MHC class II showed greater sequence polymorphism than MHC class I and there was much stronger signature of diversifying selection acting on MHC class II than class I. Phylogenetic analysis provided evidence for trans-species similarity of class II, but not class I, sequences, which is likely consistent with stronger balancing selection at MHC class II.ConclusionsRelatively high MHC diversity retained in the White-tailed Eagles from northern Poland reinforces high conservation value of local eagle populations. At the same time, our study is the first to demonstrate contrasting patterns of allelic diversity and selection at MHC class I and class II in an accipitrid species, supporting the hypothesis that different mechanisms can shape evolutionary trajectories of MHC class I and class II genes.Electronic supplementary materialThe online version of this article (10.1186/s12862-018-1338-3) contains supplementary material, which is available to authorized users.

Background Long-term ringing and telemetry studies show that the Osprey (Pandion haliaetus) is a broad-front migrant following different migratory flyways, depending on the geographical location of their breeding populations. We have investigated two distinct and declining populations of Osprey in Poland, separated by only a few hundred kilometres, and hypothesised they may exhibit two different migration routes. We followed mortality causes, comparing them between migration and stationary phases of annual cycle, as well as between two distinct populations. Methods Nineteen Ospreys, both juveniles and adults, were equipped with GPS loggers in 2017–2020 in two populations in western and eastern Poland and followed on their autumn migration. We calculated the distance they covered on the migration, number of stopover days, migration duration, daily distances covered and departure dates to compare them between age and sex classes and between the eastern and western populations. Results Ospreys from the western and eastern populations showed a partial migratory divide. While the first migrated through a western flyway, the second followed a central flyway, resulting in crossing the Mediterranean Sea in distant passes that affected the distance covered. Annual mortality reached at least 67% in juveniles and at least 57% in adults. Conclusions We showed that two distinct Osprey populations in Poland revealed a partial migratory divide, with one covering greater distances over sea and deserts over the central flyway. This might affect individual survival rates and contribute to a steeper decline in one of the populations. In order for this to be confirmed, more individuals still have to be followed.

The Major Histocompatibility Complex (MHC) genes code for key immune receptors responsible for recognition of intra- and extra-cellular pathogens (MHC class I and class II, respectively). It was hypothesized that MHC polymorphism can be maintained via fluctuating selection resulting from between-habitat variation in pathogen regimes. We examined associations between MHC class I and class II genes and habitat structure in an apex avian predator, the white-tailed eagle Haliaeetus albicilla. We genotyped MHC class I and class II genes in ca. 150 white-tailed eagle chicks from nearly one hundred nesting territories distributed across three distinct populations in Poland. Habitat structure was quantified at the levels of foraging territories and directly at the nest sites. We found strong support for associations of habitat traits with diversity and allelic composition at the MHC class II. Forest area within territory and forest productivity were identified as the major habitat predictors of MHC class II polymorphism, while other habitat traits (distance to nearest open water, grassland and water area within territory or understory presence) showed fewer associations with class II alleles. In contrast, there was little support for associations between MHC class I genes and habitat structure. All significant associations were apparent at the within- rather than between-population level. Our results suggest that extra-cellular (rather than intra-cellular) pathogens may exert much stronger selective pressure on the white-tailed eagle. Associations of habitat structure with MHC class II may reflect fluctuating (balancing) selection which maintains MHC diversity within populations.

Poland is the only European country where the Osprey population is declining due to the mortality of adult birds from poaching, which impacts not only single breeding attempts but also the Lifetime Reproductive Success (LRS) of specimens. However, what if there came an extra mortality factor in the moment of the lowest numbers of Osprey, already vulnerable in the country? In the years 2018–2020, we installed 22 trail cameras and five digital cameras (live online video feeds) on the nests. The total failure level observed in cameras (18.5%) was high. We observed, using these cameras, the extra mortality of chicks (10.7% of potentially fledged chicks) and even adult birds by unexpected predation by Northern Goshawk and White-tailed Eagle. This phenomenon is also common in the national population, as we found a total of ten cases of total losses by predators (eight or nine of them were birds of prey), including nests not covered by camera monitoring. The extra adult-predation by Goshawks means an extra drop in LRS. Those adult and chick predations are an example of exceptional catastrophic phenomena, which have been described as the direct cause of the extinction of animal populations throughout history. Only active conservation and stop poaching of the Polish population could stop the decline and save the Polish Ospreys.