Wood Warbler Phylloscopus Sibilatrix Research Articles

Auditory risk recognition is socially transmitted across territory borders in wild birds

Prey species commonly assess predation risk based on acoustic signals, such as predator vocalizations or heterospecific alarm calls. The resulting risk-sensitive decision-making affects not only the behavior and life-history of individual prey, but also has far-reaching ecological consequences for population, community, and ecosystem dynamics. Although auditory risk recognition is ubiquitous in animals, it remains unclear how individuals gain the ability to recognize specific sounds as cues of a threat. Here, it has been shown that free-living birds (Wood Warblers Phylloscopus sibilatrix) can learn to recognize unfamiliar, complex sounds (samples of punk rock songs) as cues of a threat from conspecifics holding adjacent territories during the spring breeding season. In a playback experiment, Wood Warblers initially ignored the unfamiliar sounds, but after repeatedly hearing that these sounds trigger alarm calling reaction of neighbors, most individuals showed an anti-predator response to them. Moreover, once learned soon after nestlings hatching, the anti-predator response of parents toward previously unfamiliar sounds was then retained over the entire nestlings rearing period. These results demonstrate that social learning via the association of unfamiliar sounds with known alarm signals enables the spread of anti-predator behavior across territory borders and provides a mechanism explaining the widespread abilities of animals to assess predation risk based on acoustic cues.

Wood warbler population dynamics in response to mast seeding regimes in Europe.

Mast seeding is the episodic, massive production of plant seeds synchronized over large areas. The resulting superabundance of seeds represents a resource pulse that can profoundly affect animal populations across trophic levels. Following years of high seed production, the abundance of both seed consumers and their predators increase. Higher predator abundance leads to increased predation pressure across the trophic web, impacting nonseed consumers such as the wood warbler Phylloscopus sibilatrix through increased nest predation after tree mast years. Over the past 30 years, the frequency of tree seed masts has increased, while wood warbler populations have declined in several regions of Europe. We hypothesized that increasing mast frequencies may have contributed to the observed population declines by creating suboptimal breeding conditions in years after masting. We measured reproductive output in four study areas in central Europe, which was between 0.61 and 1.24 fledglings lower in the years following masting than nonmasting. For each study area, we used matrix population models to predict population trends based on the estimated reproductive output and the local mast frequencies. We then compared the predicted with the observed population trends to assess if the frequency of mast years had contributed to the population dynamics. In Wielkopolska National Park (PL) and Hessen (DE), masting occurred on average only every 4 years and populations were stable or nearly so, whereas in Jura (CH) and Białowieża National Park (PL), masting occurred every 2 and 2.5 years, respectively, and populations were declining. The simple matrix population models predicted the relative difference among local population trends over the past 10-20 years well, suggesting that the masting frequency may partly explain regional variation in population trends. Simulations suggest that further increases in mast frequency will lead to further declines in wood warbler populations. We show that changes in a natural process, such as mast seeding, may contribute to the decline in animal populations through cascading effects.

The use of song playback experiments to study the social mechanisms of bird settlement decisions

Acoustic playback techniques involve broadcasting of various sound stimuli to simulate the presence of a certain species. In this article, we applied a variation of the method using automatic sound-reproducing stations (ASRS), which imitate the daily vocal activity of birds that is close to natural. We investigated whether acoustic playback experiments help to examine the role of social cues for bird settlement decisions, as well as to detect the presence of rare and low-density species. Using this method, we were able to increase the local density of the rustic bunting Ocyris rusticus Pallas, 1776 (Emberizidae) – species, which is currently listed in the Red Book of the Russian Federation (2001). The findings of our study show that rustic buntings preferred to settle on plots where the song imitating the presence of conspecifics was broadcasted by ASRSs. In addition, the playback method was used to examine the social structure of the population of the wood warbler Phylloscopus sibilatrix Bechstein, 1793 (Sylviidae), a small species that lives here on the northern periphery of the range. The wood warbler songs were broadcasted by ASRSs before the arrivals of the first conspecifics to the breeding area. Songs with different acoustic activity (2 or 6 trills per min) were used to create a difference between the social environments on plots. The gathered data showed a display of two different behavioral responses of individuals (males) to different song types broadcasted on plots. Based on this observation, the main hypothesis is about the existence of both conservative and opportunistic male reproductive strategies of the wood warbler. The results demonstrate the possibility of using acoustic playback techniques to study the role of acoustic signals in territorial behavior and social structure of a bird population. Understanding the ways birds make settlement decisions is essential for the development of conservation practices.

Host Cells of Leucocytozoon (Haemosporida, Leucocytozoidae) Gametocytes, with Remarks on the Phylogenetic Importance of This Character.

Leucocytozoon parasites remain poorly investigated in comparison to other haemosporidians. The host cell inhabited by their blood stages (gametocytes) remains insufficiently known. This study aimed to determine the blood cells inhabited by Leucocytozoon gametocytes in different species of Passeriformes and to test if this feature has a phylogenetic importance. We microscopically analyzed blood films stained with Giemsa from six different bird species and individuals and used PCR-based methods for parasite lineage identification. The DNA sequences obtained were applied for phylogenetic analysis. Leucocytozoon parasite from the song thrush Turdus philomelos (cytochrome b lineage STUR1), the blackbird Turdus merula (undetermined lineage), the garden warbler Sylvia borin (unknown lineage) inhabited erythrocytes, a parasite from the blue tit Cyanistes caeruleus (PARUS4) infects lymphocytes, while in the wood warbler Phylloscopus sibilatrix (WW6) and the common chiffchaff Phylloscopus collybita (AFR205) they were found inhabiting thrombocytes. Parasites infecting thrombocytes were closely related, while the parasites infecting erythrocytes were placed in three different clades, and the one found in lymphocytes was placed in a separate clade. This shows that the determination of host cells inhabited by Leucocytozoon parasites can be phylogenetically important and should be considered in future species descriptions. Noteworthy, phylogenetic analysis might be used for the prediction of which host cells parasite lineages might inhabit.

Weather impacts on interactions between nesting birds, nest-dwelling ectoparasites and ants

Weather has a dominant impact on organisms, including their life histories and interspecific interactions. Yet, for nesting birds, and the arthropods inhabiting bird nests, the direct and cascading effects of weather are poorly known. We explored the influence of ambient temperatures and rainfall on the cohabitation of dome-shaped bird nests by Wood Warblers Phylloscopus sibilatrix, their blowfly Protocalliphora azurea ectoparasites, and predatory Myrmica and Lasius ants that may provide nest sanitation. We sampled blowflies and ants in 129 nests, and measured warbler nestlings during 2018–2020 in the primeval Białowieża Forest, eastern Poland. The probability of ectoparasites occurring in nests increased with increasing ambient temperatures and declining precipitation in the early nestling stage, when adult blowflies are ovipositing. Where present, the number of ectoparasites was greater if higher ambient temperatures had prevailed in the late nestling stage, but only when ants were absent from nests. However, the nestling growth was unrelated to ectoparasite abundance or ant presence within bird nests, although it was lower at high rainfall. The results suggest that weather can have conflicting impacts on interactions between nesting birds and nest-dwelling arthropods, but birds can mostly compensate for any related costs in old-growth forest, where food is generally abundant.

Accounting for predator species identity reveals variable relationships between nest predation rate and habitat in a temperate forest songbird.

Nest predation is the primary cause of nest failure in most ground‐nesting bird species. Investigations of relationships between nest predation rate and habitat usually pool different predator species. However, such relationships likely depend on the specific predator involved, partly because habitat requirements vary among predator species. Pooling may therefore impair our ability to identify conservation‐relevant relationships between nest predation rate and habitat. We investigated predator‐specific nest predation rates in the forest‐dependent, ground‐nesting wood warbler Phylloscopus sibilatrix in relation to forest area and forest edge complexity at two spatial scales and to the composition of the adjacent habitat matrix. We used camera traps at 559 nests to identify nest predators in five study regions across Europe. When analyzing predation data pooled across predator species, nest predation rate was positively related to forest area at the local scale (1000 m around nest), and higher where proportion of grassland in the adjacent habitat matrix was high but arable land low. Analyses by each predator species revealed variable relationships between nest predation rates and habitat. At the local scale, nest predation by most predators was higher where forest area was large. At the landscape scale (10,000 m around nest), nest predation by buzzards Buteo buteo was high where forest area was small. Predation by pine martens Martes martes was high where edge complexity at the landscape scale was high. Predation by badgers Meles meles was high where the matrix had much grassland but little arable land. Our results suggest that relationships between nest predation rates and habitat can depend on the predator species involved and may differ from analyses disregarding predator identity. Predator‐specific nest predation rates, and their relationships to habitat at different spatial scales, should be considered when assessing the impact of habitat change on avian nesting success.

Reproductive success of the wood warbler Phylloscopus sibilatrix varies across Europe

Differences in population trends across a species' breeding range are ultimately linked to variation in demographic rates. In small songbirds, demographic rates related to fecundity typically have strong effects on population trends. Populations of a forest songbird, the wood warbler Phylloscopus sibilatrix, have been declining in many but not all regions of the European breeding range. We investigated if clutch size, hatching rate, nest survival and number of fledglings vary across Europe, and if nest survival is related to differences in the regionally dominant nest predator class (birds versus mammals). From 2009 to 2020, we monitored 1896 nests and used cameras at a subsample of 645 nests in six study regions: the United Kingdom (mid‐Wales, Dartmoor, the New Forest), Germany (Hessen), Switzerland (Jura mountains) and Poland (Białowieża National Park). Number of fledglings was lowest in the New Forest (1.43 ± CI 0.23), intermediate in Jura (2.41 ± 0.31) and Białowieża (2.26 ± 0.24) and highest in mid‐Wales (3.02 ± 0.48) and Dartmoor (2.92 ± 0.32). The reason for low reproductive success in the New Forest, Jura and Białowieża was low nest survival, and large clutch sizes in Białowieża did not compensate for high nest losses. High reproductive success in mid‐Wales and Dartmoor was due to high nest survival and large clutch sizes. Overall predation rates were similar everywhere despite variation between the regions in the dominant nest predator class. Unsuccessful nests in mid‐Wales were mainly predated by birds; in Dartmoor, the New Forest, Hessen and Jura similarly by birds and mammals; and in Białowieża exclusively by mammals. Regional reproductive success does not match the population trends recently reported for the wood warbler in the six study regions (i.e. high reproduction ≠ positive trend). Annual survival may be a decisive factor, but it is difficult to quantify for a nomadic species such as the wood warbler that rarely returns to the same breeding locations.

Territory‐level temperature influences breeding phenology and reproductive output in three forest passerine birds

Temperature plays an important role in determining the breeding phenology of birds in temperate climates, with higher spring temperatures associated with earlier breeding. However, the effect of localised territory‐scale temperature variations is poorly understood, with relationships between temperature and breeding phenology mostly studied using coarse‐grained climatic indices. Here, we interpolate spring temperatures recorded at 150‐m2 grid intersections encompassing 417 ha of forest to examine the influence of territory‐scale temperature, and its interaction with mean annual temperature, on territory selection, breeding phenology, clutch size and fledging success for three co‐occurring single‐brooded passerine birds using data from 672 nests over four years. All species exhibited significant trends in reproductive traits associated with territory‐scale temperature. Pied flycatchers Ficedula hypoleuca settled in cooler territories first, where they raised more fledglings. Blue tits Cyanistes caeruleus laid larger clutches in warmer territories in warm years and always laid earlier at warmer territories irrespective of annual temperature variation. Contrastingly, pied flycatcher and wood warbler Phylloscopus sibilatrix breeding phenology was earlier at warmer territories in cool years and cooler territories in warm years, with wood warbler clutch size responding similarly to this interaction. Greater previous breeding experience and higher rates of historical territory occupancy (territory quality) also predicted earlier breeding phenology and higher fledging success for pied flycatchers. We suggest that the migratory pied flycatcher and wood warbler are best synchronised with their prey availability in cooler years at a local population level. However resident blue tits match local phenology across all years, which is potentially advantageous under warmer predicted climate change scenarios. We conclude that temperature at the territory scale can be an important driver of settlement and breeding phenology and influence reproductive traits.

Loop-migration and non-breeding locations of British breeding Wood Warblers Phylloscopus sibilatrix

ABSTRACT Capsule: British breeding Wood Warblers Phylloscopus sibilatrix show a clockwise loop migration incorporating stops in southern Europe, the Sahel, and the humid forest zone of West Africa. Aims: To determine autumn and spring migration routes, the location and duration of stopover sites on migration, and the location of non-breeding areas of British breeding Wood Warblers. Methods: In 2016 and 2018 we deployed geolocators to male Wood Warblers on Dartmoor, Devon, and in the New Forest, Hampshire. We retrieved four geolocators from returning birds in 2017, 2019, and 2020. Results: Male Wood Warblers departed breeding sites in late July and stopped for most of August in central southern Europe, crossed the Sahara by a non-stop night and day flight immediately followed by a short stop, and then migrated west to a longer stopover in the Sahel. Final non-breeding destinations were in an area of West Africa covering Guinea, Sierra Leone, and Liberia. Two were tracked on spring migration, again crossing the Sahara via a non-stop flight before migrating through Western Europe to complete a clockwise loop migration back to Britain. Conclusion: All tracked Wood Warblers used stopovers for at least three weeks in three distinct regions, in central southern Europe, in the Sahel, and in the humid zone of West Africa. Although the limitations of geolocation prevent matching locations with habitat, these regions are broadly characterized by distinct forest or woodland habitat types, which differ from breeding habitat. All four tracks showed similar patterns in route, stopover behaviour, and timings, suggesting they may be representative of males in these breeding populations, and potentially of other British and western European Wood Warbler populations.

Testosterone Increase in Free-Living Young Blackcaps Sylvia atricapilla and Wood Warblers Phylloscopus sibilatrix During Post-Juvenile Molt with Possible Implications for Juvenile Dispersal

Testosterone may affect many aspects of passerine maturation including the expression of plumage coloration and spring migration in adult birds. However, how the testosterone level changes during bird development is not well known. We compared the testosterone profile during juvenile development of a sexually dimorphic species, the Blackcap Sylvia atricapilla, and a monomorphic species, the Wood Warbler Phylloscopus sibilatrix. We first tested whether testosterone influences plumage coloration and increases at the onset of molt in males of the dimorphic but not the monomorphic species. We found that a testosterone level increase occurred in both sexes and species during later stages of molt, and thus was not related to male plumage coloration. We also investigated whether the increase in testosterone level coincides with juvenile dispersal. If testosterone affects dispersal behavior in these species, both sexes should show an elevated testosterone level during dispersal, but this increase should occur earlier in the Wood Warbler, which disperses earlier than the Blackcap. In juvenile Blackcaps, the increase in testosterone level occurred on the 43-68th day after hatching, while in Wood Warblers it occurred on the 32-36th day (i.e. 11-32 days earlier). The increase in testosterone level coincided with the onset of the post-juvenile dispersal in both species. This study provides the first direct evidence of a testosterone level increase during juvenile development in two free-living migratory birds. While not correlated with species plumage coloration, this increase might affect other aspects of behavior, e.g. juvenile dispersal.

Multi-scale settlement patterns of a migratory songbird in a European primaeval forest

The drivers of animal settlement are core topics in ecology. Studies from primaeval habitats provide valuable but rare insights into natural settlement behaviour, where species are unconstrained by habitat fragmentation and modification. We examined whether territorial male songbirds (wood warblers Phylloscopus sibilatrix) exhibited clustered distributions when settling in extensive primaeval forest. We tested whether settlement patterns were consistent between years, the influence of habitat preference and the spatial extent of these processes. Remote sensing (airborne LiDAR and satellite multi-spectral) and field survey data were combined to investigate settlement at the landscape (tens of km2), plot (46–200 ha) and microhabitat (20 m diameter) scales, by sampling across 5.9 × 4.5 km of the Bialowieza National Park (Poland) during 2018–2019. We assessed bird distribution patterns using nearest neighbour analyses, and habitat preference with generalized linear mixed models. Variation in bird density between plots and years was suggestive of large-scale clustering at the landscape scale. At the local plot scale, any clustering was masked by territoriality, with birds appearing dispersed or randomly distributed. At the microhabitat scale within territories, birds preferred a tall, closed canopy forest structure and avoided a dense herb layer of wild garlic patches. Settlement appeared unconstrained by the availability of preferred habitat at the plot scale, which remained unsaturated. Wood warbler settlement apparently involved hierarchical decisions at multiple spatial scales, compatible with potential social attraction, territoriality, habitat preference or predator avoidance. Selecting an appropriate scale of analysis is an important consideration when assessing the clustering of territorial species. Many songbirds settle in clusters of breeding territories, possibly reflecting patchy habitat quality and/or social attraction. Studies of territory clustering generally come from fragmented, modified habitats, such as secondary forest, which might bias or limit bird distribution. We studied settlement patterns of male wood warblers in an extensive, primaeval forest, where they could exhibit natural behaviour. Spatial analyses of satellite, airborne and field survey data revealed a series of settlement decisions at multiple spatial scales. We found evidence supporting large-scale clustering across the forest, but at the local scale any such clusters were masked by the spacing of birds within territories. A tall forest structure was preferred within the territories, but at larger scales, some birds seemed to compromise habitat quality to settle within a cluster. The results highlight that settlement decisions of animals must be considered at appropriate spatial scales.

Patterns of Haemoproteus majoris (Haemosporida, Haemoproteidae) megalomeront development

Blood parasites of the genus Haemoproteus (Haemosporida, Haemoproteidae) are cosmopolitan and prevalent in birds. Numerous species and lineages of these pathogens have been identified. Some of the infections are lethal in avian hosts mainly due to damage of organs by tissue stages, which remain insufficiently investigated. Several closely related lineages of Haemoproteus majoris, a common parasite of passeriform birds, have been identified. One recent study described megalomeronts of unique morphology in the lineages hPHYBOR04 and hPARUS1 of H. majoris and suggested that the similar tissues stages might also be features in other phylogenetically closely related lineages of the same parasite species. This study aimed to test if (i) megalomeronts are present during the development of the lineage hPHSIB1 of H. majoris and if (ii) they are similar to the other investigated lineages of this species in regard of their morphology and location in organs. One adult wood warbler Phylloscopus sibilatrix, an Afrotropical migrant, naturally infected with H. majoris lineage hPHSIB1 was wild-caught after seasonal spring migration and screened using microscopic examination of blood films and histological sections of organs as well as using PCR-based testing. Bayesian phylogenetic analysis placed the lineages hPHSIB1, hPHYBOR04 and hPARUS1 in one, well-supported clade. Parasitaemia was high (6.5%) in the examined wood warbler, numerous megalomeronts were found in kidneys, and a few in the intestine. Megalomeronts of the lineage hPHSIB1 were morphologically hardly distinguishable from those of lineages hPHYBOR04 and hPARUS1; only negligible differences in the maturation stage of the cytomeres were seen. The kidneys were the main location site of the megalomeronts in all three lineages of this parasite species. This study shows that closely related lineages of H. majoris produce megalomeronts of similar morphology and predominant location in kidneys, while the normal function of this organ may be affected by the presence of numerous large megalomeronts. Megalomeronts of different avian Haemoproteus species are markedly variable in morphology and location, but phylogenetically closely related lineages possess cryptic megalomeronts. This finding suggests that phylogenies based on partial cytb gene could provide information for prediction of patterns of exo-erythrocytic development of closely related Haemoproteus parasites and are worthy of attention in planning haemosporidian parasite tissue stage research.

Nest predator avoidance during habitat selection of a songbird varies with mast peaks and troughs

Predation risk is vital in determining habitat quality. But landscapes of fear are complex and dynamic, such as temperate deciduous forests characterized by intermittent, synchronized production of large seed crops (mast peaks). These “resource pulses” elevate the magnitude of predation risk via direct or indirect effects, possibly altering also predator avoidance strategies of a prey. Yet, how animals manage predation risk via informed habitat selection and how this depends on ecological context remains poorly explored. We studied risk-sensitive settlement behavior of wood warbler Phylloscopus sibilatrix. We tested the hypothesis that warblers flexibly avoid predators (Eurasian jays Garrulus glandarius) during habitat selection but that their interactions are mediated by mast seeding, which is known to elevate the overall nest predation risk for warblers through its cascading effects on population dynamics of rodents and generalist predators. We used a playback experiment to manipulate warblers’ perceived risk of nest predation by jays. We replicated the experiment in two consecutive years, followed by high or low seed crop. In the spring following mast trough, risky jay plots featured low abundance, late settlement, and poor pairing success compared to other treatments. In contrast, no response differences were found in the spring following mast seeding. Our results suggest that the way wood warblers adjust habitat choices to jay predation risk is indirectly mediated by mast seeding. This highlights plasticity in predator avoidance during prey habitat selection in complex and dynamic landscapes of fear, and the potential effects of mast seeding on animal behavior via indirect interactions.Significance statementAvoiding places with high nest predation risk during settlement decisions is believed to be widespread in birds. However, the mechanistic understanding of how birds acquire and use information about ambient nest predation risk to fine-tune settlement decisions is still limited. Using a playback experiment, we show that wood warblers eavesdrop on predator calls and avoid settling at sites with high perceived risk of nest predation by Eurasian jays. However, the way warblers adjust habitat choices to the nest predation risk posed by jays appears to be affected by tree mast seeding. This is potentially due to cascading effects of masting on the population dynamics of rodents and generalist predators, creating temporal peaks of elevated nest predation risk for wood warblers. Our study highlights plasticity in predator avoidance during prey settlement behavior in temporally variable, pulsed-resources ecosystems.

Does acoustically simulated predation risk affect settlement and reproduction of a migratory passerine?

AbstractNest predation is one of the most important drivers of avian life history evolution and population dynamics. Increasing evidence suggests that birds are able to assess nest predation risk and avoid settling in high‐risk areas to increase their reproductive performance. However, the cues used for settlement decisions are poorly known in most species. Population sizes of the migratory wood warbler Phylloscopus sibilatrix are characterized by strong annual fluctuations, which are negatively correlated with the number of forest rodents. Wood warblers might avoid rodent‐rich areas to reduce predation risk arising either from rodents, from rodent‐hunting predators attracted to such areas or from predators not linked to rodents. To evaluate these hypotheses, we conducted a large‐scale field experiment to test whether wood warblers avoided settling in plots with high predation risk simulated by broadcasting vocalizations of rodents or predators. Moreover, we tested whether reproductive performance varied in relation to simulated predation risk. Settlement patterns did not differ between plots with rodent, predator and noise control treatments. Likewise, measures of reproductive performance did not seem to differ across treatments. Thus, the broadcasted vocalizations of rodents and predators did not seem to be perceived as threat by wood warblers. Alternatively, the species might use other cues than those presented here, either other acoustic cues, visual and/or olfactory cues or a combination of cue types during settlement. Further experimental investigations to pin point cues and senses relevant for settlement decisions in wood warblers and birds in general are needed to better understand their life history and population dynamics.

Trophic consequences of mast seeding for avian and mammalian seed and non-seed consumers in European temperate forests

Resource pulses are wide-ranging, influential ecosystem processes with effects permeating throughout the food web, sometimes over several years. In temperate forests, resource pulses may be triggered by mast seeding of one or several tree species, providing a key food source to a multitude of species. However, direct and indirect consequences of mast seeding for various seed and non-seed consumers, and interactions among them, are often poorly understood. Based on a 16-year data set from Germany, we evaluated several hypotheses concerning the relationships between (1) mast seeding and seed consumers, (2) seed and non-seed consumers, and (3) seed or non-seed consumers and extrinsic factors other than mast seeding. Abundances of Eurasian Jays Garrulus glandarius correlated negatively, but abundances of voles positively, with mast seeding of oak in the previous fall, while the abundances of Great Tits Parus major did not appear to be linked to mast seeding. The abundance of non-seed consumers, such as Wood Warblers Phylloscopus sibilatrix, but not Chiffchaffs Phylloscopus collybita, appeared to be linked indirectly to mast seeding of oak via voles. Specifically, Wood Warbler abundance negatively correlated with abundances of voles. Extrinsic factors other than mast seeding appeared to be unimportant. This study shows how the set of factors affecting a species at a large spatial scale may vary from the set of factors acting at smaller spatial scales, as obtained from the literature. Lastly, we illustrate how several taxa at various trophic levels of a temperate forest ecosystem in Central Europe are linked via resource pulses. Assessing ecological processes revolving around seed-based resource pulses is pivotal to understanding how changing mast seeding dynamics may alter an ecosystem.

Autumn migration and wintering site of a wood warbler Phylloscopus sibilatrix breeding in Denmark identified using geolocation

BackgroundBasic knowledge of detailed spatiotemporal migration patterns is lacking for most migratory bird species. Using the smallest available geolocator, we aim to map autumn migration and wintering areas of north European wood warblers Phylloscopus sibilatrix and compare the spatiotemporal pattern with recoveries of individuals ringed across Europe.ResultsA tracked wood warbler migrated south-south-east to sub-Saharan Africa in Sudan and then west-south-west to winter in Côte d’Ivoire. The timing and route fits well within the distribution of ring recoveries although the westward movement after the Sahara crossing is not revealed by the ring recoveries, but only few recoveries south of Sahara exist.ConclusionsThe surprising westward movement south of the Sahara supplements the overall pattern revealed by ring recoveries and aids our understanding of the connectivity and site dependence in this generally declining species.

Patterns of predator behaviour and Wood Warbler Phylloscopus sibilatrix nest survival in a primaeval forest

Understanding the foraging behaviour of predators is key to interpreting the role of anti‐predator adaptations of birds in reducing nest losses. Conducting research in primaeval habitats, with a low level of direct human interference, is particularly valuable in the understanding of predator–prey interactions. Using nest cameras, we investigated the identity and behaviour of potential and actual predators appearing at Wood Warbler Phylloscopus sibilatrix nests, and the importance of different predator groups for nest survival, in the primaeval part of Białowieża Forest (Poland). Mammals formed the main predator group (30 of 32 nest depredations), particularly medium‐sized carnivores (24 of 32), which attacked nests more frequently than merely passing by. This contrasted with other species, especially small rodents, which were commonly recorded near nests but rarely attacked them. Most nest attacks (22 of 32) took place at night and nest survival did not depend on nest visibility, indicating a reduced utility of nest concealment in defence against predators using mainly sound or olfaction when hunting. Daily nest survival declined strongly with nest progression (from egg‐laying to fledging of chicks), probably due to increased predator detection of nests containing older and louder chicks, rather than to increasing parental activity at nests during the day. The set of actual nest predators differed from some previous studies in human‐transformed habitats, showing that Wood Warblers may face different threats in modified vs. near‐pristine environments.

Changes in the area of optimal tree cover of a declining Afro‐Palaearctic migrant across the species’ wintering range

Land use change in sub‐Saharan Africa continues apace, but its role in driving the declines of Afro‐Palaearctic migrant birds is unknown. This is due partly to a lack of knowledge of migrants’ requirements on the wintering grounds, and of spatially explicit assessments of land cover change. We compared tree cover data derived from satellite remote sensing (available for the period 2000–2014) with distributional data from surveys in four West African countries for the Wood Warbler Phylloscopus sibilatrix, one such declining migrant, to determine the extent of, and change in, optimal tree cover. Wood Warblers were most likely to occur where tree cover per hectare was between 40 and 61% (optimal tree cover). Extrapolation to the whole of the wintering range indicated there was a 46.7% net increase in extent between 2000 and 2014. This was due to an alarming 27 683 km2 of previously closed forest being degraded from > 61% cover to between 40 and 61%, an area greater than that of the optimal tree cover that was lost. Increases in optimal tree cover were greatest in countries with greatest forest cover, such as Sierra Leone, Liberia, Côte d'Ivoire and Democratic Republic of Congo. The results suggest that loss of optimal tree cover in the wintering range might not be a key driver of population decline for Wood Warblers, but the degradation will probably impact species that rely upon dense tree cover.

Correction to: Nest predation and the influence of habitat structure on nest predation of Wood Warbler Phylloscopus sibilatrix, a ground-nesting forest passerine

In the original publication of the article, there is a misalignment of the last two columns in table 2. The correct Table 2 is provided below.

Occupation of wood warbler Phylloscopus sibilatrix nests by Myrmica and Lasius ants

Bird nests can provide habitats for various invertebrates, including ectoparasites, scavengers, and predators. Records of ants associating with active bird nests mostly involve the insects searching for food, with some exceptional records of ants raising their broods (eggs, larvae or pupae) within songbird nests in nest-boxes or tree cavities. We present data for a previously undocumented, but apparently regular, occurrence of ants and their broods within the active nests of a songbird, the wood warbler Phylloscopus sibilatrix (Bechstein, 1793), which builds domed nests on the ground in European forests. Systematic recording found ants, mostly Myrmica ruginodis Nylander, 1846, in 43% of 80 wood warbler nests in the primary forest of Bialowieza National Park (Poland) during the springs of 2016–2017, including ant broods in 30%. Ad hoc records from this site in 2004–2015 found ants in a further 29% of 163 nests, including broods in 20%, indicating a regular association. However, examination of 37 nests from secondary forest in Switzerland and Great Britain founds ants in only 14%, and broods in just 5%. We discuss the potential drivers and mechanisms of the observed association between breeding wood warblers and ants, including the apparent difference in frequency between the primary and secondary forests.