The influence of wind direction on the capture of the wood warbler (<Emphasis Type="Italic">Phylloscopus sibilatrix</Emphasis>), an uncommon migratory species in the western Mediterranean

The wood warbler (Phylloscopus sibilatrix) is a migratory species in the western Mediterranean wintering in the Gulf of Guinea region, West Africa. In autumn and spring, this species, along with the populations breeding in Ireland and Britain, uses the Italian peninsula as its main axis of migration. From the data of captured birds at several ringing stations in the western Mediterranean (Balearic Islands and coastal Iberian Peninsula), we analyzed capture rates of the species during spring migration from 1993 to 2007. Based on the selection of days with a significant number of captures and those without captures, we analyzed the effect of wind direction over the western Mediterranean to determine a relationship between winds and the number of captures. From a total of 663 wood warblers captured between 1993 and 2007, a total of 31 days have been selected as significant days with a high number of captures, and 31 days have been selected as no-capture days. On days of maximum captures, winds coming from an easterly direction, i.e. Algeria and Tunisia, were dominant, indicating days with a clear eastern component. Contrary to expected results, captures were also made on days when the wind direction was predominantly from a northerly direction. Analysis of the origin of the winds in north eastern Spain (western Mediterranean) revealed that the majority of northerly winds originated from Africa and not from Europe as is usual for this region. Days or periods selected as no-capture days were characterized by winds coming from a northerly (European origin) or westerly direction.     

Departure of migrating European robins, Erithacus rubecula, from a stopover site in relation to wind and rain

Migratory birds replenishing their fuel stores have to decide when to leave their stopover site for the next flight bout. We studied whether the decision to leave a stopover site depends on wind and rain conditions. From capture-recapture data of 1153 European robins collected during three autumns at a stopover site in Switzerland, we estimated the daily emigration probability with a newly developed multistate capture-recapture model that accounts for the occurrence of transients. We tested whether the variation in the daily emigration probabilities can be explained by wind speed, wind direction (both on the ground and 300 m above ground) or rain. Variation in emigration probability was largely explained by variation in wind at 300 m and rain. The emigration probability was highest (0.5) during nights with no or weak (<1.5 m/s) winds at 300 m and no rain, intermediate (0.15-0.2) on nights without rain and with medium wind (>1.5 m/s), and on nights with weak winds (<1.5 m/s) and rain; and almost zero during nights with rain and strong winds at 300 m. Wind direction at 300 m and wind conditions (speed and direction) on the ground had no influence on departure decision. We suggest that birds may consider cues other than wind speed at ground level to predict wind speed at higher altitudes, and that they consider wind direction only when aloft by selecting an optimal flight altitude. Wind speed aloft and rain appeared to be significant factors that synchronize bird migration spatially and temporally.     

Nocturnal migratory flight initiation in reed warblers Acrocephalus scirpaceus: effect of wind on orientation and timing of migration

We used radio-telemetry to study autumn migratory flight initiation and orientation in relation to wind and air pressure in a nocturnal passerine migrant, the reed warbler Acrocephalus scirpaceus at Falsterbo, southwest Sweden. The majority of the reed warblers departed in the expected migratory direction towards south of southwest, while a low number of the birds took off in reverse directions between north and east. Flight directions at departure correlated with wind directions. These correlations were particularly prominent at higher wind speeds but were absent at wind speeds below 4 m/s. Birds departing in the expected migratory direction compensated completely for wind drift. The reed warblers preferred to depart during nights with tailwinds and when air pressure was increasing suggesting that reed warblers are sensitive to winds and air pressure and select favourable wind conditions for their migratory flights. Since air pressure as well as velocity and direction of the wind are correlated with the passage of cyclones, a combination of these weather variables is presumably important for the birds' decision to migrate and should therefore be considered in optimal migration models.     

Stopover durations of three warbler species along their autumn migration route

In migrating birds, the success of migration is determined by stopover duration, the most important factor determining overall speed of migration, and fuel deposition rate. However, very little is known about stopover durations of small migrant birds, because appropriate methods for data analysis were lacking until recently. We used a new capture-recapture analysis to estimate stopover durations of 1st-year reed warblers Acrocephalus scirpaceus, sedge warblers A. schoenobaenus and garden warblers Sylvia borin at 17 stopover sites in Europe and Africa during autumn. Average stopover duration of non-moulting reed warblers was 9.5 days while moulting conspecifics stayed about twice as long. Average stopover duration of sedge warblers was 9.1 days and, in contrast to the other two species, differed between years at several sites. Garden warblers stayed 7.7 days on average. The long stopover duration of the reed warbler, resulting in slow overall migration speed, is related to its low fuel deposition rate. It can be explained by low, but predictable, food resources and an early departure during moult. Compared to the reed warbler, the stopover duration of the sedge warbler varies more between sites and probably also between years, as the supply of its preferred diet (reed aphids) is spatially and temporally unpredictable but can be superabundant. The short stopover duration of the garden warbler, leading to high overall migration speed, can be related to high fuel deposition rates, probably brought about by a change to an abundant, predictable and long-lasting fruit diet. Within species, stopover duration did not change significantly along the migration route. Hence, an increase of migration speed along the migration route, as suggested in the literature, may be caused by longer flight bouts in the south. However, it remains largely unknown which environmental and possibly endogenous factors regulate stopover duration.     

Annual Cycle and Migration Strategies of a Trans-Saharan Migratory Songbird: A Geolocator Study in the Great Reed Warbler

Recent technological advancements now allow us to obtain geographical position data for a wide range of animal movements. Here we used light-level geolocators to study the annual migration cycle in great reed warblers (Acrocephalus arundinaceus), a passerine bird breeding in Eurasia and wintering in sub-Saharan Africa. We were specifically interested in seasonal strategies in routes and schedules of migration. We found that the great reed warblers (all males, no females were included) migrated from the Swedish breeding site in early August. After spending up to three weeks at scattered stopover sites in central to south-eastern Europe, they resumed migration and crossed the Mediterranean Sea and Sahara Desert without lengthy stopovers. They then spread out over a large overwintering area and each bird utilised two (or even three) main wintering sites that were spatially separated by a distinct mid-winter movement. Spring migration initiation date differed widely between individuals (1-27 April). Several males took a more westerly route over the Sahara in spring than in autumn, and in general there were fewer long-distance travels and more frequent shorter stopovers, including one in northern Africa, in spring. The shorter stopovers made spring migration on average faster than autumn migration. There was a strong correlation between the spring departure dates from wintering sites and the arrival dates at the breeding ground. All males had a high migration speed in spring despite large variation in departure dates, indicating a time-minimization strategy to achieve an early arrival at the breeding site; the latter being decisive for high reproductive success in great reed warblers. Our results have important implications for the understanding of long-distance migrants’ ability to predict conditions at distant breeding sites and adapt to rapid environmental change.     

Inability to regain normal body mass despite extensive refuelling in great reed warblers following the trans‐Sahara crossing during spring migration

Migratory birds wintering in Africa face the challenge of passing the Sahara desert with few opportunities to forage. During spring migration birds thus arrive in the Mediterranean area with very low energy reserves after crossing the desert. Since early arrival to the breeding grounds often is of importance to maximize reproductive success, finding stopover sites with good refuelling possibilities after the Saharan passage is of utmost importance. Here we report on extensive fuelling in the great reed warbler Acrocephalus arundinaceus on the south coast of Crete in spring, the first land that they encounter after crossing the Sahara desert and the Mediterranean Sea in this area. Birds were studied at a river mouth and due to an exceptional high recapture rate (45 and 51% in two successive years), we were able to get information about stopover behaviour in 56 individual great reed warblers during two spring seasons. The large proportion of trapped great reed warbler compared to other species and the large number of recaptures suggest that great reed warblers actively choose this area for stopover. They stayed on average 3–4 d, increased on average about 3.5 g in body mass and the average rate of body mass increase was 4.8% of lean body mass d^–1. Wing length affected the rate of increase and indicated that females have a slower increase than males. The results found show that great reed warblers at this site regularly deposit larger fuel loads than needed for one continued flight stage. The low body mass found in great reed warblers (also in birds with high fat scores) is a strong indication that birds staging at Anapodaris still had not been able to rebuild their structural tissue after the strenuous Sahara crossing, suggesting that rebuilding structural tissue may take longer time than previously thought.     

Body fat influences departure from stopover sites in migratory birds: evidence from whole-island telemetry

Migration remains one of the great mysteries of animal life. Small migratory birds rely on refuelling stopovers after crossing ecological barriers such as deserts or seas. Previous studies have suggested that fuel reserves may determine stopover duration but this hypothesis could not be tested because of methodological limitations. Here, we provide evidence that subcutaneous fat stores determine stopover duration by measuring the permanence of migratory garden warblers (Sylvia borin) on a small Mediterranean island during spring migration with telemetry methods. Garden warblers with large amounts of fat stores departed the island significantly sooner than lean birds. All except one fat bird left the island on the same evening after capture, with a mean total stopover estimate of 8.8 hours. In contrast, the mean estimated total stopover duration of lean birds was 41.3 hours. To our knowledge, this is the first study that measures the true minimum stopover duration of a songbird during migration.     

Reed warbler orientation: initiation of nocturnal migratory flights in relation to visibility of celestial cues at dusk

We used radiotelemetry to investigate the time of migratory flight initiation relative to available celestial orientation cues and departure direction of a nocturnal passerine migrant, the reed warbler, Acrocephalus scirpaceus, during autumn migration. The study was carried out at Falsterbo, a coastal site in southwest Sweden. The warblers initiated migration from times well after local sunset and well into the night, corresponding to sun elevations between -4 degrees and -35 degrees, coinciding with the occurrence of stars at night. They departed in the expected migratory direction towards south of southwest with a few initiating migration in reverse directions towards northeast to east. Flight directions under overcast conditions (7-8/8) were more scattered than under clear sky conditions (0-4/8). There were fewer clouds on departure nights than on nights when the birds did not initiate migration. For birds staying longer than one night at stopover the horizontal visibility was higher and precipitation was less likely on departure nights than on the previous night. The results show that the visibility of celestial cues, and stars in particular, are important for the decision to initiate migration in reed warblers. However, cloud cover, horizontal visibility and precipitation might be correlated with other weather variables (i.e. wind or air pressure) that are also likely to be important for the decision to migrate. Copyright 2001 The Association for the Study of Animal Behaviour.     

Morning flight behavior of nocturnally migrating birds along the western basin of Lake Erie

Many species of birds that normally migrate during the night have been observed engaging in so‐called morning flights during the early morning. The results of previous studies have supported the hypothesis that one function of morning flights is to compensate for wind drift that birds experienced during the night. Our objective was to further explore this hypothesis in a unique geographic context. We determined the orientation of morning flights along the southern shore of Lake Erie's western basin during the spring migrations of 2016 and 2017. This orientation was then compared to the observed orientation of nocturnal migration. Additionally, the orientation of the birds engaged in morning flights following nights with drifting winds was compared to that of birds following nights with non‐drifting winds. The morning flights of most birds at our observation site were oriented to the west‐northwest, following the southern coast of Lake Erie. Given that nocturnal migration was oriented generally east of north, the orientation of morning flight necessarily reflected compensation for accumulated, seasonal wind drift resulting from prevailingly westerly winds. However, the orientation of morning flights was similar following nights with drifting and non‐drifting winds, suggesting that birds on any given morning were not necessarily re‐orienting as an immediate response to drift that occurred the previous night. Given the topographical characteristics of our observation area, the west‐northwest movement of birds in our study is likely best explained as a more complex interaction that could include some combination of compensation for wind drift, a search for suitable stopover habitat, flying in a direction that minimizes any loss in progressing northward toward the migratory goal, and avoidance of a lake crossing.     

Can wind help explain seasonal differences in avian migration speed?

A bird's ground speed is influenced by the wind conditions it encounters. Wind conditions, although variable, are not entirely random. Instead, wind exhibits persistent spatial and temporal dynamics described by the general circulation of the atmosphere. As such, in certain geographical areas wind's assistance (or hindrance) on migratory flight is also persistent, being dependent upon the bird's migratory direction in relation to prevailing wind conditions. We propose that, considering the western migration route of nocturnal migrants through Europe, winds should be more supportive in spring than in autumn. Thus, we expect higher ground speeds, contributing to higher overall migration speeds, in spring. To test whether winds were more supportive in spring than autumn, we quantified monthly wind conditions within western Europe relative to the seasonal direction of migration using 30 years (1978–2008) of wind data from the NCEP/NCAR Reanalysis dataset. We found that supporting winds were significantly more frequent for spring migration compared to autumn and up to twice as frequent at higher altitudes. We then analyzed three years (2006–2008) of nocturnal migratory ground speeds measured with radar in the Netherlands which confirmed higher ground speeds in spring than autumn. This seasonal difference in ground speed suggests a 16.9% increase in migration speed from autumn to spring. These results stress the importance of considering the specific wind conditions experienced by birds when interpreting migration speed. We provide a simple methodological approach enabling researchers to quantify regional wind conditions for any geographic area and time period of interest.     

A long-distance avian migrant compensates for longitudinal displacement during spring migration

In order to perform true bicoordinate navigation, migratory birds need to be able to determine geographic latitude and longitude. The determination of latitude is relatively easy from either stellar or magnetic cues [1-3], but the determination of longitude seems challenging [4, 5]. It has therefore been suggested that migrating birds are unable to perform bicoordinate navigation and that they probably only determine latitude during their return migration [5]. However, proper testing of this hypothesis requires displacement experiments with night-migratory songbirds in spring that have not been performed. We therefore displaced migrating Eurasian reed warblers (Acrocephalus scirpaceus) during spring migration about 1000 km toward the east and found that they were correcting for displacements by shifting their orientation from the northeast at the capture site to the northwest after the displacement. This new direction would lead them to their expected breeding areas. Our results suggest that Eurasian reed warblers are able to determine longitude and perform bicoordinate navigation. This finding is surprising and presents a new intellectual challenge to bird migration researchers, namely, which cues enable birds to determine their east-west position.     

Landscape movements of migratory birds and bats reveal an expanded scale of stopover

Many species of birds and bats undertake seasonal migrations between breeding and over-wintering sites. En-route, migrants alternate periods of flight with time spent at stopover--the time and space where individuals rest and refuel for subsequent flights. We assessed the spatial scale of movements made by migrants during stopover by using an array of automated telemetry receivers with multiple antennae to track the daily location of individuals over a geographic area ~20 × 40 km. We tracked the movements of 322 individuals of seven migratory vertebrate species (5 passerines, 1 owl and 1 bat) during spring and fall migratory stopover on and adjacent to a large lake peninsula. Our results show that many individuals leaving their capture site relocate within the same landscape at some point during stopover, moving as much as 30 km distant from their site of initial capture. We show that many apparent nocturnal departures from stopover sites are not a resumption of migration in the strictest sense, but are instead relocations that represent continued stopover at a broader spatial scale.     

Wind conditions affect stopover decisions and fuel stores of shorebirds migrating through the south Yellow Sea

We assessed the effects of wind conditions on stopover decisions and fuel stores of migratory shorebirds at Chongming Dongtan in the south Yellow Sea along the East Asian–Australasian Flyway. In spring and autumn, wind directions differed among altitudes and wind speed generally increased with altitude. Numbers of shorebirds were related to wind effects at low altitudes (on the ground and at 300 and 800 m above the ground), wind effects at 300 m being the best predictor of shorebird numbers. In spring, total number of shorebirds and numbers of the four most abundant shorebird species were negatively related to wind assistance at low altitudes, more birds departing when tailwinds prevailed and more arriving when headwinds prevailed. In autumn, however, total number of shorebirds and numbers of the four most abundant species were positively related to wind assistance at low altitudes, more birds departing and more arriving with tailwinds than with headwinds. When tailwinds prevailed, the number of arriving birds was higher than the number of departing birds. The fuel stores of captured shorebirds, represented by their body mass, was related to wind effects and change in wind conditions between two consecutive days in both spring and autumn, captured birds being heavier when headwinds prevailed than in tailwind conditions, and when the wind conditions became less favourable for flight between two consecutive days. Our results suggest that wind conditions affect stopover decisions and fuel stores, and thus the optimal migration and fuel deposition strategies of migratory shorebirds.     

Towards a new understanding of migration timing: slower spring than autumn migration in geese reflects different decision rules for stopover use and departure

According to migration theory and several empirical studies, long‐distance migrants are more time‐limited during spring migration and should therefore migrate faster in spring than in autumn. Competition for the best breeding sites is supposed to be the main driver, but timing of migration is often also influenced by environmental factors such as food availability and wind conditions. Using GPS tags, we tracked 65 greater white‐fronted geese Anser albifrons migrating between western Europe and the Russian Arctic during spring and autumn migration over six different years. Contrary to theory, our birds took considerably longer for spring migration (83 days) than autumn migration (42 days). This difference in duration was mainly determined by time spent at stopovers. Timing and space use during migration suggest that the birds were using different strategies in the two seasons: In spring they spread out in a wide front to acquire extra energy stores in many successive stopover sites (to fuel capital breeding), which is in accordance with previous results that white‐fronted geese follow the green wave of spring growth. In autumn they filled up their stores close to the breeding grounds and waited for supportive wind conditions to quickly move to their wintering grounds. Selection for supportive winds was stronger in autumn, when general wind conditions were less favourable than in spring, leading to similar flight speeds in the two seasons. In combination with less stopover time in autumn this led to faster autumn than spring migration. White‐fronted geese thus differ from theory that spring migration is faster than autumn migration. We expect our findings of different decision rules between the two migratory seasons to apply more generally, in particular in large birds in which capital breeding is common, and in birds that meet other environmental conditions along their migration route in autumn than in spring.     

The influence of weather on the flight altitude of nocturnal migrants in mid‐latitudes

By altering its flight altitude, a bird can change the atmospheric conditions it experiences during migration. Although many factors may influence a bird's choice of altitude, wind is generally accepted as being the most influential. However, the influence of wind is not clearly understood, particularly outside the trade‐wind zone, and other factors may play a role. We used operational weather radar to measure the flight altitudes of nocturnally migrating birds during spring and autumn in the Netherlands. We first assessed whether the nocturnal altitudinal distribution of proportional bird density could be explained by the vertical distribution of wind support using three different methods. We then used generalized additive models to assess which atmospheric variables, in addition to altitude, best explained variability in proportional bird density per altitudinal layer each night. Migrants generally remained at low altitudes, and flight altitude explained 52 and 73% of the observed variability in proportional bird density in spring and autumn, respectively. Overall, there were weak correlations between altitudinal distributions of wind support and proportional bird density. Improving tailwind support with height increased the probability of birds climbing to higher altitude, but when birds did fly higher than normal, they generally concentrated around the lowest altitude with acceptable wind conditions. The generalized additive model analysis also indicated an influence of temperature on flight altitudes, suggesting that birds avoided colder layers. These findings suggested that birds increased flight altitudes to seek out more supportive winds when wind conditions near the surface were prohibitive. Thus, birds did not select flight altitudes only to optimize wind support. Rather, they preferred to fly at low altitudes unless wind conditions there were unsupportive of migration. Overall, flight altitudes of birds in relation to environmental conditions appear to reflect a balance between different adaptive pressures.     

基于卫星跟踪大天鹅春季迁徙时间及其影响因素

候鸟春季迁徙时间与其能否顺利完成迁徙过程,以及对繁殖地的成功选择和繁殖成效密切相关,通过对大天鹅越冬地和停歇地的春季迁徙时间选择原因及其影响因素进行分析,有助于深入理解候鸟春季迁徙时间策略和栖息地保护状况。2015年2月和12月,在河南三门峡湿地捕捉了60只越冬大天鹅并佩戴卫星跟踪器,获取了详细的大天鹅在越冬地和停歇地的春季迁徙时间等信息,并进一步分析了春季迁徙时间与气候因子的相关性。结果表明,大天鹅主要在夜间22：00-0：00和0：00-1：59迁离越冬地。大天鹅迁离越冬地的日期与温度呈显著性正相关,迁离时多选择顺风且风速较小的天气。大天鹅迁离越冬地后主要是在夜间飞行,而迁离停歇地后主要是在日间飞行。越冬地的温度越高,越有利于植物的生长,大天鹅可以快速地积累能量,提前开始春季迁徙。野外调查表明,内蒙古黄河中上游作为迁徙的重要停歇地,春季两岸捕鱼、农耕和放牧等为活动较多,因此大天鹅多选择在夜间觅食补充能量,在日间迁离。最后,针对黄河流域大天鹅栖息地的保护现状,提出了禁止经济开发项目、建立保护区和开展宣传教育等保护建议。     

Fall migratory departure decisions and routes of blackpoll warblers Setophaga striata and red‐eyed vireos Vireo olivaceus at a coastal barrier in the Gulf of Maine

Each year, millions of songbirds concentrate in coastal areas during fall migration. The choices birds make at the coast about stopover habitat use and migratory route can influence both the success of their migratory journey and fitness in subsequent life stages. We made use of a regional‐scale automated radio telemetry array to study stopover and migratory flights and migratory routes of blackpoll warblers Setophaga striata and red‐eyed vireos Vireo olivaceus during fall migration in the Gulf of Maine, USA. We focused on differences between species, sexes, age groups, breeding origins, and time of year. Both species made within‐stopover relocations (i.e. ‘stopover flights’) from the coastal capture site. Stopover flights were primarily oriented inland, and were more frequent for blackpolls (87%) than vireos (44%). By studying migratory behavior at a broad spatial scale, we demonstrated that most blackpolls and vireos took coastal and offshore routes through the Gulf of Maine, despite initially relocating inland from the capture site. Though we captured blackpolls and vireos from a broad breeding range, more than 70% of migratory flights from the capture site were oriented for coastal or offshore travel for both species, suggesting that birds actively chose coastal and offshore routes, and were not simply displaced by wind drift. Later vireos oriented offshore more frequently during migratory flights from the coast, indicating that they may be more inclined towards time‐minimizing overwater flight routes and thus more exposed to coastal and offshore collision hazards than earlier conspecifics.     

Migratory stopover conditions affect the developmental state of male gonads in garden warblers (Sylvia borin)

Long-distance migrants face the challenge of a short window for reproduction that requires optimal timing and full functional gonads. Male garden warblers (Sylvia borin) meet these demands by initiating testicular recrudescence during spring migration, enabling them to reproduce immediately after arrival at the breeding grounds. In a combined field and laboratory study, we investigated testicular size, plasma levels of luteinizing hormone (LH), androstenedione (AE), 5α-dihydrotestosterone (DHT), testosterone and nocturnal migratory restlessness (Zugunruhe) under different stopover conditions. We manipulated food availability, the duration of stopover and simulated migration by food deprivation. Garden warblers showed significantly retarded testicular development after nine days of stopover under limited food conditions compared to birds that had ad libitum access to food. However, there was no significant difference in Zugunruhe between the two groups. Thus, the degree of Zugunruhe was unaffected by the quality of the stopover site and migration continued independent of the developmental state of the testis. We suggest that male garden warblers face the necessity to either compensate for slowed testicular recrudescence during the subsequent leg of migration and delay arrival at the breeding grounds, or arrive with less developed testes. Either of these may reduce annual reproductive success.     

Optimal stopover decisions under wind influence: the effects of correlated winds

Wind speed and direction have a significant effect on a flying bird's ground speed. Migrants are therefore expected to be sensitive to wind conditions and this should have consequences for optimal strategies of stopover and refuelling. Based on an earlier model of time-minimizing migration which includes wind condition, we investigate the consequences of the temporal correlation of wind conditions. Day-to-day changes in wind conditions are modelled with a two-state Markov process and an expression for the expected speed of migration is derived. The policy of the migrants is described by two parameters: a day t(g) when the birds start to leave whenever favourable conditions occur and a later day t(b)when they leave even in unfavourable winds. The model predicts that in most cases departures should be close to the date which is predicted by a wind-free deterministic model and that the birds should never leave without wind assistance. Only if the probability that the condition remains the same on the following day is close to 1 should the birds leave even in unfavourable conditions shortly after the deterministic optimal date. If the transition matrix is highly asymmetrical, i.e. if it is very probable that unfavourable conditions remain and that favourable conditions will change into unfavourable, then the birds are predicted to start using good winds several days before the deterministic optimal date. An analysis of six years of wind data from two sites in Sweden shows that wind directions on successive days are in fact correlated in all years.     

Optimal use of wind by migrating birds: combined drift and overcompensation

Migrating birds may save flying time by allowing themselves to be partially drifted by strong winds at high altitude and correcting for the displacement at low altitude under relatively weaker winds. This behaviour will be favourable with strong upper winds and with wind direction approx. 30 ° to 90 ° in relation to the goal direction (following side winds). Radar observations of drift in high altitude bird migration and visual records of low altitude overcompensation are compatible with the optimal flight behaviour of migrants at high and low altitude, respectively, as predicted from this hypothesis.