Kjell Larsson

Summary: Increasing goose populations in the northern hemisphere during the past 50 years have generated human-goose conflicts regarding agricultural damages and various ecosystem effects. The population increase, and the marked southward breeding range expansion of Barnacle Geese Branta leucopsis into temperate regions in northwestern Europe, motivated the adoption of an AEWA International Single Species Management Plan for the species. To protect the population at internationally important key sites and, if necessary, grant derogations to prevent damage, the management plan emphasised the need for updated estimates of the size of different population segments, data on demographic parameters, and assessments of the role of predators in regulating populations. In this study, the increase, and subsequent 98% decrease, of the breeding population of Barnacle Geese on the islands of Gotland and Öland, Sweden, in the central Baltic Sea during the 55-year period from 1971 to 2025 is analysed. Previous analyses of demographic parameters performed during the increase phase are supplemented by analyses of reproductive success and survival during the decline phase. The first breeding pair in the Baltic Sea region was recorded on Gotland in 1971. Thereafter, the total number of breeding pairs on Gotland and Öland increased rapidly to c.5,290 breeding pairs in 2002. At that point these birds constituted the major part of the species Baltic population. Thereafter, the number declined rapidly to only c.100 breeding pairs in 2025. Reproductive parameters such as clutch size and production of fledged young declined or remained low during the decline phase, but the lower reproductive output could not alone explain the decline of breeding pairs. Instead, declining adult survival rates due to intensive predation by increasing numbers of White-tailed Eagles Haliaeetus albicilla , in combination with increased natal dispersal rates, were found to be the major cause of the steep decline.

The breeding grounds of the North Atlantic population of Sandwich Tern Thalasseus sandvicensis range along the coasts of western Europe and in the Baltic Sea: predominantly in the Netherlands, UK, France, Germany, and Denmark, with a smaller fraction of the population breeding in southern Sweden. This population has fluctuated strongly in size in recent decades due to human activities, pollution, changing availability of food, and outbreaks of highly pathogenic avian influenza. To evaluate whether management actions to further protect the species are needed in Sweden, better knowledge about the recent development of the breeding population is required. We found that the total number of breeding pairs in Sweden between 2007 and 2014 fluctuated between c. 390 and c. 570 pairs. Later, the number increased markedly from c. 630 pairs in 2015 to c. 1,830 pairs in 2021. In 2022 and 2023, the number decreased partly due to avian influenza outbreaks and c. 1,540 pairs bred in 2023. Most pairs bred in the provinces of Blekinge, Gotland, and Skåne. The four largest colonies hosted c. 55% of the cumulative number of breeding pairs between 2007 and 2023. This aggregated distribution makes the species vulnerable to various threats.

The transport of non-petroleum substances such as vegetable oils, other bio-based oils and their refined products by chemical tankers is increasing worldwide. The majority of the non-petroleum substances carried by chemical tankers will have detrimental effects on the marine environment if accidentally spilled or discharged during tank washing procedures. Swedish Coast Guard aircrafts detected 233 discharges of floating non-petroleum substances in the Swedish territorial sea and Swedish Exclusive Economic Zone (EEZ) between 2020 and 2023. The majority of the discharges, 84%, were detected in the EEZ. About 14% of the discharges were detected within protected marine Natura 2000 sites. Together, the detected discharges covered 1071 km² of sea surface. Discharges in marine Natura 2000 sites covered 228 km². We conclude that the regulations in the IMO MARPOL Annex II convention are not strict enough to meet the objectives of EU nature legislation regarding protection of sensitive seas areas.

Summary: In 2022, highly pathogenic avian influenza (HPAI) A(H5N1) virus clade 2.3.4.4b became enzootic and caused mass mortality in Sandwich TernThalasseus sandvicensisand other seabird species across north-western Europe. We present data on the characteristics of the spread of the virus between and within breeding colonies and the number of dead adult Sandwich Terns recorded at breeding sites throughout north-western Europe. Within two months of the first reported mortalities, 20,531 adult Sandwich Terns were found dead, which is >17% of the total north-western European breeding population. This is probably an under-representation of total mortality, as many carcasses are likely to have gone unnoticed and unreported. Within affected colonies, almost all chicks died. After the peak of the outbreak, in a colony established by late breeders, 25.7% of tested adults showed immunity to HPAI subtype H5. Removal of carcasses was associated with lower levels of mortality at affected colonies. More research on the sources and modes of transmission, incubation times, effective containment, and immunity is urgently needed to combat this major threat for colonial seabirds.

A bstract: In 2022, highly pathogenic avian influenza (HPAI) A(H5N1) virus clade 2.3.4.4b became enzootic and caused mass mortality in Sandwich terns and other seabird species across northwestern Europe. We present data on characteristics of the spread of the virus between breeding colonies and the number of dead adult Sandwich terns recorded at breeding sites throughout northwestern Europe. Within two months after the first mortalities were reported, in total 20,531 adult Sandwich terns were found dead, which is >17% of the total northwestern European breeding population. Losses are likely higher, as we expect that many victims were not found (mortality rate might be up to 74% of the breeding population). Inside the colonies almost all chicks died. After the peak of the outbreak, in a colony established by late breeders, 25.7% of adults showed immunity against HPAI subtype H5. Removal of carcasses helped in reducing the spread of the disease and consequently total mortality. More research on the sources and modes of transmission, incubation times, effective containment and immunity is urgently needed to combat this major threat for colonial seabirds.

The West Siberian/North European population of Long-tailed Duck Clangula hyemalis that breeds in Fennoscandia and Northwestern Russia, and winters in the Baltic Sea, has declined by at least 65% since the 1990s and is classified as globally vulnerable. To propose effective management actions to stop the decline, knowledge about demographic parameters is required. A photo survey method by which it is possible, in winter, to collect data on sex ratios and production of first-winter birds is presented here. The plumage traits and bill patterns, which are detectable in photos taken at a distance, are described. The traits can be used to discriminate between three categories of birds: adult males, first-winter males, and females. To extrapolate results from several photo surveys and obtain population-wide estimates of demographic parameters, knowledge about non-random distributions of different bird categories is needed. It was found that different age and sex categories were distributed differently across flocks of different sizes. The required sample sizes and the possible constraints and biases related to the photo survey method are discussed.

The postnatal growth period is a crucial life stage, with potential lifelong effects on an animal's fitness. How fast animals grow depends on their life‐history strategy and rearing environment, and interspecific comparisons generally show higher growth rates at higher latitudes. However, to elucidate the mechanisms behind this gradient in growth rate, intraspecific comparisons are needed. Recently, barnacle geese expanded their Arctic breeding range from the Russian Barents Sea coast southwards, and now also breed along the Baltic and North Sea coasts. Baltic breeders shortened their migration, while barnacle geese breeding along the North Sea stopped migrating entirely. We collected cross‐sectional data on gosling tarsus length, head length and body mass, and constructed population‐specific growth curves to compare growth rates among three populations (Barents Sea, Baltic Sea and North Sea) spanning 17° in latitude. Growth rate was faster at higher latitudes, and the gradient resembled the latitudinal gradient previously observed in an interspecific comparison of precocial species. Differences in day length among the three breeding regions could largely explain the observed differences in growth rate. In the Baltic, and especially in the Arctic population, growth rate was slower later in the season, most likely because of the stronger seasonal decline in food quality. Our results suggest that differences in postnatal growth rate between the Arctic and temperate populations are mainly a plastic response to local environmental conditions. This plasticity can increase the individuals' ability to cope with annual variation in local conditions, but can also increase the potential to re‐distribute and adapt to new breeding environments.

Cultural transmission of migratory traditions enables species to deal with their environment based on experiences from earlier generations. Also, it allows a more adequate and rapid response to rapidly changing environments. When individuals break with their migratory traditions, new population structures can emerge that may affect gene flow. Recently, the migratory traditions of the Barnacle Goose Branta leucopsis changed, and new populations differing in migratory distance emerged. Here, we investigate the population genetic structure of the Barnacle Goose to evaluate the consequences of altered migratory traditions. We used a set of 358 single nucleotide polymorphism (SNP) markers to genotype 418 individuals from breeding populations in Greenland, Spitsbergen, Russia, Sweden and the Netherlands, the latter two being newly emerged populations. We used discriminant analysis of principal components, F_ST, linkage disequilibrium and a comparison of geneflow models using migrate‐n to show that there is significant population structure, but that relatively many pairs of SNPs are in linkage disequilibrium, suggesting recent admixture between these populations. Despite the assumed traditions of migration within populations, we also show that genetic exchange occurs between all populations. The newly established nonmigratory population in the Netherlands is characterized by high emigration into other populations, which suggests more exploratory behaviour, possibly as a result of shortened parental care. These results suggest that migratory traditions in populations are subject to change in geese and that such changes have population genetic consequences. We argue that the emergence of nonmigration probably resulted from developmental plasticity.