Sylvia Borin Research Articles

Molecular changes and physiological responses involved in migratory bird fuel management and stopover decisions.

Migratory birds undertake long journeys across continents to reach breeding habitats with abundant resources. These migrations are essential for their survival and are shaped by a complex interplay of physiological adaptations, behavioral cues, and gene expression patterns. Central to migration are stopovers, critical resting points where birds replenish energy stores before continuing their journey. In this study, we integrate physiological measurements, behavioral observations, and molecular data from temporarily caged migrating Garden Warblers (Sylvia borin) to gain insights into their stopover strategies and physiological adaptations after crossing the extended ecological barrier formed by the Sahara Desert and the Mediterranean Sea. Depleted individuals, marked by low body mass and flight muscle mass, showcased remarkable plasticity in recovering and rapidly rebuilding energy stores within a short 5-day stopover. Flight muscle mass increased during this period, highlighting a dynamic trade-off between muscle rebuilding and refuelling. Notably, birds prioritizing muscle rebuilding exhibited a trade-off with the downregulation of genes related to lipid transport and metabolism and at the same time showing evidence of skeletal muscle angiogenesis. Early arrivals were more motivated to depart and exhibited higher levels of physiological stress. Our study highlights the importance of understanding the adaptive responses of birds to changing environmental conditions along their migration routes.

Conserving low‐intensity farming is key to halting the declines of migratory passerines in their tropical wintering grounds

AbstractHalf of all migratory bird species have declined over the past 30 years, with intercontinental migrants declining faster than their short‐distance migratory counterparts. One potential cause of these declines is habitat loss and degradation on tropical wintering grounds, where agricultural conversion of natural habitats and intensification of traditional, low‐intensity agricultural systems are frequently occurring. Although the broad patterns of wintering migrant abundance are well understood along most flyways, how species' habitat associations vary across disturbance gradients in agricultural landscapes remains a key question, with implications for landscape‐level farm management and restoration activities. We used 328 point count locations and associated habitat assessments targeted at a cohort of eight severely declining Afro‐Palaearctic migratory passerines in the Guineo–Congolian transition zone of Western Africa to model the probability of the presence of migrants within grass, shrub, forb and forest‐covered areas. We found support for the widespread use of early successional habitats retained within traditionally managed farmland by migrants. Most species utilize scrubland on fallows within the agricultural mosaic, especially Spotted Flycatcher, Garden Warbler, Melodious Warbler, Whinchat and Common Nightingale. Only Pied Flycatcher relied upon mature forested areas. The avoidance of mature forested habitats by most species suggests that habitat requirements of severely declining migrant birds must be explicitly considered within conservation and restoration schemes, via mechanisms to retain low‐intensity farming, especially short‐term abandoned fallows that regenerate scrubby areas within the agricultural matrix. Any habitat management within the agricultural matrix should be considered in the context of the needs of local communities.

Fine-Scale Map Reveals Highly Variable Recombination Rates Associated with Genomic Features in the Eurasian Blackcap.

Recombination is responsible for breaking up haplotypes, influencing genetic variability, and the efficacy of selection. Bird genomes lack the protein PR domain-containing protein 9, a key determinant of recombination dynamics in most metazoans. Historical recombination maps in birds show an apparent stasis in positioning recombination events. This highly conserved recombination pattern over long timescales may constrain the evolution of recombination in birds. At the same time, extensive variation in recombination rate is observed across the genome and between different species of birds. Here, we characterize the fine-scale historical recombination map of an iconic migratory songbird, the Eurasian blackcap (Sylvia atricapilla), using a linkage disequilibrium-based approach that accounts for population demography. Our results reveal variable recombination rates among and within chromosomes, which associate positively with nucleotide diversity and GC content and negatively with chromosome size. Recombination rates increased significantly at regulatory regions but not necessarily at gene bodies. CpG islands are associated strongly with recombination rates, though their specific position and local DNA methylation patterns likely influence this relationship. The association with retrotransposons varied according to specific family and location. Our results also provide evidence of heterogeneous intrachromosomal conservation of recombination maps between the blackcap and its closest sister taxon, the garden warbler. These findings highlight the considerable variability of recombination rates at different scales and the role of specific genomic features in shaping this variation. This study opens the possibility of further investigating the impact of recombination on specific population-genomic features.

Breeding success of passerine birds in the forest steppe of European Russia

In the period from 1985 to 2023, the breeding success of 15 bird species was studied in the Voronezh Nature Reserve (50°21ʹ-52°02ʹ N, 39°21ʹ-39°47ʹ E), where there has been no economic activity for 100 years. The plant cover consists of pine (32.3% of the total area), oak (29.3%), aspen (19.3%), birch (5.7%) and alder (5.2%) forests. 2004 nests were monitored. Low breeding success (25.4-27.9%, determined by the modified Mayfield method) was found in the Blackbird Turdus merula , the Song Thrush T. philomelos , the Finch Fringilla coelebs and the Greenfinch Chloris chloris. From 1.6 to 1.8 fledglings fly out for one breeding attempt. The average level of breeding success (38.7-50.4%) was found in the Common Chiffchaff Phylloscopus collybita , the Red-backed Shrike Lanius collurio , Redwing Turdus iliacus , Eurasian Blackcap Sylvia atricapilla , Garden Warbler S. borin and Hawfinch Coccothraustes coccothraustes . From 2.1 to3.4 fledglings fly out for one breeding attempt. A high level of breeding success (60.6-82.5%) was found in the Great Tit Parus major , the Common Redstart Phoenicurus phoenicurus , the Pied Flycatcher Ficedula hypoleuca , the Collared Flycatcher F. albicollis and the Spotted Flycatcher Muscicapa striata . From 3.7 to 7 fledglings fly out for one breeding attempt. Due to the increased pressure of nest predators, the breeding success of birds in the Voronezh Nature Reserve is often lower than in other areas. The greatest breeding success is typical for birds that place nests in hollows and other shelters. Breeding success is significantly lower in openly nesting species, especially in birds that begin nesting before the leaves bloom on trees and shrubs.

Population trend and breeding productivity of some migrant passerines in Hungary

Abstract This study aimed to monitor the demographic changes of some closely related species based on bird ringing data from the CES (Constant Effort Sites) program in Hungary between 2007 and 2018, and to explore the reasons for these demographic changes. The CES program tracks breeding bird populations with standard methods. The studied species breeding in Hungary were from genera Sylvia, Curruca, and Phylloscopus. Among these species, the trends of some forest birds, like Eurasian Blackcap (Sylvia atricapilla), Lesser Whitethroat (Curruca curruca), Common Chiffchaff (Phylloscopus collybita) and Willow Warbler (Ph. trochilus) showed no substantial changes, and one of the open-habitat species, the Barred Warbler (Curruca nisoria) – a long distant migrant – showed decreasing trends. The Garden Warbler (Sylvia borin) – a trans-Saharan migrant but forest dweller – also experienced population declines. Short-distance migrants maintained stable populations. Common Whitethroat productivity displayed an increasing trend to compensate for population decline. Upon examining the Eurasian Blackcap, this study detected a strong relationship between the number of adult birds, productivity, and the number of adults captured the following year.

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.

Adaptive Regulation of Stopover Refueling during Bird Migration: Insights from Whole Blood Transcriptomics.

Migration is one of the most energy-demanding tasks in avian life cycle. Many birds might not have sufficient fuel stores to cover long distances, so they must stop to rest and refuel at stopover sites, especially after the crossing of large ecological barriers. There, birds undergo several behavioral, morphological, and physiological trait adjustments to recover from and prepare for their journey; however, regulation of such processes at the molecular level remains largely unknown. In this study, we used transcriptomic information from the whole blood of migrating garden warblers (Sylvia borin) to identify key regulatory pathways related to adaptations for migration. Birds were temporarily caged during spring migration stopover and then sampled twice at different refueling states (lean vs. fat), reflecting different migratory stages (stopover arrival vs. departure) after the crossing of an extended ecological barrier. Our results show that top expressed genes during migration are involved in important pathways regarding adaptations to migration at high altitudes such as increase of aerobic capacity and angiogenesis. Gene expression profiles largely reflected the two experimental conditions with several enzymes involved in different aspects of metabolic activity being differentially expressed between states providing several candidate genes for future functional studies. Additionally, we identified several hub genes, upregulated in lean birds that could be involved in the extraordinary phenotypic flexibility in organ mass displayed by avian migrants. Finally, our approach provides novel evidence that regulation of water homeostasis may represent a significant adaptive mechanism, allowing birds to conserve water during long-distance flight, mainly through protein catabolism.

Long-term changes in autumn migration timing of Garden Warblers Sylvia borin at the southern Baltic coast in response to spring, summer and autumn temperatures

Many migrant European birds have been departing their breeding grounds earlier in recent decades in response to rising temperatures from climate change. We examined long-term changes in the autumn migration timing of immature Garden Warblers using data from ringing station Bukowo-Kopań on the Polish Baltic coast in 1967–2018. We calculated an Annual Anomaly (AA) of migration and the dates when 10%, 50% and 90% of migrating birds were caught at each station. We modelled AA and the dates of these percentiles of passage for each station against the year and bi-monthly spring, summer and autumn temperatures as explanatory variables in multiple linear regression models. The overall passage (AA) of immature Garden Warblers advanced by 7 days and the dates of the 50th and 90th passage percentiles advanced by 6–11 days. Overall passage, and dates for 50% of passage occurred later the higher summer temperatures rose in Finland. We suggest favourable foraging conditions in warm summers at these breeding grounds and at first stopover sites delayed the passage because these inexperienced birds took advantage of the conditions to accumulate fuel before migrating south. The overall passage, and the dates of 10%, 50% and 90% of autumn passage occurred the earlier the higher were spring temperatures on spring migration route in Italy, and the higher were mean summer temperatures at breeding grounds in southern Sweden and Norway. We suggest a trans-generation carry-over effect, as warm springs encourage early arrival and nesting of returning adults, thus earlier broods and hatching of juveniles, which also grow faster in warm summers. These effects enable immatures to migrate earlier in autumn after a warm breeding season. The long-term increase in Europe’s spring and summer temperatures thus help explain the shift in Garden Warblers’ migration timing.

The using of data on the moult progress in the study of migration periods of the annual cycle of passerines

Using trapping and ringing of birds for studying the ecology and the demographic parameters of populations is necessary to determine as accurately as possible in which period of the annual cycle each individual was caught. The first-year passerine birds undergo juvenile migration (usually in form of postfledging dispersal) and postmoulting (autumn) migration in summer-autumn period. These stages of annual cycle are difficult to identify in trapping birds by calendar dates because the timing of migration seasons can significantly overlap in the same location. The study of moult at the Ladoga Ornithological Station made it possible to determine the states of plumage that mark juvenile and postmoulting migrations in first-year birds. The article presents two examples of the use of moult progress in the study of bird migrations. The first one is an analysis of the ratio of the terms of departure and passage in three long-distant and three short-distant migrants of passerines. It has been shown that the timing of the departure of local birds is earlier than the timing of the passage of other populations in all studied species, while the last local individuals in long-distance migrants are recorded long before the end of the passage of the species, and in short-distance migrants they can be found until its end. As another example, an analysis of long-term trends in the timing of summer-autumn movements in the Garden Warbler was performed. It is shown that for almost a fifty-year observation period, neither the timing of juvenile migration nor the timing of postmoulting migration of first-year birds has changed. At the same time, the timing of the spring passage of the species shifted to earlier. As a result, the duration of stay of the Garden Warbler in the Ladoga region has increased.

Fuel load and flight range estimation of migrating passerines in the western part of the Carpathian Basin during the autumn migration

Estimating fuel load and potential flight ranges of migrant passerines are basic issues in understanding bird migration strategies. Thirteen sub-Saharan and three pre-Saharan migrant passerine species were analysed in this study. The birds were captured at the Tömörd Bird Ringing Station in the western part of the Carpathian Basin. A general linear model with body mass as the dependent variable and fat score, muscle score and wing length as independent variables were used to estimate lean body mass (body mass without fuel deposits) and fuel load. In ten of the species studied, models considering interactions between factors fit the data better than the main-effect models. Body mass was positively correlated with the fat score in all species, with muscle score in ten species and wing length in 14 species. During autumn, fuel load tended to be larger in the sub-Saharan migrants, especially in four species which pass over the Mediterranean Sea, Common Nightingale (Luscinia megarhynchos), Icterine Warbler (Hippolais icterina), Garden Warbler (Sylvia borin) and Barred Warbler (Curruca nisoria). Nine sub-Saharan migrants, Marsh Warbler (Acrocephalus palustris), Sedge Warbler (A. schoenobaenus), Eurasian Reed Warbler (A. scirpaceus), European Pied Flycatcher (Ficedula hypoleuca), Spotted Flycatcher (Muscicapa striata), Wood Warbler (Phylloscopus sibilatrix), Willow Warbler (Ph. trochilus), Common Whitethroat (C. communis) and Lesser Whitethroat (C. curruca) had estimated flight ranges similar (<1300 km) to two pre-Saharans, European Robin (Erithacus rubecula) and Eurasian Blackap (S. atricapilla). The three short-distance migrants, including the Common Chiffchaff (Ph. collybita) with the shortest distance, had sufficient fuel load to reach their southern European wintering sites without needing to refuel at stopover sites.

Morphology, biochemistry and connectivity of Cluster N and the hippocampal formation in a migratory bird

The exceptional navigational capabilities of migrating birds are based on the perception and integration of a variety of natural orientation cues. The “Wulst” in the forebrain of night-migratory songbirds contains a brain area named “Cluster N”, which is involved in processing directional navigational information derived from the Earth´s magnetic field. Cluster N is medially joined by the hippocampal formation, known to retrieve and utilise navigational information. To investigate the connectivity and neurochemical characteristics of Cluster N and the hippocampal formation of migratory birds, we performed morphological and histochemical analyses based on the expression of calbindin, calretinin, parvalbumin, glutamate receptor type 1 and early growth response protein-1 in the night-migratory Garden warbler (Sylvia borin) and mapped their mutual connections using neuronal tract tracing. The resulting expression patterns revealed regionally restricted neurochemical features, which mapped well onto the hippocampal and hyperpallial substructures known from other avian species. Magnetic field-induced neuronal activation covered caudal parts of the hyperpallium and the medially adjacent hippocampal dorsomedial/dorsolateral subdivisions. Neuronal tract tracings revealed connections between Cluster N and the hippocampal formation with the vast majority originating from the densocellular hyperpallium, either directly or indirectly via the area corticoidea dorsolateralis. Our data indicate that the densocellular hyperpallium could represent a central relay for the transmission of magnetic compass information to the hippocampal formation where it might be integrated with other navigational cues in night-migratory songbirds.

Direct and Indirect Role of Migratory Birds in Spreading CCHFV and WNV: A Multidisciplinary Study on Three Stop-Over Islands in Italy

The annual movements of migratory birds can contribute to the spread of African ticks and tick-borne pathogens of potential public health concern across Europe. The aim of the study was to investigate their role in the possible introduction of African ticks and tick-borne pathogens into European countries during spring migration. A total of 2344 ticks were collected during three spring seasons from 1079 birds captured on three Italian stop-over islands during their northbound migration. Once identified, each tick was tested by RT-PCR for the presence of Crimean-Congo hemorrhagic fever (CCHFV), West Nile (WNV), and Usutu (USUV) viruses. Moreover, carcasses of birds found dead were collected and tested for the possible presence of WNV and USUV. Results confirmed a higher contribution of trans-Saharan migrants compared to intra-Palearctic ones and the prevalence of African tick species in the sample. CCHFV was detected for the second time in Italy in a Hyalomma rufipes, and WNV was found in two ticks of the same genus, all carried by trans-Saharan birds. WNV lineage 1 was also found in the organs of a Garden warbler. These results confirm the role of migratory birds in carrying African ticks, as well as viruses of zoonotic importance, from Africa into Europe.

Access to the sky near the horizon and stars does not play a crucial role in compass calibration of European songbird migrants.

Migratory birds use different global cues including celestial and magnetic information to determine and maintain their seasonally appropriate migratory direction. A hierarchy among different compass systems in songbird migrants is still a matter for discussion owing to highly variable and apparently contradictory results obtained in various experimental studies. How birds decide whether and how they should calibrate their compasses before departure remains unclear. A recent 'extended unified theory' suggested that access to both a view of the sky near the horizon and stars during the cue-conflict exposure might be crucial for the results of cue-conflict experiments. In this study, we performed cue-conflict experiments in three European songbird species with different migratory strategies (garden warbler, Sylvia borin; pied flycatcher, Ficedula hypoleuca; and European robin, Erithacus rubecula; juveniles and adults; spring and autumn migrations) using a uniform experimental protocol. We exposed birds to the natural celestial cues in a shifted (120deg clockwise/counterclockwise) magnetic field from sunset to the end of the nautical twilight and tested them in orientation cages immediately after cue-conflict treatments. None of the species (apart from adult robins) showed any sign of calibration even if they had access to a view of the sky and local surroundings near the horizon and stars during cue-conflict treatments. Based on results of our experiments and data from previous contradictory studies, we suggest that no uniform theory can explain why birds calibrate or do not calibrate their compass systems. Each species (and possibly even different populations) may choose its calibration strategy differently.

Особенности фауны и летнего населения птиц Ашинского природного биологического заказника (Челябинская область)

Based on the results of ornithological research conducted in the Ashinsky State Biological Reserve during several nesting seasons during the period 1990–2021, a preliminary list of bird species was compiled. It includes 107 species, the vast majority of which nest or are expected to nest. To study the bird population, the main types of habitats in the southeastern and southwestern parts of the reserve were examined. The highest indicators of species diversity (42 species) and population density (568.0 ind./km²) were found in avian associations of dark coniferousdeciduous forests. The communities of upland meadows were found to be the most impoverished (16 species and 227.5 ind./km²). Based on these indices, avian associations of floodplain habitats of the Biyanka River occupied an intermediate position (34 species and 385.5 ind./km²).). In the communities of the forest biotopes of the reserve, the chaffinch, redwing, tree pipit, willow tit, white wagtail, garden warbler, common rosefinch and greenish warbler prevailed. In the associations of upland meadows, the skylark, tree pipit, whinchat and yellowhammer dominated. Almost half (43 %) of the nesting species were representatives of the European fauna; the proportions of species of the Euro-Chinese, Siberian and Nomadic types are 19, 15 and 10 %, respectively

The European Autumn Migration Pattern of the Garden Warbler, Sylvia borin – A Basic Analysis of Orientation Cage Field Data

Abstract The introduction of an effective method for studying the local headings of migrants using ‘orientation cages’ has made it possible to study the migration patterns of small passerine nocturnal migrants. Numerous papers have been published on varied samples of local data collected using this method. A rough generalization was presented at the 2019 Meeting of the European Ornithologists Union and subsequently published in The Ring. Case studies on data collected for the Blackcap have already been published, and further studies on other species were suggested. The presented work is the next study of this kind, on the migration pattern of the Garden Warbler, using data from orientation tests collected in autumn as part of the work of the SEEN (SE European Bird Migration Network). The data comprises 2,593 tests performed at 31 ringing sites in Central/Eastern Europe and the Middle East. The paper continues the discussion of application of the method to present migration patterns in a geographically wide territory. The general data evaluation methods in this work are exactly as described in the earlier papers. The paper discusses azimuths of arrival and departure tracks at every ringing site, their linearity, number relations between departing and arriving headings, and the general pattern of migration streams followed by different groups of migrants. The hypothesis put forth earlier, stating that the arrival/departure heading axes shown in studies using orientation cages are situated linearly, is once again confirmed and can be used as a general assumption in this type of study. The average deviations are very small and are negligible for drawing general migration patterns. In some cases, however, there were deviations caused by the geographic location of the study site. This problem should be discussed in more detail when more species data become available. Nine migration streams are defined in the study area, which are presented, for simplicity, using different names and colours on maps. Southward and south-eastward streams are dominant and distributed similarly to the streams of the Blackcap. The most pronounced is a stream shown in yellow (YELLOW stream), which is directed from the wide area of central and eastern Europe to the Arabian Peninsula. The BLACK and NAVY streams are the most intriguing (running nearly longitudinally) and require a great deal of attention in further analyses of migration in the Mediterranean. Two styles of presentation of the species migration pattern are discussed, of which the location style of presentation seems to be more precise. The simpler presentation style that shows only general heading axes could be used to compare the general patterns of different species.

Learning in advance? Interspecific recognition ability in male Eurasian blackcaps

Interspecific aggression is common in bird communities. It is thought to be a consequence of learned behavior or misdirected intraspecific aggression. In the former case birds acquire the ability to identify individuals of other species by social learning of species-specific traits during interactions with heterospecifics in a shared habitat. I conducted a two-choice song playback experiment to investigate the role of associating signals from two sensory modalities, auditory and visual, in shaping the agonistic response of male Eurasian Blackcaps (Sylvia atricapilla) to simulated male Garden Warbler (S. borin) intruders. I measured focal males’ response to playbacks to the song of a Blackcap or a Garden Warbler while giving them a choice between stuffed male models of both species presented in their territories. The experiments were carried out in early spring, before the arrival of Garden Warblers from Africa. I found that male Blackcaps were able to associate species-specific songs with species-specific plumage types. The ability to associate signals characterized not only experienced after-second-year old males, but also second-year old males that had not had the opportunity to defend breeding territories against heterospecifics. This suggests that second-year old male Blackcaps acquire the ability to associate Garden Warbler song with plumage before they use this skill when defending territory during their first breeding episode. Male Blackcaps do not mistake male Garden Warblers for males of their species, but rather learn these associations during the first months of life. However, the possibility that these associations are innate cannot be excluded.

Heat loss in sleeping garden warblers (Sylvia borin) during migration.

For small songbirds, energy is often a limiting factor during migration and, for this reason, they are forced to alternate nocturnal flights with stopovers to rest and replenish energy stores. Stopover duration has a key role for a successful migration and may have an important impact on fitness. Thus, migrants need to optimize their energy consumption at this stage to reduce their permanence at the site. A recent study has shown that lean individuals reduce their metabolic rate when tucking the head in the feathers during sleep. The underlying mechanism is very likely a reduction in conductance, but the thermoregulatory benefit of the increased insulation has never been quantified yet. Here, we compared heat loss in individual migratory birds while sleeping in different postures. Using a thermal camera and a within-individual approach, we estimated that Garden Warblers can reduce their rate of heat loss by 54% by sleeping with the head tucked in the feathers. This energy saving has a relevant impact on the individual's energy balance because it can account for up to 8.69% of daily energy expenditure during stopover. Our study provides novel and important information to understand the fundamental role of thermoregulatory strategies on bird's energy management.

No evidence for the use of magnetic declination for migratory navigation in two songbird species

Determining the East-West position was a classical problem in human sea navigation until accurate clocks were manufactured and sailors were able to measure the difference between local time and a fixed reference to determine longitude. Experienced night-migratory songbirds can correct for East-West physical and virtual magnetic displacements to unknown locations. Migratory birds do not appear to possess a time-different clock sense; therefore, they must solve the longitude problem in a different way. We showed earlier that experienced adult (but not juvenile) Eurasian reed warblers (Acrocephalus scirpaceus) can use magnetic declination (the difference in direction between geographic and magnetic North) to solve this problem when they were virtually displaced from Rybachy on the eastern Baltic coast to Scotland. In this study, we aimed to test how general this effect was. Adult and juvenile European robins (Erithacus rubecula) and adult garden warblers (Sylvia borin) under the same experimental conditions did not respond to this virtual magnetic displacement, suggesting significant variation in how navigational maps are organised in different songbird migrants.

Magnetic compass of garden warblers is not affected by oscillating magnetic fields applied to their eyes

The magnetic compass is an important element of the avian navigation system, which allows migratory birds to solve complex tasks of moving between distant breeding and wintering locations. The photochemical magnetoreception in the eye is believed to be the primary biophysical mechanism behind the magnetic sense of birds. It was shown previously that birds were disoriented in presence of weak oscillating magnetic fields (OMF) with frequencies in the megahertz range. The OMF effect was considered to be a fingerprint of the photochemical magnetoreception in the eye. In this work, we used miniaturized portable magnetic coils attached to the bird’s head to specifically target the compass receptor. We performed behavioural experiments on orientation of long-distance migrants, garden warblers (Sylvia borin), in round arenas. The OMF with the amplitude of about 5 nT was applied locally to the birds’ eyes. Surprisingly, the birds were not disoriented and showed the seasonally appropriate migratory direction. On the contrary, the same birds placed in a homogeneous 5 nT OMF generated by large stationary coils showed clear disorientation. On the basis of these findings, we suggest that the disruption of magnetic orientation of birds by oscillating magnetic fields is not related to photochemical magnetoreceptors in their eyes.

Behavioural response of Eurasian Blackcaps to acoustically simulated conspecific and heterospecific male intruders

Interspecific territoriality can frequently be observed in the animal kingdom, including birds. One important factor driving the degree of interspecific territoriality is competition for resources. Other influential factors are, for instance, population density, phylogenetic relatedness, fighting asymmetries, and shared predation. Although the degree of competition is usually a composite of several factors potentially influencing competition, it is possible to determine the degree of competition without knowing all influential factors and their importance. In line with this, we used the strength of behavioural interactions between species under standardized experimental conditions. In particular, we investigated the degree of interspecific competition using the territorial song of male Eurasian Blackcaps (Sylvia atricapilla) as a low-risk response variable and the crossing flights and nearest approach distance to an intruder as high-risk defence behaviours. To simulate an interspecific intruder, we used male song playbacks of phylogenetic-related and more or less coexisting species. In line with our predictions, we found territorial defence to be most intense against conspecifics (positive control) and virtually non-existent against the allopatric Subalpine Warblers (S. cantillans) (negative control). In terms of the defence reaction to sympatric interspecific intruders, the response seemed strongest in relation to the Garden Warbler (S. borin) and weakest in relation to the Barred Warbler (S. nisoria). Our results further show that the vegetation characteristics of a territory influence behavioural responses to intruders, but only in relation to low-risk defence.