Severe Population Decline of great skua Stercorarius skua during Migratory Passage

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Data may be preliminary. 9 October 2025 V1 Latest version Share on Severe Population Decline of great skua Stercorarius skua during Migratory Passage Authors : Rafael Benjumea 0000-0002-0569-8765 [email protected] , ANTONIO SANDOVAL , and MARÍA DEL MAR DELGADO Authors Info & Affiliations https://doi.org/10.22541/au.175998254.48654038/v1 341 views 170 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract The H5N1 avian influenza (HPAI) outbreak that began in 2021 has led to widespread mortality in birds and mammals globally, significantly impacting seabird populations. This study examines the effects of H5N1 on great skuas ( Stercorarius skua ), a species particularly vulnerable due to its kleptoparasitic behaviour and reliance on communal roosting sites. By using an extensive (45329 observations) long-term (59-year) migration data from Cape Estaca de Bares (Galicia, NW Spain), we assessed whether the population declines observed in breeding colonies are mirrored in the migratory population. Migration counts revealed substantial interannual variation, with the sharpest decline in 2022 and 2023, coinciding with the HPAI outbreak. Migration rates decreased by 70% compared to pre-2021 levels, and this decline reflects similar trends seen in breeding colonies. Our findings highlight the critical role of long-term monitoring of migration studies in understanding the broader ecological implications of HPAI and similar events. Further investigations are necessary to determine how predation pressures and the virus’s effects on seabird colonies are influencing the species’ broader ecological role. Severe Population Decline of great skua Stercorarius skua during Migratory Passage Rafael Benjumea 1,2 *, ANTONIO SANDOVAL 3 AND MARÍA DEL MAR DELGADO 2 1 Biogeography, Diversity and Conservation (Department of Animal Biology), University of Málaga, Campus de Teatinos s/n, 29071 Málaga, Spain. 2 Biodiversity Research Institute (IMIB), CSIC/UO/PA, Campus de Mieres, Edificio de Investigación, 33600 Mieres (Asturias), Spain 3 Plaza de San Andrés, nº2, 4ºC. 15003 A Coruña, Spain. * Corresponding author : Rafael Benjumea Biogeography, Diversity and Conservation (Department of Animal Biology), University of Málaga, Campus de Teatinos s/n, 29071 Málaga, Spain. Phone number 0034 625 498 609 [email protected] Abbreviated page-header title : Strong declining in great skua passage. Type of article: Original Research. Rafael Benjumea ORCID ID: https://orcid.org/0000-0002-0569-8765 María del Mar Delgado Sánchez ORCID ID: https://orcid.org/0000-0002-3009-738X SUMMARY The H5N1 avian influenza (HPAI) outbreak that began in 2021 has led to widespread mortality in birds and mammals globally, significantly impacting seabird populations. This study examines the effects of H5N1 on great skuas ( Stercorarius skua ), a species particularly vulnerable due to its kleptoparasitic behaviour and reliance on communal roosting sites. By using an extensive (45329 observations) long-term (59-year) migration data from Cape Estaca de Bares (Galicia, NW Spain), we assessed whether the population declines observed in breeding colonies are mirrored in the migratory population. Migration counts revealed substantial interannual variation, with the sharpest decline in 2022 and 2023, coinciding with the HPAI outbreak. Migration rates decreased by 70% compared to pre-2021 levels, and this decline reflects similar trends seen in breeding colonies. Our findings highlight the critical role of long-term monitoring of migration studies in understanding the broader ecological implications of HPAI and similar events. Further investigations are necessary to determine how predation pressures and the virus’s effects on seabird colonies are influencing the species’ broader ecological role. Key-words : seabird migration; great skua; population declining; Avian Influenza Virus; H5N1; marine birds INTRODUCTION Since 2021, highly pathogenic avian influenza (HPAI) H5N1 is causing extensive outbreaks across the globe, producing catastrophic mortality in birds and mammals on a near-global scale (Adlhoch et al., 2021; Lambertucci et al., 2025). This includes widespread die-offs of birds, which have led to rapid population declines both regionally and globally (Banyard et al., 2022; Falchieri et al., 2022; Gorta et al., 2024; Knief et al., 2024; Lambertucci et al., 2025). The HPAI outbreak in the 2021–2022 epidemiological year was the most extensive ever recorded in Europe, with 6615 virus detections across 37 countries (European Food Safety et al., 2023). This epidemic continued into the summer of 2022, resulting in a widespread spread of the virus among colony-breeding seabirds (European Food Safety et al., 2023). In early 2023, the epidemiological situation of avian influenza had changed significantly. The number of HPAI cases detected in wild birds during the January 2023 peak was comparable to January 2022, but with the difference that in 2022, most of the cases involved waterbirds (Freath et al., 2022; European Food Safety et al., 2023), being Anseriformes the predominant group (Lean et al., 2023), whereas in 2023, nearly all the affected birds were colony-breeding seabirds (Knief et al., 2024; C. J. Tremlett et al., 2024). For example, high mortality events of northern gannet ( Morus bassanus ), great skua ( Stercorarius skua ), and several species of gulls and terns, have been observed (Falchieri et al., 2022; Freath et al., 2022). In seabirds, H5N1 transmission primarily occurs within colonies through direct or indirect local spread, with transmission at sea previously thought to be unlikely (Boulinier et al., 2016). The virus spreads more easily in places where birds socialize, such as communal baths and roosting sites, which are used daily (Camphuysen et al., 2022). However, kleptoparasitism (i.e. the stealing of food) may also play a role, as the immediate ingestion of regurgitated food from an infected bird could transfer significant amounts of viral particles (Gorta et al., 2024). Great skua breeding populations in the United Kingdom, which make up around 65% of the global population (Furness and Monaghan, 1987; Mitchell et al., 2004) have experienced a 73% decline due to H5N1-related deaths in 2021/2022 (Tremlett et al., 2024), with the very likely possibility that more birds died but were not found (Banyard et al., 2022; Falchieri et al., 2022). Supporting this, a large and significant increase in the number of recoveries of dead ringed great skuas have been reported in 2022 in the British and Irish ringing Scheme (Pearce-Higgins et al., 2023). Another case occurred at Foula island (Shetland, Scotland 60°7’60” N, 2°4’60” W), where the mortality rate of great skuas in 2022 was clearly unprecedented, with a reduction to a further 65% in breeding numbers (Camphuysen and Gear, 2022). The great skua is a kleptoparasite, as well as a predator and a scavenger, seabird species (Furness and Monaghan, 1987; Phillips et al., 1999; Wernham et al., 2002), that uses communal bath in a daily basis (Falchieri et al., 2022). It nests on islands, frequently in proximity to other seabird colonies. Its breeding colonies range spans Iceland, the Faeroe Islands, northern Scotland, Jan Mayen, Svalbard, Bear Island (Norway), and northwest Russia (Magnusdottir et al., 2012; Furness, 2015). During winter, it primarily migrates to waters off the Iberian Peninsula and northwest Africa, extending as far south as Senegal (Crane, 2005; Magnusdottir et al., 2012; Wernham et al., 2002). Although many great skuas feed on fishing discards (Phillips et al., 1997; Votier et al., 2024), it is possible that the proportion that feed as scavengers, or exclusively by preying on other birds, may have rapidly transmitted the virus in communal baths or roosting sites and subsequently, spread it throughout the colony (Falchieri et al., 2022). Great skuas may also have contracted the virus by scavenging dead, infected birds as northern gannets at sea, potentially explaining how isolated colonies across their range (from Scotland to Svalbard) became infected. Notably, the unusually high northern gannet mortality in Foula island in April 2021, could have triggered the great skua outbreak that same year (Camphuysen and Gear, 2022). Thus, skuas were significantly affected in the initial stages of this panzootic in the northern hemisphere (Camphuysen et al., 2022; Tremlett et al., 2024) and were among the first birds to be found infected with H5N1 in the Antarctic and sub-Antarctic regions (e.g., Banyard et al., 2024; Bennett-Laso et al., 2024). Up to date, most of the censuses and studies of the impact of H5N1 on great skuas have been done in the breeding colonies (Elmberg et al., 2020; Macgregor et al., 2024), but there is limited information on its effects on the migration of this species. Such a study would provide more relevant insights into how the virus may have impacted the entire great skua population and allows for a more accurate assessment of the population decline they are witnessing. Counts at strategic coastal locations are effective, and cheaper than boat surveys (Magnusdottir et al., 2012), for quantifying the entire great skua population, as they include both breeding and non-breeding individuals, the latter often being underestimated in breeding colony censuses. In the present study, we analyse a long-term dataset on the migration of the great skua at Cape Estaca de Bares (Galicia, NW coast of Spain), where extensive counts have been conducted from a coastal sea-watching site. The aim of this study is to (1) provide knowledge about the phenology and climatic factors influencing migration of the great skua on Estaca de Bares, and (2) assess whether the decline observed in the already studied colonies extends to the entire migratory population passing through Estaca de Bares, thereby determining whether the virus is acting locally or, conversely, it represents a global phenomenon affecting different colonies. METHODS Study area Estaca de Bares (43.786°N, –7.685°W), located in the province of A Coruña (Galicia, NW Spain) is the northernmost cape in the Iberian Peninsula (Fig. 1), facing the Special Protection Area ES0000554, and it is one of the best observation points in Europe for watching seabird migration from land (Huyskens and Maes, 1971; Sandoval, 2015), particularly in autumn, when hundreds of thousands of shearwaters, skuas, gannets and other pelagic seabirds pass close to shore. The cape juts prominently in the Atlantic Ocean and faces an open sea with no islands in front, offering an unobstructed horizon. The sea-watch observatory comprises steep cliffs rising about 30 meters above sea level, offering shelter from almost all wind directions and, being oriented to the north, it receives light from behind most of the time. Estaca de Bares lies along the usual migration route of seabird species that breed in NW Europe and winter beyond southern Galicia. Following a path close to the coast, different seabird species, like the great skua, can be easily identified from the observation point. Data collection The dataset comprises 59 years of land-based observations conducted at Estaca de Bares, spanning from 1965 to 2023 (both included). The counts were made by experienced counters in seabird identification. The censuses were carried out from a total of 1495 days. Censuses were performed from sunrise to sunset. From 1965 to 2009, data were extracted from previous publications (Sandoval, 2015 and references therein). Great skuas are generally easy to recognize for observers who are acquainted with their characteristic morphology, behaviour, and plumage patterns (Flood et al., 2024). During migration, great skuas do not migrate in big flocks; instead, most individuals migrate alone or in groups of two to ten (Huyskens and Maes, 1971; Sandoval, 2015), thus facilitating their identification and accurate counting. For each observation, we added the day of the year and, to study the timing of migration, we computed from 1965 to 2023 the cumulative and the day of the year quantile passage dates (i.e. Q5 and Q95), representing the day of the year when 5% and 95% of the total number of birds observed had migrated (Benjumea et al., 2024; Vansteelant et al., 2020). Further, to study the variation in the number of migrating skuas during the season and over the years, we estimated the daily passage rate as the number of great skuas migrating per hour (Benjumea et al., 2024). By estimating the migration rate, we account for the fact that the number of hours observed every day and from year to year was not the same. Notably, the most consistent monitoring effort and highest data quality have been maintained since 2004. We thus restricted our analyses to study the variation in the number of migrating great skuas from 2004 to 2023 (both years included), between the 6 th of July and the 4 th of November (both days included). For doing so, we additionally created two time periods (hereafter, H5N1 outbreak): one covering the years 2004 to 2020 without the H5N1 outbreak, and another from 2021 onward, when the virus outbreak occurred. Weather data Daily meteorological data was obtained from the ERA5-Land reanalysis dataset (Hersbach et al., 2023), provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) via the Copernicus Climate Data Store. The data were accessed and processed using the Google Earth Engine platform (Muñoz-Sabater et al., 2021) . We extracted data for a single grid point corresponding to the location of Estaca de Bares, northwestern Spain (43.786°N, –7.685°W), from 1st July to 30th November for each year between 2004 and 2023. The variables included (1) mean air temperature at 2 m (°C), calculated as the mean of hourly values per day; (2) total precipitation (mm), computed as the daily sum of hourly values; (3) mean wind speed (m/s), computed from the zonal (u) and meridional (v) components of wind at 10 m as the Euclidean norm; and (4) mean wind direction (°), derived from the same u and v components using the arctangent of their ratio, converted to degrees relative to true north, and converted into a covariate with eight categories (N,NE, E, SE, S, SW, W and NW). Statistical analysis To assess (1) the influence of weather conditions on great skua migration rate and (2) the annual variation of great skua migration rate, we fitted generalized additive mixed models (GAMMs) because the residuals of great skua migration rates followed a Negative Binomial distribution. This distribution was selected instead of the Poisson because the data showed over-dispersion. Since migration rate (i.e., the number of migratory skuas per hour) corresponded to count data without zeros, we modelled migration rate – 1 as the response variable, which allowed us to treat it as a zero-truncated Negative Binomial regression. We considered the non-linear effects of day of the year (intra-annual variation) and year (inter-annual variation) using smoothing terms based on the default thin-plate regression splines implemented in the GAMM4 package in R (Wood and Scheipl, 2014; Zuur et al. , 2014). For each non-linear effect, we checked the effective degrees of freedom (EDF), and when EDF < 2, the variable was incorporated as a linear effect (Zuur et al. , 2014). Accordingly, year, and its interaction with the factor H5N1 outbreak, was included as a linear predictor (EDF < 2). In addition, we incorporated weather variables, and their interactions, as further predictors. All predictors were standardized to enable comparison of effect sizes. To address possible biases caused by unequal sampling effort across months and years, we included month ID and year ID as random intercepts. This approach helped control for additional month- or year-related factors that might otherwise confound the results. Moreover, because migration rate could be temporally autocorrelated both within and between years, we specified an AR-1 correlation structure. Maintaining the same structure for random effects and the AR-1 correlation, we generated all candidate competing models and compared them using the Akaike Information Criterion (AIC), selecting models with ΔAIC < 2 as the most parsimonious (Burnham and Anderson, 2002). Following standard procedures, we calculated Akaike weights ( wi ) for each model to quantify the relative evidence—i.e., the probability that model i was the best approximation among the candidate set—and computed evidence ratios to compare the strongest models. Model validation was performed by examining residual patterns to ensure no influential observations or temporal dependency issues remained. All analyses were conducted in R 2024.12.0+467 (R Development Core Team, 2024). RESULTS A total number of 45329 great skuas were observed during the daytime autumn migration through Estaca de Bares between 1965 and 2023. The highest annual count of great skuas was 5540 individuals in 2010, while the single-day peak occurred on September 18, 2011, with 949 individuals recorded. As the estimated number of the total breeding population of great skua ranges between 30000 and 34999 (BirdLife International, 2018), the numbers of great skuas migrating trough Estaca de Bares represents an important portion of the total population. Migration timing was concentrated between early August and early December (Fig. 2). The earliest 5% of migrants (Q5) occurred on the 7 th of August (mean DOY (±SD) = 219 ± 2,03), the median migration (Q50) on the 30 th of September (mean DOY (±SD) = 273 ± 2,19), and the latest 5% (Q95) on the 4 th of December (mean DOY (±SD) = 339 ± 4.34), illustrating the main migration period and its variability through Estaca de Bares (Table 1). We found that this pronounced seasonal migration rate was positively associated with all winds directions, except NW. Specifically, migration rates increased significantly under low to moderate north-westerly wind speeds and with stronger southerly winds (Table 2). Further, migration rate was negatively related to temperature (Table 2). From 2004 to 2023, annual migration rates exhibited interannual variation throughout the study period (Fig. 3). The highest total migration rate was recorded in 2011 (473,13 skuas/hour), followed by 2017 (469,92 skuas/hour), whereas the lowest values were observed in 2006 (74,58 skuas/hour), 2022 (86,08 skuas/hour), and 2023 (79,11 skuas/hour) (Fig. 3A). Mean migration rates ranged from a maximum of 6,27 ± 7,35 skuas/hour (range: 0,25–29,25) in 2017 to a minimum of 1,05 ± 0,94 skuas/hour (range: 0,09–5,71) in 2022 (Fig. 3B). Notably, while no significant interannual variation was detected between 2004 and 2020 (Fig. 4A), a sharp decline in migration rate was observed during the period of the virus outbreak (Table 2, Fig. 4A), with 70% fewer great skuas counted compared to previous years (Fig. 4B). DISCUSSION The ongoing H5N1 panzootic is representing a significant threat to the great skua population. Due to its kleptoparasitic behaviour and reliance on communal roosting sites, this species is likely to be among the most severely affected seabirds. By shifting the perspective from traditional breeding-grounds analyses of the influence of the H5N1 virus on great skuas, our study provides valuable insights into the broader implications and complex dynamics of HPAI. Our results suggest that the impact of HPAI on great skuas has occurred across the entire species’ range north of Galicia, as the observed declines are of similar magnitude to those reported at breeding colonies (Camphuysen et al., 2022; Tremlett et al., 2024) and at other migration sites in the North Sea (Macgregor et al., 2024). Our study reinforces earlier hypotheses (Camphuysen et al., 2022; Gorta et al., 2024) that scavenging on carcasses of infected seabirds, such as northern gannets, has facilitated the spread of the virus, leading to severe impacts even on great skua colonies in more isolated and less exposed areas. It has been reported a great skua’s decline since 1977 attributed to the impacts of climate change (Oswald et al., 2008) and reduced discarding by the fishery (Rochet et al., 2014), leading to food shortages (Church et al., 2019; Votier et al., 2004). Since the 2010s, breeding rates have remained stable (Camphuysen et al., 2022; Dunn et al., 2023), possibly due to the great skua’s plasticity in shifting its food sources (Church et al., 2019). This is in line with the results of our study, where the number of great skuas censused has remained virtually constant over nearly two decades. However, following the emergence of the HPAI outbreak, migration activity declined sharply in Estaca de Bares. Although the virus’ impact on breeding colonies was first detected in 2021 (Banyard et al., 2022), the most dramatic results at Estaca de Bares were observed a year later. It is possible that, in a broader focus, the virus had a greater impact on all individuals in the population starting in 2022, as was also observed in breeding colonies like Foula Island (Camphuysen et al., 2022). Since the virus was first detected in 1959 in Scotland and 1996 in China, as an outcome of the intensive farming (Plaza and Lambertucci, 2024), the virus expanded to new avian species and regions, and has become widespread among wild mammals and domestic livestock, even occurring in humans (Atkinson and Baillie, 2025), causing the most severe panzootic ever recorded (Bennison et al., 2024; Lambertucci et al., 2025). In the present, reports on the World Animal Health Information System (WAHIS) suggest that millions of wild animals may have died because of H5N1, being these numbers likely to be substantially underestimated, given the considerable challenges of finding all infection cases in wild animals and in remote areas. The future impact of the virus on species remains uncertain, but it is likely to disproportionately affect those already vulnerable to extinction, species with long lifespans and low reproductive rates or whose populations have been heavily impacted. Long-term monitoring at headlands such as Estaca de Bares provides valuable insights into population trends of this and potentially other species. Further, our study suggests that the virus might also be affecting non-breeding individuals (floaters), who are sometimes not properly monitored in the breeding colonies, but are counted in migration surveys. These birds have underdeveloped immune systems that may predispose them to increased future mortality (Atkinson and Baillie, 2025). As floaters spend many nights in roosting sites (Camphuysen et al., 2022; Klomp and Furness, 1990) and disperse widely across marine regions (Weimerskirch et al., 2015; Harrison et al., 2022), these birds may play a crucial role in the virus’s spread. Moreover, dispersing individuals are crucial elements regulating population dynamics (Penteriani and Delgado, 2009). They constitute the “future” of the population, as the persistence of the reproductive segment strongly relies on the dynamics of them. If this demographic fraction, which is generally more vulnerable, is at risk (or even at greater risk) of infection and mortality, the consequences for population dynamics may be severe. Critically, such effects often remain undetectable until years later, when mitigation is no longer possible. If both adults and juveniles are affected by infection, the likelihood of recruitment decreases, thereby jeopardizing population persistence. Mass mortality events extend their impact beyond individual species, affecting broader ecological processes (Lambertucci et al., 2025). In addition, loss of top predators and scavengers can disrupt ecosystem function through trophic cascades, including mesopredator release (Baum and Worm, 2009). Thus, the decline of great skua populations may affect other species, which could experience a double impact from the pandemic: (1) directly, through infection and mortality; and (2) indirectly, if other species that regulate their populations become infected. Further studies and monitoring programmes are needed to assess the potential recovery of the great skua from the virus, as it is unlikely that this predator plays a beneficial role in maintaining healthy populations by selectively removing sick or weakened individuals from other species (Votier et al., 2004). Additionally, it remains challenging to determine whether the predation pressure exerted by great skuas on seabird colonies has a negative impact on certain species (Bearhop et al., 2001). Nevertheless, it will be also important to investigate how seabird species within breeding colonies respond within the context of the ongoing population decline of the great skua. ACKNOWLEDGEMENTS We are grateful to Ricardo Hevia, Antonio Martínez Pernas, Alfonso Valderas, Pablo Lado, Daniel López Velasco, Saúl Román, Ana Rivas, David Santamaría, Nicolás Magdalena, Lois Santos, Pablo Pita, Pablo Gutiérrez, Antonio Gutiérrez, and many others for their collaboration in the long-term surveys at Estaca de Bares. Data from 2021–2022 were collected within the ARTABRO2 and ARTABRO3 projects, Centro de Extensión Universitaria y Divulgación Ambiental de Galicia (CEIDA), with the support of Fundación Biodiversidad–Pleamar Program and FEMP funds, coordinated by Carlos Vales and Sergio París. Data from 2023 were obtained through the project “Monitoring the passage of seabirds from Estaca de Bares” (Secretaría de Estado de Medio Ambiente). FUNDING STATEMENT Centro de Extensión Universitaria y Divulgación Ambiental de Galicia (CEIDA), with the support of Fundación Biodiversidad–Pleamar Program and FEMP funds. 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Declines in UK breeding populations of seabird species of conservation concern following the outbreak of high pathogenicity avian influenza (HPAI) in 2021–2022. Bird Study 71, 293–310. https://doi.org/10.1080/00063657.2024.2438641Vansteelant, W.M.G., Wehrmann, J., Engelen, D., Jansen, J., Verhelst, B., Benjumea, R., Cavaillès, S., Kaasiku, T., Hoekstra, B., De Boer, F., 2020. Accounting for differential migration strategies between age groups to monitor raptor population dynamics in the eastern Black Sea flyway. Ibis 162, 356–372. https://doi.org/10.1111/ibi.12773Votier, S.C., Bearhop, S., Ratcliffe, N., Furness, R.W., 2004. Reproductive consequences for great skuas specializing as seabird predators. The Condor 106, 275–287.Votier, S.C., Furness, R.W., Bearhop, S., Crane, J.E., Caldow, R.W.G., Catry, P., Ensor, K., Hamer, K.C., Hudson, A.V., Kalmbach, E., Klomp, N.I., Pfeiffer, S., Phillips, R.A., Prieto, I., Thompson, D.R., 2024. Changes in fisheries discard rates and seabird communities 427, 727–730.Weimerskirch, H., Tarroux, A., Chastel, O., Delord, K., Cherel, Y., Descamps, S., 2015. Population-specific wintering distributions of adult south polar skuas over three oceans. Mar. Ecol. Prog. Ser. 538, 229–237. https://doi.org/10.3354/meps11465Wernham, C., Toms, M., Marchant, J., Clark, J., Siriwardena, G., Baillie, Stephen, 2002. The Migration Atlas: Movements of the Birds of Britain and Ireland, Movements of the Birds of Britain and Ireland. T. & A.D. Poyser. FIGURE LEGENDS Figure 1. Estaca de Bares, one of the most important seabirds migratory watch sites in land in Western Europe, is located in Galicia (NW Spain). From 2004 to 2023 (both years included), a total number of 45.329 great skuas were observed during the daytime through this migratory corridor. The composition was made using maps from Vemaps.com. Figure 2. The phenology of the great skua at Estaca de Bares from 1965 to 2023, shows an increase in passage rate in mid-September (around day 270 of the year) as evidenced by fitting the general additive model of the migration rate as a function of the smoothing factor for day in year (i.e. Day of the Year). Figure 3. Annual and inter-annual variation in great skua migration rates. (A) Annual cumulative migration rate (great skuas/hour) across the study period. (B) Inter-annual distribution of migration rates. Boxplots show the median (horizontal line), interquartile range (box), and 1.5 × IQR (whiskers), while dots represent extreme values. This figure illustrates the degree of variability within years and highlights the occurrence of sporadic high-intensity migration events. Figure 4. Variation in the migration passage of great skua at Estaca de Bares over the years. (A) Migration rate over time, modelled with a negative binomial distribution. The plot shows the effect of the HPAI outbreak (indicated by the red arrow) in 2021. The analysis demonstrates a significant decrease in migration rate post-outbreak. The data points are shown in light blue for the period without HPAI outbreak and light red for the period with HPAI outbreak, with a regression line representing the estimated migration rates over time. (B) Total annual sum of migration rate for the periods 2004–2021 and 2022–2023. Each point represents the annual total migration rate for a given year. The data indicate a marked decline (70%) in migration rates during the 2022–2023 period compared to previous years. TABLES Table 1. Phenological quantiles of great skua’s migration based on cumulative daily migration rates. The table shows the Julian day (Day of the year) and corresponding calendar date (MM/DD) for each quantile (Q5, Q25, Q50, Q75, Q95), along with the standard deviation (SD) estimated from bootstrap resampling. Q5 and Q95 represent the earliest and latest 5% of total migration, respectively, while Q25, Q50, and Q75 indicate the 25th, median, and 75th percentiles of cumulative migration. Q5 08/07 219 2.03 Q25 09/10 253 1.24 Q50 09/30 273 2.19 Q75 10/28 302 3.37 Q95 12/04 339 4.34 Table 2. Beta estimates, Standard Error and t-values for the predictor variables included in the top Generalised Additive Mixed Model. The intercept represents the baseline condition, corresponding to years centred at the mean, absence of H5N1 outbreaks, and wind direction North. Estimates for other predictor levels or continuous variables indicate deviations relative to this baseline. The model evaluates the effects of temperature, HPAI (H5N1), year, wind direction, and wind speed on the migration rate of great skuas in Estaca de Bares. (Intercept) -0.17 0.12 -1.47 Year 0.05 0.04 1.23 H5N1 Outbreak 2.17 0.84 2.57 * Temperature -0.10 0.02 -4.86 *** Wind Direction NE 0.33 0.11 2.98 ** Wind Direction E 0.39 0.11 3.39 *** Wind Dirrection SE 0.59 0.13 4.61 *** Wind Direction S 0.71 0.13 5.36 *** Wind Direction SW 0.49 0.11 4.30 *** Wind Direction W 0.31 0.11 2.75 ** Wind Direction NW -0.88 0.46 -1.92 . Wind Speed -0.09 0.09 -1.02 Year:H5N1 outbreak -1.95 0.61 -3.20 ** Wind Dir NE:Wind Speed 0.02 0.09 0.22 Wind Dir E:Wind Speed 0.04 0.09 0.45 Wind Dir SE:Wind Speed 0.14 0.11 1.25 Wind Dir S:Wind Speed 0.30 0.12 2.52 * Wind Dir SW:Wind Speed 0.04 0.09 0.45 Wind Dir W:Wind Speed -0.02 0.09 -0.21 Wind Dir NW:Wind Speed -0.74 0.32 -2.28 * Smooth terms edf F s(DOY) 1.981 122.8 <2e-16 (1) Significance levels: ( ) p ≥ 0.05; (.) p = 0.05, (*) p FIGURES Fig. 1 Fig. 2 Fig. 3 Fig. 4 Information & Authors Information Version history V1 Version 1 09 October 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords avian influenza virus great skua h5n1 marine birds population declining seabird migration Authors Affiliations Rafael Benjumea 0000-0002-0569-8765 [email protected] Universidad de Malaga - Campus de Teatinos View all articles by this author ANTONIO SANDOVAL Centro de Extension Universitaria e Divulgacion Ambiental de Galicia View all articles by this author MARÍA DEL MAR DELGADO Consejo Superior de Investigaciones Cientificas View all articles by this author Metrics & Citations Metrics Article Usage 341 views 170 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Rafael Benjumea, ANTONIO SANDOVAL, MARÍA DEL MAR DELGADO. Severe Population Decline of great skua Stercorarius skua during Migratory Passage. Authorea . 09 October 2025. DOI: https://doi.org/10.22541/au.175998254.48654038/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. 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