Aquatic Warbler Acrocephalus Paludicola Research Articles

Successful Translocation of a Long‐Distance Migrating Passerine—New Impetus for the Conservation of the Globally Threatened Aquatic Warbler

ABSTRACTTranslocations are a conservation measure that is increasingly applied to assist the recovery of animal populations threatened with extinction. Long‐distance migrating passerines, however, have been rarely addressed in translocation projects so far. One such species is the globally threatened Aquatic Warbler Acrocephalus paludicola, a habitat specialist breeding in fen mires in Central Europe and wintering in sub‐Saharan Africa. The global breeding population has severely declined during the last decades due to habitat loss. Although the implementation of conservation measures has stopped the decline in the core breeding area, peripheral populations continue to decline. The aim of this pilot study was to reveal whether translocated Aquatic Warblers will return to a distant release site after wintering in Africa. Our translocation method is based on natal habitat imprinting of juvenile passerines before their first migration. In 2018 and 2019, 50 chicks (10–12 entire broods) each year were translocated 526 km from Belarus to Lithuania at an age of about 7 days and hand‐reared with predominantly wild insects captured in the surroundings of the release site. The survival rate of chicks until soft release from outdoor aviaries was 98% (49 fledglings released) in 2018 and 100% (50 fledglings released) in 2019. In 2019, 11 Aquatic Warblers (9 males, 2 females) were resighted at the release site after returning from wintering grounds. In 2020, nine birds (6 males, 3 females) were observed, including three males from the 2018 release cohort. An average apparent first‐year survival of 0.30 was estimated. The very successful outcome gives new impetus for the restoration prospects of declined populations of the Aquatic Warbler to halt extinction at the margins of the breeding range.

Low genetic diversity and high gene flow in the Aquatic Warbler (Acrocephalus paludicola), a threatened marshland songbird with a fragmented breeding range

Breeding habitat loss and depletion of genetic diversity can have critically negative impacts on species, and especially so for habitat specialists. The Aquatic Warbler Acrocephalus paludicola is a threatened European songbird that breeds in fens, which have been lost and fragmented over centuries. We used microsatellite loci to perform a spatial and longitudinal comparison, comparing samples collected recently (contemporary) with those obtained about two decades earlier (historical) from c. 25% of the breeding range (Biebrza and Polesie) of the Aquatic Warbler. With these data we explored changes in genetic diversity, expansions and reductions of population size, population structure, and gene flow. Allelic richness, expected heterozygosity, the number of effective and private alleles, and effective population size (NE ≈ 200) were low and comparable across time and space. We detected a genetic bottleneck in contemporary Biebrza, and a less certain bottleneck in both historical and contemporary Polesie. Across space, Biebrza and Polesie were not genetically differentiated in either period. All the historical samples clustered together, but in the contemporary samples a small part of Polesie clustered separately (FST = 0.011). Across time, Biebrza and Polesie showed low but significant differentiation (FST = 0.026–0.064), and historical and contemporary groups clustered apart. A principal coordinate analysis on genetic distance detected three groups, which spanned sampling locations. In the temporal comparison, the contemporary group and contemporary Biebrza had elevated mean within‐group pairwise relatedness. We did not recover signals for sex‐biased dispersal, asymmetric gene flow or isolation‐by‐distance using the molecular data. Our results suggest that the studied populations show: (1) impoverished genetic diversity, (2) a change in allele frequencies over the two decades studied and (3) high gene flow between distant breeding sites, implying high resilience to habitat fragmentation that should facilitate the success of recolonising restored habitat patches by Aquatic Warblers.

Male song of the Aquatic Warbler, a promiscuous bird without paternal care, is more complex than previously thought

The Aquatic Warbler Acrocephalus paludicola is one of the rarest European passerines and is characterised by promiscuity, lack of pair-bonds and female-only parental care. This makes the species an important model for studying the function of avian courtship song. The song of the Aquatic Warbler consists of whistle and rattle phrases produced as discontinuous A-, B- and C-songs, which are built by a single rattle, a rattle and a whistle, and more than two phrases of both kinds, respectively. The A- and B-songs are thought to be aggressive signals in male-male interactions, while C-songs are thought to be important for female choice. Here, we analysed recordings of 40 individually marked males, and determined the phrase repertoire. The enumerated repertoire (males recorded for ≥ 10 min) ranged from 16 to 158 (mean 99), however, it did not capture the complete phrase repertoires. We then used models from species diversity ecology to estimate the actual phrase repertoire size, which ranged between 18 and 300 phrases (mean 155). The estimated repertoire was predicted by the number of C-songs. The rattle repertoire was larger than the whistle repertoire, and both positively correlated with the number of C-songs. Our study indicates that male Aquatic Warblers have highly complex phrase repertoires that vary widely in size. Their courtship song is flexible and efficient, enabling relative song complexity to be demonstrated in a short sample, thus facilitating both female attraction through the quick presentation of large phrase repertoires and rival deterrence through the production of many short and simple A- and B-songs.

Autumn Migration Strategy and Stop-Over Sites of the Globally Threatened Aquatic Warbler Acrocephalus paludicola on the Atlantic Flyway Migration Route

Knowledge about migration strategy and availability of stop-over sites of globally threatened migratory birds is crucial for the implementation of efficient conservation plan. In this study, we investigated the refuelling strategy of the globally threatened Aquatic Warbler Acrocephalus paludicola at stop-over sites during the autumn migration in West Europe and Africa (40 and 72 sites, respectively, documented in published works about stop-over ecology and sites). Based on fuel reserves of captured birds we calculated potential maximal range of non-refuelling flight from stop-over sites located on the Atlantic flyway migration route. We tried to verify the hypothesis that Aquatic Warblers are able to cross the Sahara during one non-stop flight. We found that mean range of flights was affected significantly by the stopover geographic location but not by age of the birds and stop-over habitat. Individuals staging in France are able to cover smaller distances (mean 612.7 km) compared to individuals staging in Portugal and Spain (mean 857.5 km), i.e. closer to two important geographical barriers (the Gibraltar Strait and the Sahara desert). However, maximal ranges of the heaviest individuals were longer (2370 km in France and 1998 km in Portugal). Our study suggests that Aquatic Warblers are generally not able to cross the Sahara during one non-stop flight. Only some individuals with high fat loads starting from stop-overs in Portugal may be able to cross the Sahara without refuelling. However, due to diel air temperature pattern in the desert they need to stop at least once to rest and wait for nighttime drop in temperature to continue migration. Within the Sahara there are many small patches of suitable habitats which can facilitate resting and maybe even refuelling. Observed and predicted reduction of available area of stop-over habitats may have serious negative consequences for population dynamics of many land-bird species, including the globally threatened Aquatic Warbler.

Heterozygosity and fitness in a threatened songbird: blood parasite infection is explained by single-locus but not genome-wide effects

In non-pedigreed populations, insights into effects of inbreeding can be obtained by correlations between individual heterozygosity and fitness-related traits (HFCs). Using an information-theoretic approach, we explored whether heterozygosity of microsatellite markers, measured as internal relatedness (IR), is associated with infection by blood parasites (Plasmodium, Trypanosoma, or Leucocytozoon) in the threatened Aquatic Warbler (Acrocephalus paludicola). We also explored whether any of the markers is more influential than others, or than IR, in explaining blood parasitism (single-locus effects). While we observed that IR was a relatively important predictor of Plasmodium parasitism, we did not find strong evidence for IR to correlate with infection by the identified blood parasites, accounting for sex and population effects. Therefore, our data did not support negative inbreeding effects on blood parasite infection in the Aquatic Warbler. However, we found single-locus effects, such that individuals heterozygous at AW-03 and Ase19 had lower probability of infection by blood parasites pooled together and by Plasmodium, respectively. This indicates that these two markers are in linkage disequilibrium with unknown fitness loci which are related to resisting or clearing blood parasites, and which confer a heterozygote advantage in the Aquatic Warbler. Our results add to the growing evidence that single-locus effects contribute more to HFCs than formerly recognized and have implications for Aquatic Warbler conservation.

Correlates of blood parasitism in a threatened marshland passerine: infection by kinetoplastids of the genus Trypanosoma is related to landscape metrics of habitat edge.

In birds, vector-borne parasites invading the bloodstream are important agents of disease, affect fitness and shape population viability, thus being of conservation interest. Here, we molecularly identified protozoan blood parasites in two populations of the threatened Aquatic Warbler Acrocephalus paludicola, a migratory passerine nesting in open marsh. We explored whether prevalence and lineage diversity of the parasites vary by population and whether infection status is explained by landscape metrics of habitat edge and individual traits (body mass, fat score, wing length and sex). Aquatic Warblers were infected by genera Plasmodium, Leucocytozoon and Trypanosoma, with seven, one and four lineages, and 29.9, 0.7 and 12.5% prevalence, respectively. No Haemoproteus infections were detected. Prevalence did not vary between the populations, but lineage diversity was higher in Polesie than in Biebrza for all the lineages pooled and for Plasmodium. Infection by Trypanosoma decreased with patch core area and increased with density of habitat edge. Infection status was not predicted by the individual traits. Our study is the first to show an association between edge-related landscape features and blood parasitism in an open habitat bird. This finding will support informed conservation measures for avian species of the globally shrinking marshland and other treeless habitats.

Identifying migration routes and non-breeding staging sites of adult males of the globally threatened Aquatic WarblerAcrocephalus paludicola

SummaryBirds that are long-distance migrants partition their annual cycle among a number of locations over a large spatial range. The conservation of these species is particularly complex because it requires attention to a number of different and distant habitats based on knowledge of migratory phenology, routes and staging areas. In the case of the globally threatened Aquatic WarblerAcrocephalus paludicola, a habitat specialist that breeds in Europe and spends the boreal winter in sub-Saharan Africa, non-breeding staging areas were widely unknown until recently. We applied light-level geolocators to adult male Aquatic Warblers at breeding sites in Belarus and Ukraine. Data from eight retrieved geolocators confirmed a south-west and then westward migration route through Europe via the northern Mediterranean with staging sites on the Iberian Peninsula and occasionally France and north-west Africa. In sub-Saharan Africa, final staging areas were in Mali, either in the Inner Niger Delta or at (presumably) small water bodies in the desert, with one bird most likely staying in the north of Ivory Coast south of the previously assumed range. The birds probably stayed in the final staging areas for most of the non-breeding season, but the logger on only one bird provided data throughout the entire migration cycle. Pre-nuptial migration of the latter was by a more easterly route than the southward migration. Our study suggests that conservation strategies for Aquatic Warbler north of the Sahara should include consideration of unknown staging sites in the northern Mediterranean as well as on the Iberian Peninsula and in north-west Africa. South of the Sahara, our study demonstrates the importance of the Sahel for the conservation of the Aquatic Warbler, including both the major floodplains of the Niger River and small Sahelian wetlands that are under pressure from human development.

Conservation of disappearing cultural landscape\u2019s biodiversity: are people in Belarus willing to pay for wet grassland restoration?

Abandonment of traditional farming practices, such as hay-making and pasturing, has resulted in rapid loss of open wet grassland habitats in Europe. The globally threatened Aquatic Warbler (Acrocephalus paludicola L.) is a bird species that occurs almost exclusively in open fen mires, which have virtually disappeared in Western Europe, but still persist locally in Eastern Europe. Focusing on the world’s most important breeding site for Aquatic Warbler, the Zvaniec fen mire in Belarus, we estimated Belarusian citizens’ willingness-to-pay for adequate conservation management of this fen mire and its focal species the Aquatic Warbler. Results from a discrete choice experiment indicated that Belarusian citizens were willing to pay for appropriate conservation programmes of the Zvaniec fen mire. Scything and mechanical mowing were preferred compared to controlled burning, and especially over herbicide treatment of encroaching shrubs. Conservation management was preferred over legal protection of wetland areas without management. Respondents considered such passive conservation to be insufficient to maintain open fen mire habitat and gave a higher priority to active conservation management programmes. These preferences are consistent with evidence-based knowledge about what is effective conservation management for the Aquatic Warbler. Given the gradual disappearance of Europe’s traditional cultural landscapes, we discuss the challenge to fund the maintenance of this biocultural biodiversity legacy.

Mercury contamination, a potential threat to the globally endangered aquatic warbler Acrocephalus paludicola

Mercury (Hg) contamination is considered a global concern for humans and wildlife, and although the number of studies dealing with that issue continues to increase, some taxonomic groups such as small passerine birds are largely understudied. In this paper, concentration of mercury in the aquatic warbler (Acrocephalus paludicola) feathers, a globally threatened passerine species, was examined. The concentration differences between two ages and sexes were investigated. The comparison of feathers taken on autumn migrants of two age categories act as a comparison of the species’ exposure within the two different areas (European breeding or African wintering grounds). The average Hg concentration for all sampled individuals [2.32 μg/g dw (range 0.38–12.76)] is relatively high, compared with values found in other passerine species. An age difference was found, with first-year individuals displaying higher mercury concentrations than adults. This indicates that birds are exposed to mercury pollution during the breeding season, i.e., in the continental floodplains of eastern Europe. The average Hg concentration in feathers grown on the breeding grounds was 3.88 ± 2.59 μg/g dw, closer to the critical value of 5 μg/g dw, which is considered to impair the health of individuals. The findings suggest that mercury pollution may constitute a threat so far neglected for the endangered aquatic warbler.

Spatial Variation in Long-Term Trends in a Metapopulation of the Globally Threatened Aquatic WarblerAcrocephalus paludicolain Poland

Abstract. The Polish breeding population (3,200–3,250 males) of the globally threatened Aquatic Warbler Acrocephalus paludicola represents almost 25% of the global population. Except for the relatively stable large population in the Biebrza valley in north-east Poland less is known about population trends of peripheral populations in western, central and south-eastern regions of the country and whether trends differ depending on region. We investigated the long-term population dynamics in 38 small populations between 1969–2013 in the four Polish regions. Summarizing the trends of all small populations of Aquatic Warblers showed a significant decline in total number of individuals and declining number of populations over time. However, population trends were distinctly different in the different regions, with stable dynamics in south-east, moderate decline in north-east and sharp decline in the central and western regions. During the study period 19 out of 38 populations became extinct (11 populations in t...

Prevalence and diversity of Plasmodium and Haemoproteus parasites in the globally-threatened Aquatic Warbler Acrocephalus paludicola.

The diversity and prevalence of malaria parasites of the genera Plasmodium and Haemoproteus were determined in the globally-threatened Aquatic Warbler Acrocephalus paludicola. Birds were sampled during migration in Portugal and at the wintering quarters in Senegal and parasites were detected using molecular methods. Only three generalist parasite lineages (Plasmodium) were found. There were no significant differences in the prevalence of parasites between sexes in Europe, but adults had higher prevalence than first-year birds, and birds in Europe had higher prevalence than those captured in Africa. When comparing with other Acrocephalus species and taking sample size into account, Aquatic Warblers had the lowest prevalence and, together with another threatened species, the Seychelles Warbler Acrocephalus sechellensis, the lowest diversity of malaria parasites. We hypothesize that the low diversity of parasites and absence of specialist lineages of Aquatic Warblers are caused by its small population size and fragmented distribution. Furthermore, Aquatic Warblers' extreme habitat specialization may decrease their exposure to malaria parasites, but other explanations such as high mortality (which would constraint the sampling of infected birds) or, in contrast, very efficient immunological system in clearing the infections cannot be ruled out. This study contributes to explain variation in prevalence and diversity of malaria parasites among hosts.

Use of an abandoned gravel-pit in inland Spain as a stopover site for the Aquatic Warbler Acrocephalus paludicola

Stopover sites are of vital importance for migrating birds, especially long-distance migrant species. This study describes a wetland in NW Spain, as a particular habitat used by the Aquatic Warbler, one of the most threatened passeriformes in Europe, during postnuptial migration. It is a recently abandoned gravel-pit with an artificial water regime, a large area of standing water and a mosaic of vegetation predominated by a rush-meadow of Juncus effusus and a cattail community of Typha domingensis, a priori very favourable conditions for the Aquatic Warbler. The Acrola index value for this locality is very high, and together with some of the results including mean stopover period, fattening rate, interannual site fidelity and calculated potential flight distances, indicates that the mosaic of plant communities offers good conditions as a refuelling site for the Aquatic Warbler. Nevertheless, in only 3 years a process of homogenization has been observed in the vegetation, where rush-meadow cover has decreased noticeably and cattail and willow cover have increased. This process caused a decrease in the Acrola index throughout 2008–2010, as well as a significant decrease in the importance of this wetland with regard to others in Spain. Significant differences were found in body condition between the two age groups: adults are heavier and obtain more body fat, which provides them with greater potential flight autonomy. Although gravel pits can act as alternative wetlands, rapid changes in plant communities succession requires more active management than in the case of natural wetlands.

Optimal habitat conditions for the globally threatened Aquatic Warbler Acrocephalus paludicola in eastern Poland and their implications for fen management

To identify optimal habitat for the Aquatic Warbler Acrocephalus paludicola, a flagship species of fen mires, we related numbers of singing males to habitat variables in all core breeding sites in eastern Poland. The density of male Aquatic Warblers increased with increasing ground cover by water and mosses and litter layer height, and was highest where vegetation was 60–90 cm tall. Male densities also increased with the biomass of arthropods > 10 mm length, estimated by sweep netting, and with the abundance of spiders, estimated by pan trapping. We suggest that habitat management should take into account species‐specific morphological adaptations, nest safety and arthropod productivity. Prevention of vegetation succession is a conservation priority for open fen mires. However, modern management practices to achieve this, especially mowing using tracked vehicles, should be evaluated and optimized to ensure that such practices do not adversely affect the long‐term development of moss cover and litter structure.

Effect of Habitat Burning on the Number of Singing Males of the Aquatic WarblerAcrocephalus paludicola

Abstract. Wildfires can be the most influential phenomena in landscape dynamics and play an important role in determining avian populations. However, the intentional burning of habitats is still a controversial management practice and is legally prohibited in many countries. On the other hand, fires can positively control, or even slow down vegetation succession of reeds and bushes, and, in particular cases, may sustain a habitat for open marsh nesting specialists. One such species is the Aquatic Warbler Acrocephalus paludicola, a globally threatened habitat specialist that breeds in open fens in Central and Eastern Europe. Because bush and reed encroachment threaten many suitable breeding areas, habitat management is necessary to maintain the open wetlands that Aquatic Warblers require for nesting. To assess whether burning was beneficial, we analysed Aquatic Warbler numbers and distribution in the Chelm calcareous marshes in eastern Poland on plots in different successional stages after accidental fires...

Does the Aquatic Warbler Acrocephalus paludicola show differential migration by age during the autumn in the Iberian Peninsula?

The Aquatic Warbler Acrocephalus paludicola is a globally threatened European migratory songbird that passes through the Iberian Penninsula during its migration from Western Europe to Africa. The aim of our study was to determine the spatial migratory pattern of the species according to age classes during the post-nuptial passage in the Iberian Peninsula by analysing ringing data. The results confirm the existence of differential migration by age according to a longitudinal geographical gradient and suggest that the more westerly the wetlands, the smaller the proportion of juveniles that use them. Our findings also highlight the existence of a very broad migratory front. These results have implications for the conservation of different areas of migratory passage.

Age and Sex Differences in Fuel Load and Biometrics of Aquatic WarblersAcrocephalus paludicolaat an Autumn Stopover Site in the Loire Estuary (NW France)

The aquatic warbler Acrocephalus paludicola is the only globally threatened passerine breeding in Europe. We studied morphometric traits of 176 individuals captured at one of the key European autumn stopover sites, the Loire estuary on the Atlantic coast of France, in relation to age (first-year and older) and sex (based on molecular determination). We found significant sex differences in wing length, third primary feather length and body mass, with males being longer-winged and heavier than females; adult females also had deeper bills. However, high overlap in all measurements between the sexes meant the best discriminant functions based on wing length and bill depth (adults) and wing length (inmatures) correctly classified only 87% and 75% of individuals, respectively. The mean potential non-stop flight range of autumn staging birds at the Loire estuary with high fuel load was estimated at 1,178 km for adults and 926 km for immatures. We conclude that in autumn migrant aquatic warblers probably do not fly directly to wintering grounds in west Africa. Instead they gain significant body mass for onward migration at a few key stopover sites in western Europe and the southern Mediterranean region. Conservation of a series of important refuelling stopover sites, especially wet grassland habitats, along the migration route is therefore essential for effective protection of the aquatic warbler.

Ecology of Aquatic WarblersAcrocephalus paludicolain a Fall Stopover Area on the Atlantic Coast of France

The Aquatic Warbler Acrocephalus paludicola is the only endangered songbird in continental Europe. This trans-Saharan migratory bird significantly transits along the French Atlantic coastline during post-breeding migration and the right bank of the Gironde estuary has been identified as an important stopover site. We studied the spatial occupancy strategies of stationing individuals during August of three successive years (2010–2012). We characterized habitat use by radio-tracking individuals revealing relatively small foraging ranges (6.6 ± 2.6 ha on average) with only 1 ha actually exploited (core area), and a relatively high habitat fragmentation rate. Capture-mark-recapture analyses assessed the average stopover duration of individuals and body-mass variations during their stopover. The estimated average stopover duration was 6.46 ± 0.46 days (95% confidence interval: 4.4–9.6). Lean birds tended to forage significantly more than stout birds: on average, they gain 2.81 ± 0.89% of their initial mass each day whereas stout birds only gain 0.12 ± 0.56%. Analyses of droppings characterized the local diet. We noticed that Aquatic Warbler preferentially used partially-flooded or flooded habitats with heterogeneous and rather low vegetation, such as bulrush beds or bulrush beds mixed with reed beds. Orthoptera, Araneae and Hymenoptera represented the largest contributions to the consumed biomass (64.7%, 13.4% and 8.9% respectively). The importance of the fuel deposition rate of lean birds reflects the importance of the estuary as a stopover site for the species. It means that the available resources allow birds to replenish and continue their migration route. However, the sustainability of the site's functionality is questioned because of the evolution of habitats (erosion, rise in water levels and changes in food web).

Model-Based Selection of Areas for the Restoration of Acrocephalus paludicola Habitats in NE Germany

The global Aquatic Warbler (Acrocephalus paludicola, Vieillot, 1817) population has suffered a major decline due to the large-scale destruction of its natural habitat (fen mires). The species is at risk of extinction, especially in NE Germany/NW Poland. In this study, we developed habitat suitability models based on satellite and environmental data to identify potential areas for habitat restoration on which further surveys and planning should be focused. To create a reliable model, we used all Aquatic Warbler presences in the study area since 1990 as well as additional potentially suitable habitats identified in the field. We combined the presence/absence regression tree algorithm Cubist with the presence-only algorithm Maxent since both commonly outperform other algorithms. To integrate the separate model results, we present a new way to create a metamodel using the initial model results as variables. Additionally, a histogram approach was applied to further reduce the final search area to the most promising sites. Accuracy increased when using both remote sensing and environmental data. It was highest for the integrated metamodel (Cohen's Kappa of 0.4, P<0.001). The final result of this study supports the selection of the most promising sites for Aquatic Warbler habitat restoration.

Modelling habitat suitability in the Aquatic Warbler wintering ground Djoudj National Park area in Senegal

The only well-studied wintering ground of the globally threatened Aquatic Warbler Acrocephalus paludicola is the ‘Djoudj area’ in Senegal. This study identifies potential Aquatic Warbler habitats within that area and gives an estimate of the size of the local wintering population. A land-cover map was generated by classifying high-resolution satellite images. Overlaying it with presence–absence data from field surveys and using logistic regression models (GLMM), we derived the presence probability of the Aquatic Warbler in the study area. Using presence probability as a proxy of habitat suitability we identified 4 729 ha of potential habitat. We calculated the population density of the Aquatic Warbler by applying a Poisson mixed model to our land-cover raster data and estimate a total population size of 776 individuals (range 260–4 057), i.e. 0.37–19.8% of the world population. However, this estimate is complicated by uncertainties including the unknown efficiency of the bird-catching method, clumped presence of individuals, the variability of water levels and other seasonal changes. An effective management of Aquatic Warbler habitats in the Djoudj area should include extensive grazing, the control of litter accumulation and shrub growth, and the removal of plant material from the channels to improve water circulation.

Differential autumn migration of the aquatic warbler Acrocephalus paludicola

Relatively little attention has been paid to sex differences in the migration of birds in autumn. We studied the autumn migration strategy of molecularly sexed males and females in the globally threatened aquatic warbler Acrocephalus paludicola. We captured 176 birds at a stopover site in the Loire estuary at Donges, France. The median date for the passage of adults was 8 days earlier in males than females, although the timing of migration in first-year males and females was similar. This indicates that males, who are without parental duties, can start their migration earlier than females and first-year birds. Adults were significantly heavier than immature birds but did not have higher fat scores. In both age categories, more males (two to three times more) were captured. However, various factors (including tape-luring) can affect observed sex ratio.