Top-down control of a marine mesopredator: Increase in native white-tailed eagles accelerates the extinction of an endangered seabird population

1. Bottom-up control is an important regulator of marine mesopredators such as seabirds. The prevalence of top-down control on these species is however less well understood. In particular, how native predators affect seabird populations has rarely been quantified. 2. Here, we investigate how an increase in white-tailed eagles in northern Norway, a stronghold for the species, affected a local population of 25,000 pairs of black-legged kittiwakes, a red-listed seabird, during a 42-year period ending with colony extinction. We use a natural experiment of two neighbouring colonies with/without eagle predation to disentangle the effects of eagles from local kittiwake foraging conditions (using size of young herring as a proxy). 3. At the colony where eagle predation occurred, and in contrast to the eagle-free colony, kittiwake breeding success and population size declined with increased eagle abundance , the latter more strongly under poor foraging conditions. Breeding success increased with foraging conditions at both colonies. 4. Simple population modelling shows that although conditions were insufficient to sustain the eagle-exposed colony, the increased abundance of eagles sped up its extirpation by many years. 5. Policy implications. Our study shows that top-down effects from avian predators can be significant regulators of seabird populations, challenging their conservation where native, often protected, predators are rising. Such effects, and their possible interaction with other factors, must also be accounted for when using seabird demographic traits as environmental indicators and when developing more flexible and effective management and action plans.Top-down control of a marine mesopredator: Increase in native white-tailed eagles accelerates the extinction of an endangered seabird populationpublishedVersio

Snowy Owl (Bubo scandiacus) males select the highest vantage points around nests.

In July 2007 we collected a total of 309 Snowy Owl (Bubo scandiacus) pellets from 35 vantage points in an area where we located four Snowy Owl territories in northern Norway. The number of pellets found at each vantage point ranged from 1 to 39 (mean: 8.8, median: 6, SD + 1.8). While the four nests were situated 513, 522, 524 and 529 m asl respectively (mean 522 m, SD + 4.1), the vantage points with pellets were situated from 515 to 590 m asl (mean: 550.2 m, median: 548 m, SD + 16.3). We found the highest number of pellets at the highest vantage points. The increase of pellets with higher elevation above the nest site indicates that Snowy Owls spent more time on the highest vantage points in the territory. Although some pellets may have been cast by the females before they started incubating, the majority of pellets were most likely cast by males. We hypothesise that the presence of elevated mounds, rocks or heights around the nest site of Snowy Owls may be an important feature for the Snowy Owls when selecting the breeding territory, increasing the male´s ability to scrutinize his territory for both prey and possible threats to the nest and himself.publishedVersio

Home range and excursive post-breeding movements of Eurasian Eagle-owls revealed by GPS satellite transmitters

Investigating space use of wild birds provides important knowledge of bird behavior and ecology, which is crucial in the management and conservation of threatened species. In the present study, we used GPS satellite telemetry to investigate space use and movements of seven adult Eurasian Eagle-Owls (Bubo bubo) in Norway during breeding and post-breeding seasons. Breeding adults had a mean home range size of 42.9 km2 (SD ± 35.1 km2; 95% kernel density estimation Had hoc), and five individuals performed long (>20 km) excursive movements away from their breeding territories during autumn. Such wide-ranging behavior has not previously been reported for adults of the nominate subspecies B. b. bubo. The study demonstrates the need to investigate seasonal movements of species, populations, and age groups even if they are considered residents, and provides valuable information for management and conservation of the Eurasian Eagle-Owl.publishedVersionPaid open acces

Winter distribution of Steller's Eiders in the Varangerfjord, northern Norway

Arctic warming and decreasing sea-ice cover along the Siberian coast in the Arctic Ocean leads to greater accessibility for operations such as oil drilling and traffic of tankers. This implies increasing risks for Steller’s Eider Polysticta stelleri wintering, moulting and staging along the coasts of the Varanger Peninsula, Norway. Steller’s Eiders were surveyed by monthly counts during winter and early spring 2016/2017 to investigate numbers and distribution throughout the winter. The highest number of wintering Steller’s Eiders was found in January, representing ~7 % of the European population. In February–April numbers were lower, but at a rather stable level. We found relatively little variation in distribution between months, although the birds were more evenly distributed along the coast later in winter. Mean flock size was significantly larger in January than in March and April. Feeding behaviour was exclusively observed in shallow water, generally at depths of up to 6 m. Areas of focus for an oil pollution emergency plan are pointed out and discussed

Movements of Tundra Bean Goose Anser fabalis rossicus neck-banded in northern Scandinavia

Based on 1420 neck-band reports we could map the movements of Tundra Bean Geese marked in northern Scandinavia. With few exceptions, the maps show a distinct migration pattern through Sweden to the main wintering area in NE Scania. In mild winters, only a few individuals extend their migration into continental Europe, but during severe winters, a large proportion of the sightings are from Germany and Denmark.Based on 1420 neck-band reports we could map the movements of Tundra Bean Geese marked in northern Scandinavia. With few exceptions, the maps show a distinct migration pattern through Sweden to the main wintering area in NE Scania. In mild winters, only a few individuals extend their migration into continental Europe, but during severe winters, a large proportion of the sightings are from Germany and Denmark

Snowy Owl nest failures caused by blackfly attacks on incubating females

Blackflies (Simuliidae) have been shown to influence birds negatively in several ways. In 2011, during a peak lemming year, we found that two of 28 nests of Snowy Owls Bubo scandiacus in Northern Norway and Finland failed due to blackfly attacks on the incubating female. The observations also suggest that adult survival may be influenced by these insects. The owls’ choice of breeding habitat can be decisive in their vulnerability to blackflies.Blackflies (Simuliidae) have been shown to influence birds negatively in several ways. In 2011, during a peak lemming year, we found that two of 28 nests of Snowy Owls Bubo scandiacus in Northern Norway and Finland failed due to blackfly attacks on the incubating female. The observations also suggest that adult survival may be influenced by these insects. The owls' choice of breeding habitat can be decisive in their vulnerability to blackflies

Life-cycle analysis of an endangered migratory goose to assess the impact ofconservation actions on population recovery

Evaluating the effectiveness of conservation actions is challenging for migratory species because a population can be impacted anywhere along its route. Conservation actions for the critically endangered Fennoscandian lesser white-fronted goose population include culling of red foxes in the breeding area and habitat improvements and reduction of illegal hunting in the non-breeding areas. One goal of the predator control strategy is to prevent adult birds from using an autumn migration route through western Asia, where mortality is believed to be higher than on the migration route through eastern Europe. We used 23 years of count data obtained at different staging areas to parameterize a seasonal state-space model describing the full-annual cycle dynamics of this population and evaluate whether the recent population recovery was linked to these conservation efforts. The results did not provide evidence that predator control influenced population recovery, as survival on the European route did not appear higher than on the allegedly riskier Asian route. However, adult survival at staging areas on both routes and at wintering sites may have improved in the last decade, suggesting a positive effect of the other conser- vation initiatives. These results emphasize the importance of including the non-breeding dynamics in population assessments of migratory species and highlight the challenge of evaluating the efficacy of separate conservation actions when a proper experimental design is unfeasible. Our study, which is a unique case of cross-national, coordinated conservation efforts, exemplifies how to model complex population dynamics to assess the influ- ence of costly conservation initiatives. Goose management State-space model Management evaluation Lesser whitefronted goose Unmarked individuals Non-breeding dynamic

Effects of satellite transmitters on survival in Snowy Owls Bubo scandiacus

The use of tracking devices to monitor birds is extensive, but the effects of such instruments on equipped individuals are still insufficiently taken into account. Here we evaluate potential effects of backpack-mounted satellite transmitters (platform terminal transmitters; PTTs) on survival of 28 Snowy Owls Bubo scandiacus. Six confirmed deaths were all probably related to natural and human-induced causes. Although PTT operational time was significantly shorter than expected lifetime of Snowy Owls, five owls were observed alive after transmissions ceased. Additionally four PTTs stopped due to low battery levels, indicating end of transmitter life and not owl mortality. We found no evidence of mortality caused by PTTs, but sample sizes are relatively low and detrimental effects on equipped Snowy Owls cannot be excluded. We recommend caution when instrumenting large owls.publishedVersionNivå

Density‑dependent winter survival of immatures in an irruptive raptor with pulsed breeding

Highly mobile predators can show strong numerical responses to pulsed resources, sometimes resulting in irruptions where large numbers of young invade landscapes at a continental scale. High production of young in irruption years may have a strong influence on the population dynamics unless immature survival is reduced compared to non-irruption years. This could occur if subordinate individuals (mainly immatures) are forced into suboptimal habitats due to density-dependent effects in irruption years. To test whether irruptive individuals had lower survival than non-irruptive ones, we combined necropsy results (N = 365) with telemetry (N = 185) from more than 20 years to record timing and causes of mortality in snowy owls (Bubo scandiacus), which irrupt into eastern North America during winter following high breeding output caused by lemming peaks in the Arctic. Mortality was more than four times higher in irruption years than non-irruption years, but only for immatures, and occurred disproportionately in early winter for immatures, but not adults. Mortality was also higher in eastern North America, where owl abundance fluctuates considerably between years, compared to core winter regions of the Arctic and Prairies where populations are more stable. Most mortality was not due to starvation, but rather associated with human activity, especially vehicle collisions. We conclude that immature snowy owls that irrupt into eastern North America are limited by density-dependent factors, such as increased competition forcing individuals to occupy risky human-altered habitats. For highly mobile, irruptive animals, resource pulses may have a limited impact on population dynamics due to low subsequent survival of breeding output during the nonbreeding season.publishedVersio