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.     

Around the Mediterranean: an extreme example of loop migration in a long‐distance migratory passerine

An important issue in migration research is how small‐bodied passerines pass over vast geographical barriers; in European–African avian migration, these are represented by the Mediterranean Sea and the Sahara Desert. Eastern (passing eastern Mediterranean), central (passing Apennine Peninsula) and western (via western Mediterranean) major migration flyways are distinguished for European migratory birds. The autumn and spring migration routes may differ (loop migration) and there could be a certain level of individual flexibility in how individuals navigate themselves during a single migration cycle. We used light‐level loggers to map migration routes of barn swallows Hirundo rustica breeding in the centre of a wide putative contact zone between the northeastern and southernwestern European populations that differ in migration flyways utilised and wintering grounds. Our data documented high variation in migration patterns and wintering sites of tracked birds (n = 19 individuals) from a single breeding colony, with evidence for loop migration in all but one of the tracked swallows. In general, two migratory strategies were distinguished. In the first, birds wintering in a belt stretching from southcentral to southern Africa that used an eastern route for both the spring and autumn migration, then shifted their spring migration eastwards (anti‐clockwise loops, n = 12). In the second, birds used an eastern or central route to their wintering grounds in central Africa, shifting the spring migration route westward (clockwise loops, n = 7). In addition, we observed an extremely wide clockwise loop migration encompassing the entire Mediterranean, with one individual utilising both the eastern (autumn) and western (spring) migratory flyway during a single annual migration cycle. Further investigation is needed to ascertain whether clockwise migratory loops encircling the entire Mediterranean also occur other small long‐distance passerine species.     

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.     

Migration speeds among eleven species of long-distance migrating passerines across Europe, the desert and eastern Africa

Based on phenology, passage and median dates gathered from large number of study sites, we measured autumn and spring migration speeds of eleven long distance migratory passerines in four different ecogeographic sectors: Europe, desert, north-eastern and eastern Africa. Results demonstrate that, during the southward autumn migration, late-departing species, such as lesser whitethroat Sylvia curruca , garden warbler S. borin , spotted flycatcher Muscicapa striata , whitethroat S. borin , and willow warbler Phylloscopus trochilus cover their migration route with a slower average migration speed across Europe than do early migrating species. During spring migration, late-departing species (marsh warbler Acrocephalus palustris , garden warbler, spotted flycatcher, red-backed shrike Lanius collurio ) across north-eastern Africa showed a higher speed than early migrating species. Our results show overall shorter migration duration estimates in spring than autumn. Sector-wise seasonal comparisons of duration indicate that migration journey in the African and desert sectors are covered in a relatively shorter time in spring than in autumn. Periods required to cover the distance between northern latitude breeding grounds and desert during both seasons were equivalent.     

Migration of red‐backed shrikes from the Iberian Peninsula: optimal or sub‐optimal detour?

The current Northern Hemisphere migration systems are believed to have arisen since the last glaciation. In many cases, birds do not migrate strait from breeding to non‐breeding areas but fly via a detour. All western European populations of red‐backed shrikes Lanius collurio are assumed to reach their southern African wintering grounds detouring via southeast Europe. Based on theoretical considerations under an optimality framework this detour is apparently optimal. Here, we use individual geolocator data on red‐backed shrikes breeding in Spain to show that these birds do indeed detour via southeast Europe en route to southern Africa where they join other European populations of red‐backed shrikes and return via a similar route in spring. Disregarding potential wind assistance, the routes taken for the tracked birds in autumn were not optimal compared to crossing the barrier directly. For spring migration the situation was quite different with the detour apparently being optimal. However, when considering potential wind assistance estimated total air distances during autumn migration were overall similar and the barrier crossing shorter along the observed routes. We conclude that considering the potential benefit of wind assistance makes the route via southeast Europe likely to be less risky in autumn. However, it cannot be ruled out that other factors, such as following a historical colonisation route could still be important.     

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.     

The effect of wind, season and latitude on the migration speed of white storks Ciconia ciconia, along the eastern migration route

The relation between wind, latitude and daily migration speed along the entire migration route of white storks was analysed. Mean daily migration speed was calculated using satellite telemetry data for autumn and spring migration of white storks from their breeding grounds in Germany and Poland to wintering grounds in Africa and back. The National Center for Environmental Prediction (NCEP) reanalysis data were used to systematically fit 850 mb wind vectors to daily migration speed along the migration route. White storks migrated significantly faster and had a shorter migration season in autumn (10 km/h) compared to spring (6.4 km/h). In autumn mean daily migration speed was significantly slower in Europe (8.0 km/h) than in the Middle East (11.1 km/h) and Africa (11.0 km/h). In spring mean daily migration speed was significantly faster in Africa (10.5 km/h) as birds left their wintering grounds than in the Middle East (4.3 km/h). Migration speed then increased in Europe (6.5 km/h) as birds approached their breeding grounds. In both spring and autumn tailwind (at 850mb) and latitude were found to be significant variables related to daily migration speed.     

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.     

Migration routes and strategies in a highly aerial migrant, the common swift Apus apus, revealed by light-level geolocators

The tracking of small avian migrants has only recently become possible by the use of small light-level geolocators, allowing the reconstruction of whole migration routes, as well as timing and speed of migration and identification of wintering areas. Such information is crucial for evaluating theories about migration strategies and pinpointing critical areas for migrants of potential conservation value. Here we report data about migration in the common swift, a highly aerial and long-distance migrating species for which only limited information based on ringing recoveries about migration routes and wintering areas is available. Six individuals were successfully tracked throughout a complete migration cycle from Sweden to Africa and back. The autumn migration followed a similar route in all individuals, with an initial southward movement through Europe followed by a more southwest-bound course through Western Sahara to Sub-Saharan stopovers, before a south-eastward approach to the final wintering areas in the Congo basin. After approximately six months at wintering sites, which shifted in three of the individuals, spring migration commenced in late April towards a restricted stopover area in West Africa in all but one individual that migrated directly towards north from the wintering area. The first part of spring migration involved a crossing of the Gulf of Guinea in those individuals that visited West Africa. Spring migration was generally wind assisted within Africa, while through Europe variable or head winds were encountered. The average detour at about 50% could be explained by the existence of key feeding sites and wind patterns. The common swift adopts a mixed fly-and-forage strategy, facilitated by its favourable aerodynamic design allowing for efficient use of fuel. This strategy allowed swifts to reach average migration speeds well above 300 km/day in spring, which is higher than possible for similar sized passerines. This study demonstrates that new technology may drastically change our views about migration routes and strategies in small birds, as well as showing the unexpected use of very limited geographical areas during migration that may have important consequences for conservation strategies for migrants.     

Winds at departure shape seasonal patterns of nocturnal bird migration over the North Sea

On their migratory journeys, terrestrial birds can come across large inhospitable areas with limited opportunities to rest and refuel. Flight over these areas poses a risk especially when wind conditions en route are adverse, in which case inhospitable areas can act as an ecological barrier for terrestrial migrants. Thus, within the east-Atlantic flyway, the North Sea can function as an ecological barrier. The main aim of this study was to shed light on seasonal patterns of bird migration in the southern North Sea and determine whether departure decisions on nights of intense migration were related to increased wind assistance. We measured migration characteristics with a radar that was located 18 km off the NW Dutch coast and used simulation models to infer potential departure locations of birds on nights with intense nocturnal bird migration. We calculated headings, track directions, airspeeds, groundspeeds on weak and intense migration nights in both seasons and compared speeds between seasons. Moreover, we tested if departure decisions on intense migration nights were associated with supportive winds. Our results reveal that on the intense migration nights in spring, the mean heading was towards E, and birds departed predominantly from the UK. On intense migration nights in autumn, the majority of birds departed from Denmark, Germany and north of the Netherlands with the mean heading towards SW. Prevailing winds from WSW at departure were supportive of a direct crossing of the North Sea in spring. However, in autumn winds were generally not supportive, which is why many birds exploited positive wind assistance which occurred on intense migration nights. This implies that the seasonal wind regimes over the North Sea alter its migratory dynamics which is reflected in headings, timing and intensity of migration.     

Migration strategies and annual space‐use in an Afro‐Palaearctic aerial insectivore – the European nightjar Caprimulgus europaeus

Obligate insectivorous birds breeding in high latitudes travel thousands of kilometres during annual movements to track the local seasonal peaks of food abundance in a continuously fluctuating resource landscape. Avian migrants use an array of strategies when conducting these movements depending on e.g. morphology, life history traits and environmental factors encountered en route. Here we used geolocators to derive data on the annual space‐use, temporal pattern and migratory strategies in an Afro‐Palaearctic aerial insectivorous bird species – the European nightjar Caprimulgus europaeus. More specifically, we aimed to test a set of hypothesises pertaining to the migration of a population of nightjars breeding in south‐eastern Sweden. We found that the birds wintered across the central and western parts of the southern tropical Africa almost entirely outside the currently described wintering range of the species. The nightjars performed a narrow loop migration across Sahara, with spring Sahel stopovers significantly to the west of autumn stops indicative to an adaptive response to winds during migration. To our surprise, the migration speed was faster in the autumn (119 km d^{? 1}) than in the spring (99 km d^{? 1}), possibly due to the prevailing wind regimes over the Sahara. The estimated flight fraction in both autumn (14%) and spring (12%) was almost exactly as the theoretically predicted 1:7 time relationship between flights and stopovers for small birds. The temporal patterns within the annual cycle indicate that individuals follow alternative spatiotemporal schedules that converge towards the breeding season. The positive relationship between the spatially and temporally distant winter departure and breeding arrival suggests that individuals´ temporal fine‐tuning to breeding may be constrained, leading to potential negative fitness consequences.     

Seasonal variation in migration strategies used to cross ecological barriers in a nearctic migrant wintering in Africa

Ecological barriers such as oceans, mountain ranges or glaciers can have a substantial influence on the evolution of animal migration. Along the migration flyway connecting breeding sites in the North American Arctic and wintering grounds in Europe or Africa, nearctic species are confronted with significant barriers such as the Atlantic Ocean and the Greenland icecap. Using geolocation devices, we identified wintering areas used by ringed plovers nesting in the Canadian High‐Arctic and investigated migration strategies used by these nearctic migrants along the transatlantic route. The main wintering area of the ringed plovers (n = 20) was located in western Africa. We found contrasting seasonal migration patterns, with ringed plovers minimizing continuous flight distances over the ocean in spring by making a detour to stop in Iceland. In autumn, however, most individuals crossed the ocean in one direct flight from southern Greenland to western Europe, as far as southern Spain. This likely resulted from prevailing anti‐clockwise winds associated with the Icelandic low‐pressure system. Moreover, the plovers we tracked largely circumvented the Greenland icecap in autumn, but in spring, some plovers apparently crossed the icecap above the 65°N. Our study highlighted the importance of Iceland as a stepping‐stone during the spring migration and showed that small nearctic migrants can perform non‐stop transatlantic flights from Greenland to southern Europe.     

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.     

BIRD MIGRATION OVER THE MALTESE ISLANDS

The present paper is based on data obtained during several years' observations and three recent surveys. Little has previously been published on migration through the Maltese Islands. The geographical, vegetational and climatic factors of the islands are discussed in so far as they affect the migrants. Visible migration is seen with anticyclonic weather and westerly winds. Birds are found grounded after night migration in cyclonic weather with southwesterly or easterly winds, much larger numbers and variety being seen with the latter. When there is a deterioration in the weather during the night, a large influx of birds is seen on the following morning, and in addition large flocks of migrating Turtle Doves are seen. Several trans-Saharan migrants may pass in smaller numbers during autumn than spring, but the difference may be more apparent than real because in early autumn the birds may depart after only a very short stay, and a few conspicuous species are absent or scarce. By contrast several species which winter north of the Sahara pass only or in much larger numbers during autumn, and these more than make up for those which are absent or rare. There is no evidence from bird ringing that in spring Malta regularly gets birds from Tunisia, at any rate from that part covered by the ringing stations (Cap Bon, Enfidaville, Gabes). The migrants which pass through Malta probably originate from an area in North Africa around Tripoli and some way westwards of it. During autumn the bulk of recoveries is from eastern European countries with a smaller percentage from northern and central Europe. Several species or groups of species are dealt with individually. In the discussion stress is laid on the very close relationship between migration and weather, especially the wind component. The comparatively small numbers of birds seen at Malta probably form part of a larger movement travelling on a broad front. It is argued that the large “falls” of migrants in bad weather result from drift acting on a mass passing mainly to one side or the other of the islands. Since much larger densities are seen with easterly than with westerly winds, it follows, if the hypothesis of drift be correct, that the numbers of birds travelling to the east of Malta are larger than those to the west of it. Moreover, since day migrants are seen with westerly winds and the bulk of night migrants with easterly ones, it is inferred that day migrants normally pass to the west, and the bulk of night migrants to the east, of the Maltese Islands. A parallel is drawn between the autumn migration and the performance of racing pigeons which are flown from the north and NE at this season.     

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 VISIBLE MIGRATION OF RAPTORS OVER THE MALTESE ISLANDS

Whilst the large migrations of raptors at the Bosphorus and at the Straits of Gibraltar have been documented in some detail, the movements which take place across the Mediterranean itself have been neglected. This paper reports observations of the visible migration of raptors over the Maltese Islands during 1969–73. The largest numbers of raptors were recorded both in spring and autumn during contrary winds or overcast conditions. Normally very few were seen before the early afternoon at either season, in spite of the much shorter minimum sea-crossing in autumn. Large passages occurred in winds with easterly or westerly components. It is therefore concluded that eastward drift does not greatly affect the numbers seen in Malta, as had been suggested by De Lucca (1969); rather that most influxes occurred in the late afternoon during unfavourable meteorological conditions. At other times most migration was probably above the visible range. The number of raptors observed on passage in the Maltese Islands was small compared with movements at the Bosphorus or at Gibraltar. Nevertheless, the numbers of the narrow-winged species (i.e., Ospreys, harriers and falcons) compare favourably with records at the narrow crossings. The only large soaring species commonly seen in Malta was the Honey Buzzard. Evidence is presented which suggests that the volume of Honey Buzzard migration across the central Mediterranean may be much larger than was formerly realized.     

Investigating connectivity and seasonal differences in wind assistance in the migration of Common Sandpipers

Many migratory bird species have undergone recent population declines, but there is considerable variation in trends between species and between populations employing different migratory routes. Understanding species-specific migratory behaviours is therefore of critical importance for their conservation. The Common Sandpiper Actitis hypoleucos is an Afro-Palaearctic migratory bird species whose European populations are in decline. We fitted geolocators to individuals breeding in England or wintering in Senegal to determine their migration routes and breeding or non-breeding locations. We used these geolocator data in combination with previously published data from Scottish breeding birds to determine the distributions and migratory connectivity of breeding (English and Scottish) and wintering (Senegalese) populations of the Common Sandpiper, and used simulated random migrations to investigate wind assistance during autumn and spring migration. We revealed that the Common Sandpipers tagged in England spent the winter in West Africa, and that at least some birds wintering in Senegal bred in Scandinavia; this provides insights into the links between European breeding populations and their wintering grounds. Furthermore, birds tagged in England, Scotland and Senegal overlapped considerably in their migration routes and wintering locations, meaning that local breeding populations could be buffered against habitat change, but susceptible to large-scale environmental changes. These findings also suggest that contrasting population trends in England and Scotland are unlikely to be the result of population-specific migration routes and wintering regions. Finally, we found that birds used wind to facilitate their migration in autumn, but less so in spring, when the wind costs associated with their migrations were higher than expected at random. This was despite the wind costs of simulated migrations being significantly lower in spring than in autumn. Indeed, theory suggests that individuals are under greater time pressures in spring than in autumn because of the time constraints associated with reproduction.     

Converging migration routes of Eurasian hobbies Falco subbuteo crossing the African equatorial rain forest

Autumn migration of adult Eurasian hobbies Falco subbuteo from Europe to southern Africa was recorded by satellite telemetry and observed routes were compared with randomly simulated routes. Two non-random features of observed routes were revealed: (i) shifts to more westerly longitudes than straight paths to destinations and (ii) strong route convergence towards a restricted area close to the equator (1 degree S, 15 degrees E). The birds migrated south or southwest to approximately 10 degrees N, where they changed to south-easterly courses. The maximal spread between routes at 10 degrees N (2134 km) rapidly decreased to a minimum (67 km) close to the equator. We found a striking relationship between the route convergence and the distribution of continuous rainforest, suggesting that hobbies minimize flight distance across the forest, concentrating in a corridor where habitat may be more suitable for travelling and foraging. With rainforest forming a possible ecological barrier, many migrants may cross the equator either at 15 degrees E, similar to the hobbies, or at 30-40 degrees E, east of the rainforest where large-scale migration is well documented. Much remains to be understood about the role of the rainforest for the evolution and future of the trans-equatorial Palaearctic-African bird migration systems.     

MIGRATION ACROSS THE SOUTHERN NORTH SEA STUDIED BY RADAR PART 5. MOVEMENTS IN AUGUST, WINTER AND SPRING, AND CONCLUSION

Arrivals of gulls S.W. seem almost confined to late July.
In August, the main movements are W. by day and night of Lapwings and S.S.E. by night of warblers. Sand Martins migrated at dawn directly from a large roost at 34 m.p.h.
In midwinter, migration occurred in at least one direction almost every morning and night. "Hard-weather movements" west or south are initiated primarily by easterly or northerly winds respectively, and not by cold. The corresponding return movements, which sometimes follow immediately, arc initiated primarily by westerly or southerly winds respectively, but warmth has an influence in February. In midwinter, various species are evidently ready to travel in either the autumn or the spring direction, and wind-direction is the paramount factor determining which occurs.
The big eastward departures in March and April occur mainly with westerly winds and in warm weather, both factors being important, but proportionately more departures occur against strong head or cross-winds than in autumn. The general weather situation has no direct influence.
Starlings migrated directly from their roosts on a few mornings each spring, normally at sunrise but on one occasion some did so much earlier.
In late April and May the chief movements are N.N.W. arrivals and onward passages of British summer visitors, N.E. departures of shore-birds, and N.N.E. departures of small passerines, presumably en route between Iberia and Norway. These movements occur mainly with southerly winds, but sometimes against the wind.
"Reversed movements" W. or S.S.E. are much more frequent in spring than has previously been supposed. They occur exclusively with easterly or northerly winds respectively (like the midwinter movements).
The frequency of each migratory movement throughout the year is summarized in Table 6.
The main findings of the 5-year study are reviewed under four heads in the Conclusion.     

The trans‐equatorial loop migration system of Eleonora’s falcon: differences in migration patterns between age classes,regions and seasons

Internal factors such as experience (e.g. age) and motivation for breeding, and external ones such as environmental conditions (e.g. meteorology and landscape characteristics) can promote differences in migratory behaviour and routes among seasons, regions and populations. Using satellite telemetry we investigated whether such differences occur and which factors promote them among migrating Eleonora’s falcons breeding in the Mediterranean area (Spain and Croatia) and wintering in Madagascar. We found that during autumn migration no age differences occur when crossing the Sahara desert, but in the remaining African regions, juveniles were more prone than adults to fly at a slower and more tortuous rate, as well as exhibiting longer stop‐overs, particularly in the Sahel region. Such differences might be promoted by a lower foraging and pre‐migratory fattening efficiency in juveniles. During spring, routes were significantly more eastern than during autumn, resulting in a loop migration occurring in all studied populations. This could be accounted by seasonal variation in the distribution of trophic resources. Our results show that Eleonora’s falcons integrate spatially seasonal changing resources on a continental scale throughout their annual cycle, changing their movement patterns in response to internal (age) and external (habitat) factors. This loop migration pattern may prove to be widespread among other Palearctic trans‐continental migratory bird species.