Trends of autumn counts at Iberian migration bottlenecks as a tool for monitoring continental populations of soaring birds in Europe

Migration monitoring may allow us to detect population trends over large geographic areas because the pattern of change in migrant counts may be expected to follow the pattern of change in population size. We analysed recent regional European population trends of migratory soaring birds from rates of change in migration counts over the Strait of Gibraltar (Spain) during the years (1999–2013). An additional bottleneck (Organbidexka, France) within the same migratory route and period was also considered. We estimated count trends by fitting a log-generalized linear model to the time series of each species counts. The counts in Organbidexka were used to test the consistency in the observed trends over the Strait of Gibraltar. Migration counts of White and Black Storks, Black Kites, Short-toed and Booted Eagles as well as Egyptian Vultures showed a linear increase over the Strait of Gibraltar throughout the 15-year period. In contrast, Honey Buzzard numbers remained stable. Trends were highly consistent with those recorded in Organbidexka. We suggest that the larger slopes for the trends in Organbidexka when compared with the Strait reflect an increasing tendency in these species to overwinter in southern Europe. A combination of complementary data sets collected at different bottleneck sites within the European–African flyway system may become a fundamental tool for the investigation of migratory patterns and population trends and changes of soaring migrant birds all over Europe.     

Narrow sea crossings present major obstacles to migrating Griffon Vultures Gyps fulvus

The flight behaviour of Griffon Vultures Gyps fulvus was studied at a major migration bottleneck, the Strait of Gibraltar in southernmost Spain, during the autumns of 2004 to 2007. The 14‐km‐wide sea channel significantly impeded the southern migration of the species into Africa, with many birds attempting repeated passage for weeks before crossing, and others not crossing at all and overwintering in Southern Spain. Water‐crossing attempts were restricted to times between 11:00 and 14:00 h on days with light or variable winds, or on days with strong winds from the north or west. No crossing attempts were made on days with strong winds from the south or east. Vultures attempted to cross the Strait in large flocks and never attempted to do so alone. Although 29% of the birds soared during crossing attempts, at least until they flew beyond visible range of approximately 4 km, most engaged in considerable flapping flight when attempting to cross. Overall, birds flying over water flapped more than 10 times as frequently as those flying over land prior to crossing attempts. Vultures did not flap continuously, but intermittently in brief bouts of flapping interspersed with periods of gliding or soaring flight. The number of flaps per bout over water was significantly greater than the number of flaps per bout over land. Vultures flying over water that flapped at rates of 20 flaps or more per minute typically aborted attempted crossings and returned to Spain in intermittent flapping and gliding flight. There are numerous reports of Vultures falling into the Strait and drowning while attempting to cross, as well as reports of returning Vultures collapsing on the beach having reached Spain in spring ( Barrios Partida 2006 ). Our observations indicate that passage of Griffon Vultures at the Strait of Gibraltar is limited by the species’ over‐water flapping‐flight abilities, including its inability to flap continuously for even short periods of time. We suggest that even relatively short sea crossings represent significant obstacles to migrating Vultures and discuss the implications of this limitation on the distribution and abundance of the species.     

Impact of wind farms on soaring bird populations at a migratory bottleneck

Collision with turbines at wind farms is expected to have a greater impact on birds at particular sites where high concentrations of individuals occur, such as migration bottleneck areas. The Strait of Gibraltar (southern Spain) has long been recognized as the most important bottleneck in western Europe for soaring bird migration. Moreover, this area is within one of the most important potential areas for wind energy generation in Spain. Here, we examine monthly migratory soaring bird abundance in relation to long-term avian mortality rates at 21 wind farms located near the Strait of Gibraltar using zero-inflated hurdle negative binomial and gamma models. Best fit models included an effect of season in the collision mortality rates and in the proportion of adult individuals within the total deaths. However, monthly bird abundance was not directly related to the number of fatalities over the year. The accumulated fatalities during autumn migration constitute a small percentage (1%) of the total migrating population size. Moreover, mortality peak during autumn migration is largely attributable to juvenile birds. In contrast, the number of fatalities coinciding with the breeding period constitutes a substantial proportion (6%) of the local population, and it involved substantial losses among adult birds. Our results show that wind farms probably have an individually low impact on the migratory population of soaring birds. On the contrary, annual losses among adult local birds are remarkably high considering the small size of the local populations, and they may have population level effects.     

Is there a connection between weather at departure sites,onset of migration and timing of soaring‐bird autumn migration in Israel?

Aims Different aspects of soaring‐bird migration are influenced by weather. However, the relationship between weather and the onset of soaring‐bird migration, particularly in autumn, is not clear. Although long‐term migration counts are often unavailable near the breeding areas of many soaring birds in the western Palaearctic, soaring‐bird migration has been systematically monitored in Israel, a region where populations from large geographical areas converge. This study tests several fundamental hypotheses regarding the onset of migration and explores the connection between weather, migration onset and arrival at a distant site. Location Globally gridded meteorological data from the breeding areas in north‐eastern Europe were used as predictive variables in relation to the arrival of soaring migrants in Israel. Methods Inverse modelling was used to study the temporal and spatial influence of weather on initiation of migration based on autumn soaring‐bird migration counts in Israel. Numerous combinations of migration duration and temporal influence of meteorological variables (temperature, sea‐level pressure and precipitable water) were tested with different models for meteorological sensitivity. Results The day of arrival in Israel of white storks, honey buzzards, Levant sparrowhawks and lesser spotted eagles was significantly and strongly related to meteorological conditions in the breeding area days or even weeks before arrival in Israel. The cumulative number of days or cumulative value above or below a meteorological threshold performed significantly better than other models tested. Models provided reliable estimates of migration duration for each species. Main conclusions The meteorological triggers of migration at the breeding grounds differed between species and were related to deteriorating living conditions and deteriorating migratory flight conditions. Soaring birds are sensitive to meteorological triggers at the same period every year and their temporal response to weather appears to be constrained by their annual routine.     

Regional and seasonal flight speeds of soaring migrants and the role of weather conditions at hourly and daily scales

Given that soaring birds travel faster with supportive winds or in good thermal soaring conditions, we expect weather conditions en route of migration to explain commonly observed regional and seasonal patterns in the performance of soaring migrants. We used GPS‐loggers to track 13 honey buzzards and four Montagu's harriers for two to six migrations each. We determined how tailwinds, crosswinds, boundary layer height (a proxy for thermal convection) and precipitation affected hourly speeds, daily distances and daily mean speeds with linear regression models. Honey buzzards mostly travel by soaring while Montagu's harriers supplement soaring with flapping. Therefore, we expect that performance of harriers will be less affected by weather than for buzzards. Weather conditions explained between 30 and 50% of variation in migration performance of both species. Tailwind had the largest effect on hourly speeds, daily mean speeds and daily travel distances. Honey buzzards travelled significantly faster and farther, and Montagu's harriers non‐significantly faster, under better convective conditions. Honey buzzards travelled at slower speeds and shorter distances in crosswinds, whereas harriers maintained high speeds in crosswinds. Weather conditions varied between regions and seasons, and this variation accounted for nearly all regional and seasonal variation in flight performance. Hourly performance was higher than predicted at times when we suspect birds had switched to intermittent or continuous flapping flight, for example during sea‐crossings. The daily travel distance of Montagu's harriers was determined to a significant extent by their daily travel time, which differed between regions, possibly also due to weather conditions. We conclude with the implications of our work for studies on migration phenology and we suggest an important role for high‐resolution telemetry in understanding migratory behavior across entire migratory journeys.     

Revealing patterns of nocturnal migration using the European weather radar network

Nocturnal avian migration flyways remain an elusive concept, as we have largely lacked methods to map their full extent. We used the network of European weather radars to investigate nocturnal bird movements at the scale of the European flyway. We mapped the main migration directions and showed the intensity of movement across part of Europe by extracting biological information from 70 weather radar stations from northern Scandinavia to Portugal, during the autumn migration season of 2016. On average, over the 20 nights and all sites, 389 birds passed per 1 km transect per hour. The night with highest migration intensity showed an average of 1621 birds km^–1 h^–1 passing the radar stations, but there was considerable geographical and temporal variation in migration intensity. The highest intensity of migration was seen in central France. The overall migration directions showed strong southwest components. Migration dynamics were strongly related to synoptic wind conditions. A wind‐related mass migration event occurred immediately after a change in wind conditions, but quickly diminished even when supporting winds continued to prevail. This first continental‐scale study using the European network of weather radars demonstrates the wealth of information available and its potential for investigating large‐scale bird movements, with consequences for ecosystem function, nutrient transfer, human and livestock health, and civil and military aviation.     

Effect of wind,thermal convection,and variation in flight strategies on the daily rhythm and flight paths of migrating raptors at Georgia's Black Sea coast

Every autumn, large numbers of raptors migrate through geographical convergence zones to avoid crossing large bodies of water. At coastal convergence zones, raptors may aggregate along coastlines because of convective or wind conditions. However, the effect of wind and thermal convection on migrating raptors may vary depending on local landscapes and weather, and on the flight strategies of different raptors. From 20 August to 14 October 2008 and 2009, we studied the effect of cloud development and crosswinds on the flight paths of raptors migrating through the eastern Black Sea convergence zone, where coastal lowlands at the foothills of the Pontic Mountains form a geographical bottleneck 5‐km‐wide near Batumi, the capital of the Independent Republic of Ajaria in southwestern Georgia. To identify key correlates of local aggregation, we examined diurnal variation in migration intensity and coastal aggregation of 11 species of raptors categorized based on size and flight strategies. As reported at other convergence zones, migration intensity of large obligate‐soaring species peaked during the core period of thermal activity at mid‐day. When clouds developed over interior mountains and limited thermal convection, these large obligate‐soaring species aggregated near the coast. However, medium‐sized soaring migrants that occasionally use flapping flight did not aggregate at the coast when clouds over the mountains weakened thermal convection. Numbers of alternate soaring‐flapping harriers (Circus spp.) peaked during early morning, with these raptors depending more on flapping flight during a time of day with poor thermal convection. Small sparrowhawks (Accipiter spp.) aggregated at the coast during periods when winds blew offshore, suggesting aggregation caused by wind drift. Thus, weather conditions, including cloud cover and wind speed and direction, can influence the daily rhythm and flight paths of migrating raptors and, therefore, should be accounted for before inferring population trends from migration counts.     

Long‐term changes in autumn migration dates at the Strait of Gibraltar reflect population trends of soaring birds

A growing body of work shows that climate change is the cause of a number of directional shifts in the spring phenology of migratory birds. However, changes in autumn phenology are well studied and their consistency across species, as well as their link with population trends, remains uncertain. We investigate changes in the autumn migration dates of 11 species of soaring birds over the Strait of Gibraltar over a 16‐year period. Using models corrected for phylogeny, we assessed whether ecological and morphological characteristics, as well as population trends, account for interspecific shifts in migration times. We recorded different phenological changes in different periods of the migration season and suggest that these differences are due to age‐dependent responses. The variable best predicting advances in migration dates was population trend: species that did not advance their autumn migration dates were those showing a decline in their European breeding populations. We repeated our tests on a dataset representing the migration date of soaring birds across the Pyrenees Mountains and found that population trends at this site also predicted phenological shifts. Our results suggest that flexibility in migratory strategy and population trends may be related, such that different adaptive capacity in migration timing may be more relevant than other ecological traits in determining the conservation status of migratory birds in Europe and perhaps other regions.     

Non‐breeding areas and timing of migration in relation to weather of Scottish‐breeding common sandpipers Actitis hypoleucos

The number of breeding common sandpipers has declined in Britain due to poorer return rates from non‐breeding areas. To investigate little known aspects of their annual cycle, breeding common sandpipers were fitted with geolocators to track their migrations and determine their non‐breeding areas. Ten tagged birds left Scotland on 9 July (median dates and durations are given throughout the abstract). Short‐term staging was carried out by some birds in England and Ireland, then for longer by most birds in Iberia before continuing to West Africa, arriving on 28 July. Six birds spent most of the non‐breeding season (October–February) on the coast of Guinea‐Bissau, suggesting that this is a key area. Single birds occurred in Sierra Leone, Guinea, the Canary Islands and western Sahara. The southward migration from Scotland took 17.5 d (range 1.5–24 d), excluding the initial fuelling period. The first northward movement from Africa was on 12 April. Staging occurred in either Morocco, Iberia or France. Arrival in Scotland was on 2 May. The northward migration took 16 d (range 13.5–20.5 d). The main migration strategy involved short‐ and medium‐range flights, using tail‐winds in most cases. Variation in strategy was associated with departure date; birds that left later staged for shorter durations. Coastal West Africa provides two major habitats for common sandpipers: mudflats associated with mangroves and rice fields. Although the area of mangrove has been depleted, the scale of loss has probably been insufficient to account for the decline in sandpiper numbers. Rice fields are expanding, providing feeding areas for water‐birds. Meteorological data during the migrations suggest that the weather during the southward migration is unlikely to contribute to a population decline but strong cross‐winds or head‐winds during the northward migration to the breeding grounds may do so.     

Traveling or stopping of migrating birds in relation to wind: an illustration for the osprey

Although it is often assumed that birds strongly prefer tailwindsfor their migratory flights, we predict that a strategy of nowind selectivity (traveling independently of winds) may be morefavorable than wind selectivity (traveling on tailwind occasionsbut stopping to rest under headwind occasions) for birds withlow energy costs of travel relative to rest and for birds thatcannot use stopover time for efficient fuel deposition. We testthis prediction by analyzing the daily traveling or stoppingas recorded by satellite tracking of five ospreys Pandion haliaetus,a species often using energy-saving thermal soaring, duringtheir migration between northern Europe and Africa. Besideswind, precipitation is another weather factor included in theanalyses because thermal soaring migrants are expected to stopand rest in rainy weather. In logistic regression analyses,taking into account the effects of latitude, behavior on previousday, season, date, and individual for discriminating betweentraveling and stopping days, we found a lack of influence ofwinds, suggesting that the ospreys travel or stop without regardto wind. This lack of wind selectivity under light and moderatewinds is in agreement with our prediction. We expect a low degreeof wind selectivity and thus regular flights under headwindsalso among other types of birds that cannot use stopping timefor efficient foraging and fuel deposition. We also found anunexpected lack of influence of precipitation, possibly becauseof relatively few instances with rainfall in combination withpoor geographic precision for estimates of this weather variable.     

Variable detours in long‐distance migration across ecological barriers and their relation to habitat availability at ground

Migration detours, the spatial deviation from the shortest route, are a widespread phenomenon in migratory species, especially if barriers must be crossed. Moving longer distances causes additional efforts in energy and time, and to be adaptive, this should be counterbalanced by favorable condition en route. We compared migration patterns of nightingales that travelled along different flyways from their European breeding sites to the African nonbreeding sites. We tested for deviations from shortest routes and related the observed and expected routes to the habitat availability at ground during autumn and spring migration. All individuals flew detours of varying extent. Detours were largest and seasonally consistent in western flyway birds, whereas birds on the central and eastern flyways showed less detours during autumn migration, but large detours during spring migration (eastern flyway birds). Neither migration durations nor the time of arrival at destination were related to the lengths of detours. Arrival at the breeding site was nearly synchronous in birds flying different detours. Flying detours increased the potential availability of suitable broad‐scale habitats en route only along the western flyway. Habitat availability on observed routes remained similar or even decreased for individuals flying detours on the central or the eastern flyway as compared to shortest routes. Thus, broad‐scale habitat distribution may partially explain detour performance, but the weak detour‐habitat association along central and eastern flyways suggests that other factors shape detour extent regionally. Prime candidate factors are the distribution of small suitable habitat patches at local scale as well as winds specific for the region and altitude.     

Limitations and mechanisms influencing the migratory performance of soaring birds

Migration is costly in terms of time, energy and safety. Optimal migration theory suggests that individual migratory birds will choose between these three costs depending on their motivation and available resources. To test hypotheses about use of migratory strategies by large soaring birds, we used GPS telemetry to track 18 adult, 13 sub‐adult and 15 juvenile Golden Eagles Aquila chrysaetos in eastern North America. Each age‐class had potentially different motivations during migration. During spring, the migratory performance (defined here as the directness of migratory flight) of adults was higher than that of any other age‐classes. Adults also departed earlier and spent less time migrating. Together, these patterns suggest that adults were primarily time‐limited and the other two age‐classes were energy‐limited. However, adults that migrated the longest distances during spring also appeared to take advantage of energy‐conservation strategies such as decreasing their compensation for wind drift. During autumn, birds of all age‐classes were primarily energy‐minimizers; they increased the length of stopovers, flew less direct routes and migrated at a slower pace than during spring. Nonetheless, birds that departed later in autumn flew more directly, indicating that time limitations may have affected their decision‐making. During both seasons, juveniles had the lowest performance, sub‐adults intermediate performance and adults the highest performance. Our results show age‐ and seasonal variation in time and energy‐minimization strategies that are not necessarily exclusive of one another. Beyond time and energy, a complex suite of factors, including weather, experience and navigation ability, influences migratory performance and decision‐making.     

Effect of weather conditions on the spring migration of Eurasian Woodcock and consequences for breeding

Migration is a critical period of time with fitness consequences for birds. The development of tracking technologies now allows researchers to examine how different aspects of bird migration affect population dynamics. Weather conditions experienced during migration are expected to influence movements and, subsequently, the timing of arrival and the energetic costs involved. We analysed satellite‐tracking data from 68 Eurasian Woodcock Scolopax rusticola fitted with Argos satellite tags in the British Isles and France (2012–17). First, we evaluated the effect of weather conditions (temperature, humidity, wind speed and direction, atmospheric stability and visibility) on migration movements of individuals. Then we investigated the consequences for breeding success (age ratio) and brood precocity (early‐brood ratio) population‐level indices while accounting for climatic variables on the breeding grounds. Air temperature, wind and relative humidity were the main variables related to migration movements, with high temperatures and northward winds greatly increasing the probability of onward flights, whereas a trend towards greater humidity over 4 days decreased the probability of movement. Breeding success was mostly affected by climatic variables on the breeding grounds. The proportion of juveniles in autumn was negatively correlated with temperature in May, but positively correlated with precipitation in June and July. Brood precocity was poorly explained by the covariates used in this study. Our data for the Eurasian Woodcock indicate that, although weather conditions during spring migration affect migration movements, they do not have a major influence on subsequent breeding success.     

THE AUTUMN MIGRATION OF SOARING BIRDS AT THE BOSPHORUS

The migration of soaring birds was observed at Küçük Çamlica at the southern end of the Bosphorus between 14 July and 8 November 1966. Simultaneous watches were also carried out at other points on the Bosphorus on a number of dates. The largest movements of birds of prey occurred on days of light northeasterly winds, the largest movements of storks on days of light winds with a southerly component. On most days the stream of migrants appeared to be concentrated over the southern end of the Bosphorus. Migration frequently occurred right throughout the day, though the peak period was usually not spread over more than three hours. Figures are given for the daily times of migration of the commonest soaring birds. Daily counts of soaring birds (storks, raptors and Cranes) migrating over the Bosphorus at Küçük Çamlica are given. The main species found migrating were (with total number recorded in brackets) White Stork Ciconia ciconia (207,145), Black Stork C. nigra (6,194), Honey Buzzard Pernis upivorus (8,997), Buzzard Buteo buteo (12,949), "Spotted" Eagle Aquila clanga/pomarina (4,309) and sparrowhawk Accipiter nisus / brevipes (5,224). The autumn migration of 1966 is discussed in detail in a systematic list. Buzzards B. buteo were recorded in large numbers for the first time at the Bosphorus, and were the commonest bird of prey. Cranes Grus grus were also recorded for the first time. Comparison is made between our results and those of previous workers, though differences of coverage rule out any firm conclusions.     

Individual variation in orientation promotes a 3000‐km latitudinal change in wintering grounds in a long‐distance migratory raptor

Migrating juvenile birds rely on endogenous information in choosing the direction in which to fly, but such input may be overridden by social interactions with experienced individuals. We tagged seven juvenile Short‐toed Eagles Circaetus gallicus with GPS transmitters in southern Italy. This trans‐Saharan migrant flies mainly by soaring and is therefore not well adapted to performing long water crossings. Five of the seven tagged juveniles used the longer but apparently safer route towards the Strait of Gibraltar, and two migrated along a southerly trajectory and subsequently spent the winter in Sicily, apparently forced to do so by the 150‐km‐wide Sicily Channel. One of these individuals took the longer route the following autumn. These results, combined with long‐term (15 years) visual field observations involving thousands of individuals, suggest that inexperienced Short‐toed Eagles may learn their migratory routes from experienced adults, whereas some of them migrate south in response to an innate orientation instinct. Transport costs, inherited information and geography apparently interact, forcing some Short‐toed Eagles to winter 3000 km to the north of the majority of their conspecifics.     

Weather conditions promote route flexibility during open ocean crossing in a long-distance migratory raptor

Weather conditions are paramount in shaping birds’ migratory routes, promoting the evolution of behavioural plasticity and allowing for adaptive decisions on when to depart or stop during migration. Here, we describe and analyze the influence of weather conditions in shaping the sea-crossing stage of the pre-breeding journey made by a long-distance migratory bird, the Eleonora’s falcon (Falco eleonorae), tracked by satellite telemetry from the wintering grounds in the Southern Hemisphere to the breeding sites in the Northern Hemisphere. As far as we know, the data presented here are the first report of repeated oceanic journeys of the same individuals in consecutive years. Our results show inter-annual variability in the routes followed by Eleonora’s falcons when crossing the Strait of Mozambique, between Madagascar and eastern continental Africa. Interestingly, our observations illustrate that individuals show high behavioural plasticity and are able to change their migration route from one year to another in response to weather conditions, thus minimising the risk of long ocean crossing by selecting winds blowing towards Africa for departure and changing the routes to avoid low pressure areas en route. Our results suggest that weather conditions can really act as obstacles during migration, and thus, besides ecological barriers, the migratory behaviour of birds could also be shaped by “meteorological barriers”. We briefly discuss orientation mechanisms used for navigation. Since environmental conditions during migration could cause carry-over effects, we consider that forecasting how global changes of weather patterns will shape the behaviour of migratory birds is of the utmost importance.     

Adverse wind conditions during northward Sahara crossings increase the in‐flight mortality of Black‐tailed Godwits

Long‐distance migratory flights are predicted to be associated with higher mortality rates when individuals encounter adverse weather conditions. However, directly connecting environmental conditions experienced in‐flight with the survival of migrants has proven difficult. We studied how the in‐flight mortality of 53 satellite‐tagged Black‐tailed Godwits (Limosa limosa limosa) during 132 crossings of the Sahara Desert, a major geographical barrier along their migration route between The Netherlands and sub‐Saharan Africa, is correlated with the experienced wind conditions and departure date during both southward and northward migration. We show that godwits experienced higher wind assistance during southward crossings, which seems to reflect local prevailing trade winds. Critically, we found that fatal northward crossings (15 deaths during 61 crossings) were associated with adverse wind conditions. Wind conditions during migration can thus directly influence vital rates. Changing wind conditions associated with global change may thus profoundly influence the costs of long‐distance migration in the future.     

Regional wind patterns likely shape a seasonal migration detour

Migrating animals should optimise time and energy use when migrating, travelling directly to their destination. Detours from the most direct route may arise however because of barriers and weather conditions. Identifying how such situations arise from variable weather conditions is crucial to understand population response in the light of increased anthropogenic climate change. Here we used light-level geolocators to follow Cyprus wheatears for their full annual cycle in two separate years migrating between Cyprus, over the Mediterranean and the Sahara to winter in north–east sub-Saharan Africa. We predicted that any route detours would be related to wind conditions experienced during migration. We found that spring migration for all birds included an eastern detour, whilst autumn migrations were direct across the Sahara. The direct autumn migration was likely a consequence of consistent tail-winds, whilst the eastern detour in spring is likely to be more efficient given the wind conditions which are against a direct route. Such variable migration routes shaped by coincidence with prevailing winds are probably common suggesting that some birds may be able to adapt to future changes in wind conditions.     

Soaring across continents: decision‐making of a soaring migrant under changing atmospheric conditions along an entire flyway

Thermal soaring birds reduce flight‐energy costs by alternatingly gaining altitude in thermals and gliding across the earth's surface. To find out how soaring migrants adjust their flight behaviour to dynamic atmospheric conditions across entire migration routes, we combined optimal soaring migration theory with high‐resolution GPS tracking data of migrating honey buzzards Pernis apivorus and wind data from a global numerical atmospheric model. We compared measurements of gliding air speeds to predictions based on two distinct behavioural benchmarks for thermal soaring flight. The first being a time‐optimal strategy whereby birds alter their gliding air speeds as a function of climb rates to maximize cross‐country air speed over a full climb– glide cycle (V_opt). The second a risk‐averse energy‐efficient strategy at which birds alter their gliding air speed in response to tailwinds/headwinds to maximize the distance travelled in the intended direction during each glide phase (V_bgw). Honey buzzards were gliding on average 2.05 ms^{– 1} slower than V_opt and 3.42 ms^{– 1} faster than V_bgw while they increased air speeds with climb rates and reduced air speeds in tailwinds. They adopted flexible flight strategies gliding mostly near V_bgw under poor soaring conditions and closer to V_opt in good soaring conditions. Honey buzzards most adopted a time‐optimal strategy when crossing the Sahara, and at the onset of spring migration, where and when they met with the best soaring conditions. The buzzards nevertheless glided slower than V_opt during most of their journeys, probably taking time to navigate, orientate and locate suitable thermals, especially in areas with poor thermal convection. Linking novel tracking techniques with optimal migration models clarifies the way birds balance different tradeoffs during migration.