Search

Migratory behaviour in birds shows a remarkable variability at species, population and individual levels. Short-distance migrants often adopt a partial migratory strategy and tend to have a flexible migration schedule that allows a more effective response to extreme environmental variations. Weather seasonality and environmental heterogeneity have been reported as significant factors in the diversification of migratory behaviour for Mediterranean migrants, but relatively few studies investigated the migration patterns of non-passerine birds migrating within the Mediterranean basin. In this study, we investigated the migratory strategy of 40 Eurasian Stone-curlews Burhinus oedicnemus tagged with geolocators and GPS-GSM tags and belonging to continental and Mediterranean populations of the Italian peninsula. The proportion of migrants was higher in continental populations, but we observed a significant variability also within Mediterranean populations. All birds spent the winter within the Mediterranean basin. Continental Stone-curlews departed earlier in spring and later in autumn and covered longer distances than those from Mediterranean areas. The speed of migration did not change between seasons for continental birds, while Mediterranean individuals migrated faster in spring. The likelihood of departure for autumn migration of GPS-tagged birds increased when temperatures were near or below 0 °C suggesting that Stone-curlews tend to delay departure until weather conditions worsen abruptly. As a consequence of global warming in the Mediterranean, the frequency of migratory birds in the considered populations might decrease in the near future. This could affect the distribution of species throughout the year and should be taken into account when targeting conservation measures.

Este artículo contiene 20 páginas, 6 figuras, 4 tablas., Aim: Over the last decades, the study of movement through tracking data has grown exceeding the expectations of movement ecologists. This has posed new challenges, specifically when using individual tracking data to infer higher- level distributions (e.g. population and species). Sources of variability such as individual site fidelity (ISF), en-vironmental stochasticity over time, and space-use variability across species ranges must be considered, and their effects identified and corrected, to produce accurate estimates of spatial distribution using tracking data. Innovation: We developed R functions to detect the effect of these sources of vari-ability in the distribution of animal groups when inferred from individual tracking data. These procedures can be adapted for their use in most tracking datasets and tracking techniques. We demonstrated our procedures with simulated datasets and showed their applicability on a real-world dataset containing 1346 year- round migratory trips from 805 individuals of three closely related seabird species breeding in 34 colonies in the Mediterranean Sea and the Atlantic Ocean, spanning 10 years. We detected an effect of ISF in one of the colonies, but no effect of the environmental stochasticity on the distribution of birds for any of the species. We also identified among-colony variability in nonbreeding space use for one species, with significant effects of popu-lation size and longitude. Main conclusions: This work provides a useful, much- needed tool for researchers using animal tracking data to model species distributions or establish conservation measures. This methodology may be applied in studies using individual tracking data to accurately infer the distribution of a population or species and support the deline-ation of important areas for conservation based on tracking data. This step, designed to precede any analysis, has become increasingly relevant with the proliferation of studies using large tracking datasets that has accompanied the globalization process in science driving collaborations and tracking data sharing initiatives., We thank the following institutions for funding: EU H2020 pro-gramme through grant 634495; Seventh Framework Programme (Research Executive Agency) through Marie Curie Career Integration Grant 618841 (FP7-PEOPLE-2013- CIG); ESFRI LifeWatch Project; LIFE programme of the European Commission through projects LIFE10 NAT/MT090 and LIFE11 NAT/IT/000093; Ministerio de Ciencia e Innovación/Ministerio de Economia y Competitividad (Spain) through projects CGL2009- 11278/BOS, CGL2013-42585-P, C G L 2 0 1 3 - 4 2 2 0 3 - R , C G L 2 0 16 - 7 8 5 3 0 - R a n d C G L 2 0 17- 8 52 10 - P ; Organismo Autónomo de Parques Nacionales (Spain) through pro-ject 1248/2014; Fundação para a Ciência e a Tecnologia (MCTES, Portugal) through projects MARE-UID/MAR/04292/2019; IF/00502/2013/CP1186/CT0003, PTDC/BIA-ANM/3743/2014, PTDC/MAR-PRO/0929/2014, UID/AMB/50017/2019 and UIDP/50017/2020 + UIDB/50017/2020 (to CESAM); Office Français de la Biodiversité (France), through the Programme PACOMM, Natura2000 en mer; Hellenic Bird Ringing Centre; MSDEC (Malta). VMP was supported by pre-doctoral contract BES-2014- 068025 of the Spanish Ministerio de Industria, Economía y Competitividad; MM by grant SFRH/BPD/47047/2008 from the Portuguese Foundation for Science and Technology; JMRG by Ph.D. grant AP2009-2163 from the Spanish Ministerio de Educación; GDO and MMü by Ornis italica and by the Regione Siciliana and Assessorato Risorse Agricole e Alimentari thoriugh a grant to the Ringing Unit of Palermo; VHP by grant SFRH/BPD/85024/2012 from the Portuguese Foundation for Science and Technology; VN by grant SFRH/BPD/88914/2012 from the Portuguese Foundation for Science and Technology; and JN by the Spanish National Programme Ramón y Cajal (RYC-2015- 17809); GK and SX were partially funded by the Operational Program “Environment and Sustainable Development” (EPPERAA) of the National Strategic Reference Framework (NSRF) 2007-2013, co- financed by the ERDF and Greek EDP; FdF by a Ph.D. grant from the Coordination for the Improvement of Higher Education Personnel (CAPES—Brazilian government agency; Bex Process 1307/13-4); ZZ by a PhD grant from the University of Barcelona (APIF/2012); MCF by a PhD grant from the University of Barcelona; and RR by post-doctoral contracts of the PLEAMAR programme from MINECO and Fundación Biodiversidad (2017/2349), and Ministerio de Ciencia, in-novación y Universidades (RYC-2017- 22055). This publication is part of the project I+D+i/PID2020-117155GB-I00, funded by MCIN/ AEI/10.13039/501100011033.

Air quality monitoring in cities is significant for both human health and environment. Here, an innovative miniaturized active air sampler wearable by free-flying birds is presented. The device integrates a GPS logger and atmospheric calibrated sensors allowing for high spatiotemporal resolution measurements of carbon dioxide (CO2) concentration, barometric pressure, air temperature, and relative humidity. A field campaign, carried out from January to June 2021, involved the repeated release of homing pigeons (Columba livia) from downtown Rome (Italy), to sample the air on their way back to the loft, located in a rural area out of the city. The measurements suggest the importance of green urban areas in decreasing CO2 levels. Moreover, a positive relation between CO2 levels, relative humidity, and air temperature was revealed. In contrast, a negative relation with distance from the point of release, month, and time of day was found. Flight speed and the altitude of flight were related to rising CO2 levels. The easy use of such devices paves the way for the application of miniaturized air samplers to other synanthropic species (i.e., gulls), making birds convenient biomonitors for the urban environment.

There are growing pressures on marine biodiversity. Seabirds in particular are one the most-threatened groups. The black-vented shearwater (Puffinus opisthomelas) is endemic to Mexican islands and the only shearwater living its entire life cycle in the California Current System, one of the most productive large marine ecosystems in the world. Marine Protected Areas (MPAs) in this region, however, were designed without consideration of accurate data on seabird distributions. Here, 57 black-vented shearwaters were GPS-tracked from their main breeding colony (95% of the global population) over four seasons (2016–2019) to estimate their at-sea distribution. Two methods were applied to identify priority conservation areas: the approach developed by BirdLife International to identify marine Important Bird and Biodiversity Areas and a method using expectation-maximization binary clustering to identify core foraging areas. One potential marine Important Bird and Biodiversity Area close to the breeding colony and five core foraging areas were identified. These priority conservation areas were largely beyond the bounds of the current MPA network in the region. Our results detail opportunities for improving the implementation of conservation and management measures in the California Current System region with respect to seabirds. The approach of combining site identification methods can be applied to other seabird species for which high-resolution tracking data are available and can help guide conservation action plans and MPA design.

Parent–offspring recognition (POR) is fundamental in colonial birds when the potential intermingling of chicks is higher due to the large number and proximity of nests. In species with isolated nests, where chick presence in the nest is strong contextual evidence of kinship, there might be circumstances when the parent might doubt the identity of the chick, but not enough to reject it. Olfactory-based recognition of conspecifics and nest sites in birds has gained strong evidence suggesting a potential role of olfaction in POR. Despite that, there are no studies testing it. We used Scopoli's shearwater (Calonectris diomedea) as model colonial single-brooded species with a developed olfactory sense, usually breeding in well-spaced nests with low probability of chicks mixing. We tested the parent's ability to selectively respond to their own chick as opposed to a simpler rule of feeding any chick found in the nest by means of chick-fostering experiments. We designed two cross-fostering experiments using chicks of different ages to test whether the ability of parents to recognize a related chick develops over time, possibly after acquiring an own distinctive odour. Finally, we also manipulated nests’ odour to disentangle the confounding effect of nest site recognition from POR when parents return at night. All experimental chicks were adopted by parents as the weight and bill growth of cross-fostered chicks did not differ significantly from the control group. We recorded a small difference in weight when foster chicks were inside an odour-manipulated nest; although we did not record weight loss in experimental chicks, only a steeper increase in weight was observed in control chicks. In conclusion, adoption in Scopoli's shearwater seems to follow the rule "if the young is in my nest, accept it" proposed by Beecher (1991) for species with spatially separated nests and low chick mobility. Fundação para a Ciência e Tecnologia - FCT info:eu-repo/semantics/acceptedVersion

The European honey buzzard Pernis apivorus is a summer visitor in Europe, wintering mostly in West-central Africa. Previous studies concerning timing of autumn migration in relation to sex groups provided contrasting results. In particular, a field survey made in southern Sweden did not report differences in timing, while a satellite study via GPS tracking on six adults, three males, and three females, revealed that the latter departed earlier. The aim of this 4-year study is to further investigate the timing of autumn migration in this species carrying out observations at the Strait of Messina, a bottleneck located along the Central Mediterranean flyway, between August 10th and September 30th 2016–2019. Adult European honey buzzards concentrated the passage between late August early September, with females passing on average 5 days earlier than males. It is suggested that a different role of sexes concerning exhibition of territorial displays during the late breeding season, would explain differences between sex groups in timing of both moult and autumn migration.

The Western marsh harrier Circus aeruginosus is a partial migrant with populations breeding in eastern and northern Europe migrating south and wintering mostly in sub-Saharan Africa. During autumn ...

Migratory behaviour in birds shows a remarkable variability at species, population and individual levels. Short-distance migrants, in particular, often adopt a partial migratory strategy and tend to have a rather flexible migration schedule which allows them to respond more effectively to extreme environmental variations, like those due to climate change. Weather seasonality and environmental heterogeneity at regional and local scales have been reported as significant factors in the diversification of migratory behaviour for some species of Mediterranean migrants. Relatively few studies, however, investigated the migration patterns of non-passerine birds migrating within this area. In this study we investigated the migratory strategy of the Eurasian Stone-curlew Burhinus oedicnemus using data collected on 40 individuals tagged with geolocators and GPS-GSM tags, belonging to two continental and two Mediterranean populations of the Italian peninsula. The proportion of migrants was significantly higher in continental populations, but we observed a significant variability also within Mediterranean populations. GPS-tagged migrants traveled less than 1000 km, spending the winter within the Mediterranean basin. Continental Stone-curlews i) departed earlier in spring and later in autumn and ii) covered longer distances than those from Mediterranean areas. The speed of migration did not change between seasons for continental birds, while Mediterranean individuals tended to migrate faster in spring. The likelihood of departure for autumn migration of GPS-tagged birds significantly increased when temperatures were near or below 0 °C suggesting that Stone-curlews tend to delay departure weather conditions worsen abruptly. Thus it can be speculated that the frequency of migratory birds in the considered populations may decrease in the near future due to the effect of global warming in the Mediterranean. This could have a significant effect on the distribution of species throughout the year and should be taken into account when targeting conservation measures.

Abstract Conventional bio-logging techniques used for ethological studies of seabirds have their limitations when studying detailed behaviours at sea. This study uses animal-borne video cameras to reveal fine-scale behaviours, associations with conspecifics and other species and interactions with fishery vessels during foraging of a Mediterranean seabird. The study was conducted on Scopoli's shearwaters (Calonectris diomedea) breeding in Linosa island (35°51′33″ N; 12°51′34″ E) during summer 2020. Foraging events were video recorded from a seabirds' view with lightweight cameras attached to the birds' back. Foraging always occurred in association with other shearwaters. Competitive events between shearwaters were observed, and their frequency was positively correlated to the number of birds in the foraging aggregation. Associations with tunas and sea turtles have been frequent observations at natural foraging sites. During foraging events, video recordings allowed observations of fine-scale behaviours, which would have remained unnoticed with conventional tracking devices. Foraging events could be categorised by prey type into “natural prey” and “fishery discards”. Analysis of the video footage suggests behavioural differences between the two prey type categories. Those differences suggest that the foraging effort between natural prey and fishery discards consumption can vary, which adds new arguments to the discussion about energy trade-offs and choice of foraging strategy. These observations highlight the importance of combining tracking technologies to obtain a complete picture of the at-sea behaviours of seabirds, which is essential for understanding the impact of foraging strategies and seabird-fishery interactions. Graphical abstract

Habitat characteristics determine the presence and distribution of trophic resources shaping seabirds' behavioural responses which may result in physiological consequences. Such physiological consequences in relation to foraging strategies of different life-history stages have been little studied in the wild. Thus, we aim to assess differences in oxidative status, condition (fat stores, i.e. triglyceride levels, TRI), stress (Heterophil/Lymphocyte (H/L) ratio), and leukocyte profiles between incubation and chick rearing highlighting the role of foraging strategies in a seabird (Calonectris diomedea). Chick rearing was more energetically demanding and stressful than incubation as demonstrated by high stress levels (H/L ratio and leukocytes) and lower body stores (assessed by TRI and the increment of weight) due to the high energy requirements of rearing chicks. Also, our results make reconsider the simplistic trade-off model where reproduction increases metabolism and consequently the rate of oxidative stress. In fact, high energy expenditure (VeDBA) during chick rearing was correlated with low levels of oxidative damage likely due to mechanisms at the level of mitochondrial inner membranes (uncoupling proteins or low levels of oxygen partial pressure). Further (more distant) and longer (more days) foraging trips were performed during incubation, when antioxidants showed low levels compared to chick rearing due to incubation fasting, a change in diet, or a combination of these factors; but unlikely because of oxidative shielding since no relation was found between oxidative damage and antioxidant capacity. Males showed higher numbers of monocytes which were positively correlated with antioxidant capacity compared to females, suggesting sexual differences in immune profiles. Species-specific costs and energetic demands of different breeding phases trigger behavioural and physiological adjustments., This study was funded by Deutsche Forschungsgemeinschaft (grant QU148/17-1). In addition, RCC received financial aid from an FPI and a mobility grant from the Spanish Ministry of Economy and Competitiveness (grants BES2012-059299 and EEBB-I-16-11566).

We aimed to describe how debris originated from coastal cities and fisheries circulates and accumulates along the Argentine continental shelf and its potential interaction with southern giant petrels (SGP, Macronectes giganteus). We used tracking data of 31 SGPs (adults and juveniles) from Patagonian colonies. Lagrangian simulations of particles were released from coastal cities and fisheries. Oceanographic features together with plastic input generated a corridor of debris through the Argentine shelf with areas of high debris accumulation, exposing SGP to plastic consumption. During chick provisioning trips 93.9% of petrel's locations overlapped with areas of plastic accumulation. Although early developmental stages were more exposed to particles from cities, the exposure of petrels (all classes) to debris from fisheries was 10% higher than from cities. Measures to reduce debris from fisheries, would reduce plastic ingestion by giant petrels. Proper management of open sky dumpsters would reduce plastic consumption by chicks and juveniles.

Top marine predators are key components of marine food webs. Among them, long-distance migratory seabirds, which travel across different marine ecosystems over the year, may experience important year-round changes in terms of oceanographic conditions and availability of trophic resources. We tested whether this was the case in the Scopoli’s shearwater (Calonectris diomedea), a trans-equatorial migrant and top predator, by sampling birds breeding in three environmentally different regions of the Mediterranean Sea. The analysis of positional data and stable isotopes (δ13 C and δ15N) of target feathers revealed that birds from the three regions were spatially segregated during the breeding period while they shared non-breeding areas in the Atlantic Ocean. Isotopic baseline levels of N and C (meso-zooplankton) were significantly different among marine regions during breeding. Such variation was reflected at the higher trophic levels of pelagic and demersal fish muscles as well as in shearwater feathers grown in the Mediterranean. δ15N- and δ13C-adjusted values of shearwaters were significantly different among populations suggesting that birds from different breeding areas relied on prey species from different trophic levels. Conversely, the non-breeding spatial and isotopic niches overlapped greatly among the three populations. Shearwater trophic niches during breeding were narrower and segregated compared to the non-breeding period, revealing a high plasticity in trophic resource use. Overall, this study highlights seasonal and region-specific use of trophic resources by Scopoli’s shearwater, suggesting a broad trophic plasticity and possibly a high adaptability to environmental changes. Fundação para a Ciência e Tecnologia - FCT info:eu-repo/semantics/publishedVersion

The identification of when, how and where animals feed is essential to estimate the amount of energy they obtain and to study the processes associated with prey search and consumption. We combined the use of animal-borne video cameras and accelerometers to characterise the body and head movements associated to four types of prey capture behaviours in the Magellanic Penguin (Spheniscus magellanicus). In addition, we evaluated how the K-Nearest Neighbour (K-NN) algorithm recognized these behaviours from acceleration data. Finally, we compared the total capture and the capture per unit time (CPUT) derived by identifying prey capture events using the K-NN algorithm to that derived by counting undulations in the dive profile (“wiggles”). During captures, body and head movements were highly variable in the tridimensional space. Energy expenditure (i.e., VeDBA values) during diving periods with prey captures was from three to four times higher than during controls diving periods (i.e., with no capture events). The K-NN classification resulted effective and showed accuracy scores above 90% when considering both head and body related features. In addition, when captures were estimated using the K-NN method, the CPUT was similar or higher to that estimated by counting wiggles. Our study contributes to the knowledge of the trophic ecology of this species and provides an alternative method for estimating prey consumption in the Magellanic Penguin and other diving seabirds.

The flight speeds that animals should adopt to minimise energy expenditure in different scenarios can be predicted by the curve of power against speed. However, relatively few studies have examined how animals respond to the landscape below them, which could affect speed through modifications in climb rate and perceived predation risk. We equipped homing pigeons(Columba livia)with high frequency loggers to examine how flight speed varies in relation to topography and land cover, predicting that these parameters may have a substantial impact on flight speed and hence cost. Pigeons showed mixed evidence for an energy saving strategy, as they minimised climb rates by starting their ascent ahead of hills, but selected rapid speeds in their ascents. Birds did not modify their speed substantially in relation to land cover, but higher speeds were observed during descending flight, highlighting the importance to consider the rate of change in altitude before estimating power use from speed. Finally, we document an unexpected variability in speed and altitude over fine scales; a source of substantial inefficiency. We suggest this may a form of protean behaviour adopted to reduce predation risk when flocking is not an option, and that such a strategy could be widespread.

Durante la migracion de primavera, algunas decenas de miles de abejeros europeos que invernan en el Africa subsahariana cruzan el Mediterraneo central para llegar a sus zonas de reproduccion en Europa central y oriental. Al hacerlo, concentran el paso a traves del canal de Sicilia, pero eligen diferentes rutas migratorias en respuesta a las diferentes condiciones del viento. Este estudio investigo la influencia de las condiciones del viento local y regional en los patrones de movimiento de esta especie a lo largo del estrecho de Mesina, un cuello de botella migratorio ubicado entre el este de Sicilia y el sur de Italia continental, donde algunas aves rapaces son victimas de la caza furtiva cada primavera. Se realizaron observaciones simultaneas en cuatro puntos de observacion, tres en el lado siciliano y uno en el limite continental (lado calabres). Los vientos del noroeste prevalecieron en el Estrecho durante los dias con pico de migracion. No obstante, los patrones locales levemente diferentes de vientos horizontales y verticales en cada sitio de observacion afectaron la formacion de bandadas y dieron forma al paso de las aves rapaces a traves de este cuello de botella, ampliando el frente de migracion. Los resultados confirman que la magnitud de la migracion primaveral del abejero europeo en el Estrecho se ve fuertemente afectada por los patrones de viento en el canal de Sicilia. En particular, los migrantes se concentran en este cuello de botella despues de cruzar el Canal con vientos del noroeste el dia anterior. En conclusion, al interpretar el comportamiento migratorio tanto a escala local como regional, este trabajo puede ayudar a planificar un seguimiento mas eficiente de los abejeros europeos a traves del Estrecho y mejorar la ubicacion de las acciones de conservacion.

International audience; The Mediterranean basin is a hotspot of mercury (Hg) contamination owing to intense anthropogenic emissions, volcanic activity and oligotrophic conditions. Little work has been done to assess the sources of Hg exposure for seabirds and, particularly, the physiological consequences of Hg bioaccumulation. In this study, we (i) describe the individual and temporal variation in blood concentration of total Hg (THg) over three breeding seasons, (ii) identify the factors that affect the THg exposure and (iii) determine the individual- and population-level connections between THg and blood-based markers of oxidative status in chicks of Scopoli’s shearwaters (Calonectris diomedea) breeding on the island of Linosa in the southern Mediterranean. We carried out the work on chicks near fledging because they are fed with prey captured near the colony, thus their Hg levels reflect local contamination. The concentration of THg in erythrocytes varied from 0.23 to 4.29 µg g-1 dw. Chicks that were fed upon higher trophic level prey (i.e., higher δ15N values) had higher THg levels. Individual variation in THg concentrations was not explained by parental identity, sex nor δ13C values. There was significant variation in THg among chicks born from the same mother in different years. We found significant correlations between THg and markers of oxidative status; however, these correlations were no longer significant when we took into account the annual variation in mean values of all metrics. Males with higher values of body condition index had higher blood THg, while THg and body condition index were not correlated in females. Our data indicate that THg levels were moderate to high if compared to other seabirds. However, there is little evidence for harmful short-term detrimental effects owing to THg exposure.

1. Advances in information technology are increasing the use of radar as a tool to investigate and monitor bird migration movements. We set up a field campaign to compare and validate outputs from different radar systems.2. Here we compare the pattern of nocturnal bird migration movements recorded by four different radar systems at a site in southern Sweden. Within the range of the weather radar (WR) Ängelholm, we operated a “BirdScan” (BS) dedicated bird radar, a standard marine radar (MR), and a tracking radar (TR).3. The measures of nightly migration intensities, provided by three of the radars (WR, BS, MR), corresponded well with respect to the relative seasonal course of migration, while absolute migration intensity agreed reasonably only between WR and BS. Flight directions derived from WR, BS and TR corresponded very well, despite very different sample sizes. Estimated mean ground speeds differed among all four systems. The correspondence among systems was highest under clear sky conditions and at high altitudes.4. Synthesis and applications. While different radar systems can provide useful information on nocturnal bird migration, they have distinct strengths and weaknesses, and all require supporting data to allow for species level inference. Weather radars continuously detect avian biomass flows across a wide altitude band, making them a useful tool for monitoring and predictive applications at regional to continental scales that do not rely on resolving individuals. BirdScan and marine radar’s strengths are in local and low altitude applications, such as collision risks with man‐made structures and airport safety, although marine radars should not be trusted for absolute intensities of movement. In quantifying flight behaviour of individuals, tracking radars are the most informative.

Billions of migrating birds travel between their breeding and over-wintering areas twice a year, encountering various environmental conditions along their migration routes. Wind is of utmost importance for birds as wind speed and direction may strongly affect the birds’ flight speed and metabolism. Specifically, tailwinds were found to initiate flight and facilitate higher migration intensity and faster migratory movement while lowering the energetic cost of flight. Using radar, field observations and local meteorological measurements, we studied spring migrating raptors in a migration bottleneck in Italy near the Strait of Messina, between Sicily and Calabria. We explored the effects of wind on flight speed and the intensity of migration in soaring migrants. We found that the birds’ ground speed was positively affected by tailwind speed, and thus, tailwind likely allowed the birds to reach their destination faster. In addition, bird airspeed decreased under increasing tailwind speed, presumably lowering the energetic cost of flight. These findings are in line with predictions of optimal migration flight theory, yet, tailwind had an unexpected negative effect on migration intensity. We suggest that tailwind conditions induced a change in route selection by the migrants at a regional scale, causing a local decline in migration intensity. This change involves the undertaking of cross sea flight rather than overland detour. Furthermore, we found a modular response to crosswinds as birds compensated for winds blowing towards the sea and drifted when winds blew towards land. Our findings suggest that migrating raptors respond to en route wind conditions and coastline geography by adjusting several features of their flight in a manner that will increase their travel speed, reduce the energetic cost of flight, and permit a safe journey. Soaring birds exploit tailwinds to move faster and presumably to reduce the energetic cost of flight during migration. Using radar, we tested how bird flight is affected by tailwind and further predicted that the due to the advantages of using tailwinds, as increased number of birds migrating over the study area. We found that radar-tracked migrating birds flew faster with tailwinds over the study area, but despite this advantage, surprisingly low number of birds migrated when tailwinds prevailed. Furthermore, birds seem to drift with winds blowing from the sea to the land, but not in the opposite direction, suggesting that birds try to avoid drifting over the sea where it is riskier to fly. Hence, our study highlights the complexity of migrant flight behavior in relation to the wind and suggests a flexible response to different wind conditions and coastline geography at multiple spatiotemporal scales.

International audience; Many large birds rely on thermal soaring flight to travel crosscountry. As such, they are under selective pressure to minimise the time spent gaining altitude in thermal updrafts. Birds should be able to maximise their climb rates by maintaining a position close to the thermal core through careful selection of bank angle and airspeed; however, there have been few direct measurements of either parameter. Here, we apply a novel methodology to quantify the bank angles selected by soaring birds using on-board magnetometers. We couple these data with airspeed measurements to parameterise the soaring envelope of two species of Gyps vulture, from which it is possible to predict 'optimal' bank angles. Our results show that these large birds respond to the challenges of gaining altitude in the initial phase of the climb, where thermal updrafts are weak and narrow, by adopting relatively high, and conserved, bank angles (25-35 deg). The bank angle decreased with increasing altitude, in a manner that was broadly consistent with a strategy of maximising the rate of climb. However, the lift coefficients estimated in our study were lower than those predicted by theoretical models and wind-tunnel studies. Overall, our results highlight how the relevant currency for soaring performance changes within individual climbs: when thermal radius is limiting, birds vary bank angle and maintain a constant airspeed, but speed increases later in the climb in order to respond to decreasing air density.