weather and bird migration - british birds · pdf filecult or even impossible. in reality, the...

238 © British Birds 98 • May 2005 • 238-256

ABSTRACT Bird migration in and through Britain and Ireland shows greatvariation, both seasonally and annually. Much depends on atmospheric

conditions, which can equally hinder and assist migration, both on a broadscale (the distribution, extent, intensity and movement of pressure patterns),and at a local level (temporal and spatial changes in wind, visibility, cloud andprecipitation).All the parameters of bird migration, including timing, routes,speed and duration of migration, are affected, and modified, by a number ofmeteorological factors, probably the most influential being wind speed and

direction.This paper reviews the influence that weather has on bird migrationin Britain and Ireland, and discusses examples of the major types of migration.

A number of unusual or exceptional migration events are discussed andillustrated with appropriate weather charts.

Weather and bird migration

Norman Elkins

Richard Johnson

IntroductionMigration is one topic that never fails to appealto the whole spectrum of those with an interestin birds, from the inveterate ‘twitcher’ to thegeneral public. Bird migration, defined simplyas regular movements from one area to another,generally occurs in response to the availabilityof food resources and encompasses an enor-mous variety of methods. These range from

long-distance expeditions traversing half theplanet, such as those by Arctic Terns Sterna par-adisaea and Barn Swallows Hirundo rustica, tolocal movements in response to the temporaryfreezing of freshwater bodies. Differences inmigration strategy and behaviour result insome species undertaking nocturnal migration,while others move mainly during daylighthours. These differences operate not only

between species, but also among populations ofthe same species. The continued proliferation ofbooks and papers on the subject, in particularthe widely acclaimed Migration Atlas(Wernham et al. 2002), is testament to thisinterest but also to the continuing search foranswers to the many unanswered questions.

Britain and Ireland are exciting places toobserve migration, being a cluster of essentiallymaritime countries straddling the dividebetween the North Atlantic and the North Sea.Small islands and island groups off the northand west coasts have magnetic appeal forbirders seeking scarce migrants and rarities. Interms of bird migration, the nearest significantlandmasses are Scandinavia to the northeast,Iceland to the northwest, and continentalEurope to the east and south. Despite the vast-ness of the North Atlantic, this provides a routefor seabird migration and also occasionalweather conditions conducive to transatlanticmovements of landbirds. Their geographicalposition has resulted in Britain and Irelandbecoming a vital stopover for both regularmigrants and vagrants. The conditions underwhich these birds migrate change from day today, and from season to season, giving rise to

the variety of migrants with which we are sofamiliar, and providing a complex and ever-changing backdrop to our birding.

Weather and its effectsWeather can be defined as variations in atmos-pheric conditions over short time periods, i.e.hourly, daily or perhaps weekly. It differs fromclimate, which describes the average state ofatmospheric conditions over a longer timescale,generally of many years. Long-term changes inthe frequency and intensity of weather parame-ters can, however, also be responsible forclimate change. The effects of weather on birdmigration have been well described (e.g. Aler-stam 1990, Berthold 2001, Elkins 2004),although innumerable cases arise in which themeteorological influences on specific move-ments are not immediately obvious. Bothspring and autumn migration in Britain andIreland involve inbound and outbound move-ments as well as passage migration, whichinclude normal and involuntary movements. Allare subject to prevailing weather conditionsover the areas of inception, travel and arrival.Weather can change rapidly, both spatially andtemporally, making analyses of migration diffi-

239British Birds 98 • May 2005 • 238-256

126. Nocturnal migrants, such as these Red Knots Calidris canutus, are more selective than diurnal migrantsabout the weather conditions for migration, for the simple reason that to navigate using the setting sun or star

patterns requires skies which are at least partly cloud-free. Robin Chittenden www.harlequinpictures.co.uk

cult or even impossible. In reality, the relation-ship between a migratory movement and theprevailing weather is frequently obscure. Manyfirst-recorded dates of less common migrantsmay be linked more to onward passage than toactual arrival dates. Nonetheless, specificweather situations regularly affect migration incertain ways throughout the year and carefulanalysis can clarify the perceived patterns ofarrival or departure.

Autumn migrationDeparture from breeding groundsThe majority of birds leavingBritain and Ireland in autumnare summer visitors departing toavoid the winter, when manyinvertebrates (aerial, foliage andsoil) become scarce or inacces-sible, and habitats may becomeuntenable (for example as freshwater freezes or snow covers theuplands). In general, the autumndeparture of most species fromthe breeding grounds goes unno-ticed, being apparent only in thecase of diurnal migrants such ashirundines (Hirundinidae),pipits Anthus and finches

(Fringillidae). These groups generally move inquiet weather and light winds (often head-winds), when coasting movements can bewidely observed. For example, during a five-dayanticyclonic period at the beginning of Sep-tember 2003, huge numbers of hirundinespassed Spurn, East Yorkshire, on 3rd and 4th.Estimated counts included 45,000 Barn Swal-lows and 15,000 House Martins Delichonurbicum, while smaller numbers were recordedat Dungeness, Kent, on the English Channel


127. Reed Warbler Acrocephalus scirpaceus, aboard a ferry in the Bay of Biscay, August 2001. Passerine migrantsmake landfall in unexpected places when weather conditions are against them, and when weakened to the

point of near-exhaustion are not choosy about a perch. David Tipling/Windrush

Fig. 1. A typical fine, anticyclonic weather system, with light northerlyairflow over Britain, initiating southward autumn migration;

00.00 hrs, 1st September 2003.

coast, on the same dates. Most of these birdsnavigate using landmarks, often coasting andcrossing bays when the distant shore is visible.Inland migration can also be observed, withbirds following river valleys and other geophys-ical features that lie along the preferred direc-tion of migration.

Nocturnal migrants tend not to use cuesfrom the land above which they fly, and arerather more choosy about the weather, sincenavigation using the setting sun or star patternsnecessarily requires skies which are at leastpartly cloud-free. In addition, winds blowingapproximately in the direction of travel conferan added bonus and, indeed, are vital for somelong-distance migrants (see fig. 1). A south-ward-moving cold front, for example, not onlyintroduces tailwinds (often strong) with anortherly component, but also a marked fall intemperature that may trigger a reduction infood resources. Other groups, such as seabirds,are less dependent on the weather, but theirroutes are still affected by wind speed and direc-tion.

Migrants from the northApart from the departure of breeding birds,huge numbers of other migrants pass through

Britain and Ireland in autumn. Many speciesarrive here for the winter from breedinggrounds in the Canadian Arctic, Greenland andIceland, including Whooper Swans Cygnuscygnus, Pink-footed Anser brachyrhynchus andBarnacle Geese Branta leucopsis and waders,while others, such as Northern WheatearsOenanthe oenanthe, merely pass through. Anillustration of the conditions experienced bygeese moving south in autumn is shown in fig.2, when a deep, slow-moving depression wascentred over the English Channel on 30th Sep-tember and 1st October 1998, with cloudy, weteasterly winds prevailing over most of Britain. Aperiod of easterly winds had, however, alsoaffected Iceland from 22nd September, lastinguntil 29th, when a small anticyclone built tem-porarily, backing the wind from a light north-easterly to northwesterly over the island. Birdstook advantage of this tailwind, and encoun-tered the easterly winds farther south duringthe next two days. This resulted in their arrivaloff the western fringes of Scotland, rather thanthe normal eastern and northern landfall, and abattle eastwards into the wind to reach land.Huge numbers of geese arrived in the WesternIsles and on the western mainland of Scotlandduring this period (including 15,000 Pinkfeet at


128. Like other species of geese, these Pink-footed Geese Anser brachyrhynchus will have waited for goodweather conditions (ideally, clear skies with a following wind) before leaving their northern breeding grounds forthe journey to wintering grounds in eastern Britain, although unforeseen bad weather en route may hamper their

progress, as described in the text. Robin Chittenden www.harlequinpictures.co.uk

Ardmore Point, Clyde, on the morning of 1stOctober), but stayed only briefly.

Trans-European migrantsSpecies departing from Scandinavia andnorthern Eurasia take a variety of headings.Many head west to winter in Britain andIreland, including many waterfowl, WoodcocksScolopax rusticola and thrushes (Turdidae). Rel-atively few of the longer-distance migrants fromthis source pass through in autumn, however, astheir normal route takes them SSW across the

North Sea or through Europe.Only when weather conditionsare unfavourable do they appearin Britain. Although most speciesfollow similar routes from year toyear and spend the winter in thesame region, ringing has shownthat some individuals may winterin completely different areas indifferent years. For example,Snow Buntings Plectrophenaxnivalis from Greenland mayspend one winter in NorthAmerica and the next in Europe,while some Redwings Turdusiliacus from Scandinavia alter-nate between wintering areas in

Britain and the Black Sea region (Alerstam1990). The trigger for this ‘wandering’ may bethe prevalent wind direction at the start of theirmigration.

The same prolonged easterlies that deflectedthe Pink-footed Geese in 1998 (above) had beenestablished since 23rd September. At times, theorigin of this weather system extended east intowestern Russia, and brought considerable falls ofscarcer passerine migrants. These includedShore Larks Eremophila alpestris, BluethroatsLuscinia svecica, Yellow-browed Warblers Phyllo-

scopus inornatus, Red-breastedFlycatchers Ficedula parva andGreat Grey Shrikes Lanius excu-bitor along the entire length ofthe North Sea coastline. Associ-ated with these were severalnational rarities, including aspectacular run of at least 25Rustic Buntings Emberiza rusticaand no fewer than four Pallas’sGrasshopper Warblers Locustellacerthiola in the Northern Isles,establishing beyond doubt thatthis airstream was drawing inbirds from central Siberia. Com-moner inbound migrants arrivedthroughout eastern Britainduring this period, some speciesin large numbers, includingRobins Erithacus rubecula, SongThrushes T. philomelos, Red-wings, Blackcaps Sylvia atri-capilla, Goldcrests Regulus regulusand Siskins Carduelis spinus.

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Fig. 2. Synoptic chart, 00.00 hrs, 1st October 1998.

129. Shore Lark Eremophila alpestris, Holy Island, Northumberland,October 2004. Shore Larks were one of several scarce migrants which

appeared in higher-than-usual numbers along North Sea coasts inSeptember 1998 during a period of prolonged easterly winds.

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Significant arrivals and major fallsThrushes frequently arrive in huge numbers inOctober. After a week of strong southwesterlywinds over southern Scandinavia in lateOctober 1995, a brief ridge of high pressuremoved north into the region and over theNorth Sea on 29th. Thus, an adverse weathersituation changed suddenly into one in whichenormous numbers of delayed migrants wereable to depart, taking advantage of clearingskies and only light crosswinds over thenorthern North Sea. Redwing passage in north-east Scotland increased rapidly, reaching a peakon 30th with at least 125,000 arriving overSutherland, 75,000 of these in one half-hourperiod around midday. Similarly, a strongWNW airstream in October 1986, followingtwo weeks of adverse weather between Green-land and Britain, allowed the immediate andrapid passage of ‘Greenland’ Wheatears O. o.leucorhoa across the northeast Atlantic. A massarrival of 10,000-20,000 Northern Wheatearson the Isle of Man resulted in the partialblocking of a road and the covering of adjacentfields (Thorpe & Spencer 1992).

Observations such as these, of enormousnumbers of birds on passage, are uncommon,and occur only when migration is affected byadverse weather. One of the largest falls onrecord occurred on 3rd September 1965, andwas centred along the coast of East Anglia,where an estimated half a million birds landedalong a 40-km stretch of Suffolk coastline(Davis 1966). Fig. 3 shows the synoptic situa-tion on 3rd September 1965, and conditionstypical for a major fall. A depression over theAlps moved northwest towards Britain on 2nd,preceded by strong north or northeast windswith extensive cloud and rain from southernEngland to Denmark. A persistent ridge of highpressure lay over southern Norway and Sweden.By dawn on 3rd September, thick frontal cloudcovered the southern North Sea and by 12.00hrs was associated with a complex depression.Migrants leaving southern Scandinavia in goodconditions were deflected to the west by theeasterly winds, and encountered poor weatherover the southern North Sea. Birds beganarriving on the coast between Yorkshire andNorfolk, but it was farther south that the extra-ordinarily concentrated falls took place a littlelater. During the afternoon of the 3rd, changingwind directions over the East Anglian coast,accompanied by heavy rain, brought huge

numbers of migrants tumbling from the sky.Migrants oriented to the SSW had encounteredfrontal cloud over the sea, with landfall forsome dependent on the wind flow into whichthey penetrated. On arrival, many wereexhausted and of low weight, with largenumbers being washed up dead on the tideline.This disorientated flight in heavy rain andstrong winds over the North Sea had clearlybeen extremely debilitating. The sheer magni-tude of the fall was overwhelming. One experi-enced observer, walking a 4-km section of shoreearly on 4th September, estimated 15,000Common Redstarts Phoenicurus phoenicurus,8,000 Northern Wheatears, 4,000 Pied Fly-catchers F. hypoleuca, 3,000 Garden Warblers S.borin, 1,500 Whinchats Saxicola rubetra, 1,500Tree Pipits Anthus trivialis, 1,000 Willow War-blers Phylloscopus trochilus and 500 CommonWhitethroats Sylvia communis. Common Red-



130. Common Redstart Phoenicurus phoenicurus,Norfolk, September 1995. In the extraordinary fall of migrants in East Anglia in early September 1965,Common Redstart was one of the most abundantspecies, with birds even landing on people in the

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starts were so numerous that they descendedfrom the skies into any available cover, andsome were recorded as perching on the shoul-ders of town-dwellers. The fall was short-livedand, with breaking cloud and veering winds, themajority of these migrants had moved onwithin a few days.

Another amazing autumn event concernsthe annual arrival of Goldcrests into Britain.These tiny birds often arrive in large numbersalong the North Sea coast, during an incrediblefeat of migration from the Scandinavian forests,where a proportion of the population is resi-dent. The migrants provide a stock that canboost the resident Scandinavian populationafter severe winters, while in mild seasons thosethat overwinter can breed earlier and generallymore successfully than the returning migrants.In mid October 1982, a massive immigrationtook place along the Scottish coast south ofAberdeen, with more than 15,000 on the Isle ofMay, Fife, on 11th October. This was anotherclassic fall situation, with an anti-cyclone over Scandinavia andwestern Russia, a fresh easterlyairflow over the North Sea and aslow-moving occlusion extend-ing from eastern Scotland tosouthern Sweden.

Islands, overshooting andreorientationAlthough migrants often arrivealong the entire length of the eastcoast, arrivals in the NorthernIsles are seemingly larger andmore exciting. This is probably a

result of two factors. Firstly, migrants findingthemselves in the gaps between the islands maytend to veer off track to head towards a visiblepiece of land, where they become more concen-trated. During poor visibility and darkness,those birds in the gaps funnel through towardsthe open Atlantic. Many of these are believed toreorientate from a westerly track to a south orsoutheasterly track at dawn, when they becomeaware of nearby land. Secondly, winds with aneasterly component on the north and eastflanks of a depression, and the accompanyingpoor weather, are more frequent further north,by reason of the tracks the depressions take.

On 15th October 2001, a warm sectormoved north across northeast Scotland,bringing southeast winds, rain and low cloud.Observers at sea close to Orkney watched manyhundreds of thrushes, mostly Redwings andFieldfares T. pilaris, and presumably of Scandi-navian origin, flying low over the waves fromthe northwest and into the wind. These birdswere thought to have overshot the islandsduring the night, and were reorientating atdawn when land became visible (Eric Meekpers. comm.; fig. 4).

Farther west, arrivals (albeit now reduced insize) of Eurasian passerines also occur alongwest coasts, again thought to be reorientedmovements. Examples of these occurred inautumn 2003, when a scattering of Eurasianvagrants occurred throughout the Western Isles,and a few reached as far west as Co. Donegal.

Arrivals from SiberiaThe appearance of vagrants from Siberia is anannual event in Britain, although numbers andvariety fluctuate markedly from year to yearand depend on the development of anticyclonic



Fig. 3. Synoptic chart, 12.00 hrs,3rd September 1965.

Fig. 4. Synoptic chart, 00.00 hrs, 15th October 2001.

conditions across Scandinaviaand farther east, creating an east-erly airflow across Europe.Classic anticyclonic conditionsprevailed over northern Europein September 2003, when pres-sure at least 4 hPa (1 hectoPascal= 1 millibar) higher than normalextended from Britain east intowestern Russia. Both Septemberand October experienced periodsof east and southeasterly winds(most prevalent in late Sep-tember and mid October) andwarm airflows (which often coin-cided with those dates and were additionallypresent in mid September). The incrediblenumber and variety of rare migrants duringthese months and the relative paucity of accom-panying commoner migrants suggests that thelatter had little difficulty in maintaining theirmigration routes and tracks, while the formerwere probably non-oriented dispersing birdswandering well beyond their normal range.Many of the rarest species involved originatedfrom no closer than northwest Russia and theUral Mountains. During 5th-12th September, apersistent anticyclone extended across northernand eastern Europe north of 50°N. This main-tained fine weather, which, combined with asoutheasterly airflow across the Baltic Sea, gavethe region its highest temperatures for themonth. It also immediately preceded the begin-ning of an arrival of Yellow-browed Warblers,but it was not until the onset of slack winds andimproving weather over the North Sea from27th September that Asian vagrants began toarrive in force. Between 28th September and 1stOctober, rarities along the east coast includedRed-flanked Bluetail Tarsiger cyanurus, White’sThrush Zoothera dauma, Pallas’s GrasshopperWarbler, Lanceolated Warbler Locustella lanceo-lata, an eastern Savi’s Warbler L. luscinioidesfusca [still under consideration by BBRC] andSykes’s Warbler Hippolais rama, as well as themore regular species such as Olive-backed PipitAnthus hodgsoni, Citrine Wagtail Motacilla cit-reola, Arctic Warbler P. borealis and anotherinflux of Yellow-browed Warblers. This excep-tional fall suggests an origin from the imme-diate near continent, consisting of birds thathad previously trickled westwards during theearlier anticyclonic spell in September. Theeasterly airflow from Europe dissipated on 2nd

October, with an occlusion bringing a strongwest or northwest airflow over Britain during3rd-10th, which lasted until mid month innorthern Europe. The development of an anti-cyclone over the North Sea from 11th October(see fig. 5) produced a southeasterly airflowover Britain that backed to the northeast by19th as the high pressure shifted east anddecayed. Another influx of Asian vagrantsbegan from 11th, comprising many Pallas’s LeafP. proregulus, Hume’s P. humei, Radde’s P.schwarzi and Dusky Warblers P. fuscatus, TaigaFlycatcher Ficedula albicilla [still under consid-eration], Pechora A. gustavi and Olive-backedPipits, Red-flanked Bluetail, SiberianRubythroat Luscinia calliope and White’sThrush. Other eastern vagrants recorded duringSeptember and October 2003 included Paddy-field Warbler Acrocephalus agricola andIsabelline Shrike Lanius isabellinus, both specieswith breeding ranges centred on Central Asia,suggesting that the origin of the birds involvedextended across a wide latitudinal range.Mediterranean species such as SubalpineWarbler S. cantillans, Melodious Warbler H.polyglotta and Woodchat Shrike L. senator alsoappeared in Britain, illustrating the disparateorigin of birds wandering across Europe duringthis period.

Previously, the appearance of these primarilyAsian breeding species that normally winter insouthern and Southeast Asia has been corre-lated with the distribution and intensity of thesemi-permanent winter high-pressure zone thatbegins to form over Siberia in early autumn.This anticyclone creates an extensive easterlyflow on its southern flank that reaches west intothe Black Sea region in October. However, mostmigrants depart from the source region in



Fig. 5. Synoptic chart, 00.00 hrs, 15th October 2003.

August and September, well before this anticy-clone is established. There remains considerabledoubt over the role that weather plays in stimu-lating this migration, and also the numbersinvolved, although passage may initially bestimulated by fine weather and assisted by east-erlies. The eventual westward movement acrossRussia and northern Europe depends on syn-optic situations similar to those influencingEuropean migrants. Reasons for the anomalousdirection in which these migrants move are alsodifficult to determine. Several factors may beimplicated, including habitat and populationchanges, magnetic anomalies in the sourceregions, reverse migration, straightforward dis-orientation and (possibly most likely)exploratory movements. In the latter, non-ori-ented migrants may disperse in all directionsbut only those following their habitat zonewould survive. As habitat zones are more or lesslatitudinal, an approximate westward move-ment would result, having the appearance ofreverse orientation.

North American landbirdsWhile northern Britain is better placed than thesouth to act as a landfall for drifted migrantscrossing the North Sea in autumn, it is not sofor vagrants from the opposite direction. Thesouthwestern regions of Britain and Ireland arethe classic landfall for North American land-birds caught up in fast-moving depressions thatdevelop over their western Atlantic flyway inlate autumn and cross the Atlantic rapidly. Sincethese features are typical of mid latitudes, areasfarther north receive fewer transatlanticvagrants. Depressions affecting areas from Scot-

land northwards are more likely to have deep-ened over the mid Atlantic and slowed down.Unlike migrants in western Europe, neitherNorth American transoceanic migrants noreastern coastal migrants have the advantage of alandfall when drifted by westerlies. When thesewinds are strong, coastal birds are drifted out tosea and are able to return to the coast only withthe expenditure of considerable energy. Manylate-autumn migrants in North America crossthe western Atlantic from Nova Scotia andnortheastern USA to the West Indies and SouthAmerica in a single flight. Small landbirds areunable to sustain this long passage unlessassisted by a favourable northwesterly wind.Large numbers of migrants head southeast inthe northwesterly winds, eventually overtakingthe weakening cold front responsible for gener-ating this wind flow. Some cold frontal zonesretain their identity along the marked marinethermocline at the edge of the warm GulfStream and generate frontal waves. A frontalwave intensifies the associated cloud, precipita-tion and winds in the warm air, which may dis-orientate migrants enough to initiate drifteastwards towards the open ocean and atransatlantic crossing in strong, warm south-westerly winds (Elkins 1979).

Other vagrant landbirds are thought toencounter such frontal waves as reversemigrants heading northeast in warm airstreamsfrom a southerly point. Unlike the northernspecies on southward migration over the sea,these have a more southern distribution andmigrate rather earlier, including Red-eyed VireoVireo olivaceus (the most abundant of all NorthAmerican landbirds reaching Britain and

Ireland) and BaltimoreOriole Icterus galbula.This mechanism oftransatlantic crossing issupported by events in1995. The northernboundary of a tropical airmass over the easternseaboard of the USA, dueto Hurricane ‘Opal’ overthe Gulf of Mexico, hadalready caused a signifi-cant fall of migrants inBermuda, along with largemovements of Monarchbutterflies Danaus plex-ippus in late September. In



Fig. 6. Transatlantic vagrancy in October 1995, showing location of Hurricane ‘Opal’.

the second week of October 1995, rapidlymoving transatlantic frontal depressionsbrought at least 11 Red-eyed Vireos toBritain as well as other vagrant species andmany Monarch butterflies (Elkins 1999;fig. 6).

One particular pressure pattern oftenerroneously associated with transatlanticmigration is the tropical storm (or hurri-cane, as such storms are known in theNorth Atlantic). A hurricane is anextremely intense cyclonic storm, whichdiffers from an intense depression in that itdoes not involve different air masses and there-fore has no fronts (see Hurricane ‘Opal’ in fig.6). Hurricanes rely on high sea temperatures toprovide the moisture and energy to sustainthem. They develop at low latitudes and movewest, then northwest over the Caribbean Searegion. Some make landfall (as Hurricane ‘Opal’did) but others will curve northeast to movealong the USA’s eastern seaboard. Some maydrift out into the North Atlantic (see Hurricane‘Noel’ in fig. 6) and cross autumn migrantroutes. While there is no doubt that huge falls ofmigrants descend on coasts, islands and ship-ping during the passage of hurricanes, thestorm itself moves relatively slowly with itsviolent winds circulating round the stormcentre, often affecting a vast area. Any birdcaught up in these winds would also circum-navigate the centre (if indeed it survives the tur-bulence and torrential precipitation) and finallysuccumb to exhaustion. The only link thattransatlantic migrants may have to hurricanes isin the final stages when the storm evolves into adeep depression by engaging with cold air,which generates a frontal system. Even then, awind flow capable of carrying a migrant rapidlyacross the North Atlantic would only developahead of the cold front, well away from theparent depression. Nevertheless, these deeper,more mature depressions are considered to bethe probable mechanism for vagrancy fromNorth America to Greenland and Iceland(Elkins 2004).

The locations of several birds recorded inautumn 2003 illustrate that not all Americanlandbirds travel along a direct transatlanticroute. For example, two Catharus thrushes wererecorded in Shetland on 27th September 2003at the start of the large Eurasian vagrant fall,while other Nearctic vagrants arrived in Scot-land during October 2003, several during the

second wave of eastern vagrants discussedabove. With no obvious links to suitabletransatlantic airflows, this illustrates how diffi-cult it can be to correlate individual vagrantswith weather patterns. The time and place ofarrival, and how long they have remained unde-tected, are often unknown. Autumn 2003 saw atleast seven North American passerines reachingIceland, where individuals were recordedbetween 6th and 10th October, probably deliv-ered by two active depressions that crossed theregion on 4th-5th October (fig. 7) and on 8th-9th October. It is possible that Iceland was theprovenance of the Red-eyed Vireo on the islandof Barra, Western Isles, on 5th October, which,although travelling in the same air mass, mayhave ‘missed’ Iceland. The records of Americanpasserines in Scotland later that monthoccurred during southeasterly winds and werepossibly wandering individuals dispersingwithin Britain after having made landfall else-where (see fig. 5).

Other North American birdsNorth American waders cross the North Atlanticas frequently as landbirds, but there are subtledifferences in the mechanics by which they doso. The chief distinction is the altitude and rangeof the two groups. Waders (even the smallerspecies) generally fly higher, faster and further,and are therefore often subject to different windregimes. Most waders from Arctic NorthAmerica migrate to South America. While it istrue that weather conditions under which theydepart are normally similar to those used bylandbirds, by climbing to higher altitudes theyreach wind systems whose strength and direc-tion are at variance with the low-level windsinfluencing landbird migration. For example,the northwesterly post-cold-frontal airflow sonecessary to small songbirds flying across the



Fig. 7. Synoptic chart, 00:00 hrs, 5th October 2003.

extreme western Atlantic towards the WestIndies rarely extends to wader migration alti-tudes during the early stages of the lives of theassociated depressions. Indeed, winds back (i.e.change counter-clockwise) with height, so thatabove, say, 3 km altitude, these northwesterliesbecome westerly in direction and strengthen.Higher still, winds may be southwesterly andreach jet-stream velocities in excess of 30 m/sand frequently above 50 m/s. Consequently,North American wader arrivals in northwestEurope are thought not to be related to the rapidmovement of developing depressions across theAtlantic. It is probable that their flight at higheraltitude is influenced by the powerful southwestto westerly winds described above, and theirwider geographical and temporal distribution inBritain and Ireland does suggest that wadersarrive at a higher level, and with superior fuelreserves than the smaller songbirds, whichappear chiefly in the southwest, and often in anexhausted state (Elkins 1988).

SeabirdsSeabird migration is rather less dependent onthe weather than that of landbirds, and patternsof migration are more related to feeding oppor-tunities and mode of flight. For example, terns(Sternidae) and Kittiwakes Rissa tridactyla tendto migrate in quiet, anticyclonic weather, whiletubenoses need stronger winds to assist their

mode of flight. Seabird movements are invari-ably observed only from the coast and many goundetected farther out to sea. Additionally,migration and feeding movements are often dif-ficult to separate, especially for the more pelagicspecies such as Fulmars Fulmarus glacialis andshearwaters.

Of particular interest are those species thatbreed in the South Atlantic and spend theaustral winter in the northern hemisphere,including Sooty Shearwater Puffinus griseus,Great Shearwater P. gravis and Wilson’s Storm-petrel Oceanites oceanicus. The Sooty Shear-water is the most numerous of these species inBritish waters, occurring from midsummeronwards. From dates of observation, it seemsthat most birds arrive off western Ireland andScotland in late July and August, with move-ments around the Northern Isles and into theNorth Sea chiefly during August and Sep-tember. The largest coastal movements are asso-ciated with onshore winds, so those observedalong the west coast occur in westerly winds,with large movements off the east coast beingrecorded mainly in periods with a strong east-erly element to the wind, or during clearingweather in post-frontal north or northwesterlywinds. The large movements (over 2,000 atsome sites) along the east coast of Britain on22nd September 2002 occurred in just suchconditions. In the Northern Isles they occur



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131 & 132. Intense Atlantic depressions in autumn occasionally bring small numbers of pelagic seabirds inland toreservoirs and other large waterbodies, notably Sabine’s Gulls Larus sabini (left, Heysham, Lancashire, September

1988) and Leach’s Storm-petrels Oceanodroma leucorhoa (right, Crosby, Merseyside, September 2004).Sabine’s Gulls were particularly associated with a storm in 1987, which brought unprecedented numbers (300+) to southern and eastern England, while 2,000+ occurred in western France in September 1993,

courtesy of the tail-end of Hurricane ‘Floyd’.

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typically in wind directions anywhere betweenwest and southeast, often in disturbed frontalweather. Most large passages of skuas (Stercor-ariidae), Manx Shearwaters P. puffinus andFulmars are also associated with such winds.

Overland movements of seabirds, mainlyinvolving terns, skuas and Kittiwakes, are notunusual. Flocks of these species have beenobserved passing west along the Forth-Clydegap, southwest down the Great Glen and inlandover northeast England. The weather duringsuch movements suggests that they may beseeking lighter wind regimes as strong windsabate in the east, or using fine anticyclonicweather to navigate by sight from the east coastto the west. In the latter situation, winds oftenhave a northeast (tailwind) component. Furtherafield, high-level overland skua passage isnormal over both the Eurasian and the NorthAmerican landmasses. Gulls regularly migrateoverland, but autumn storms not infrequentlybring small numbers of typically pelagic speciesto inland reservoirs and lakes in centralEngland. Of particular note are species consid-ered scarce even along the coast, such as GreyPhalarope Phalaropus fulicarius and Sabine’sGull Larus sabini.

In October 1987, an intense depressionmoved northeast across southern England,bringing with it unprecedented numbers ofSabine’s Gulls and Grey Phalaropes (Elkins &Yésou 1998). Large numbers of both speciesfeed in and migrate through the Bay of Biscayduring autumn, and the timing and distributionof sightings following this storm suggested thatmany had drifted over the sea within the rela-tively calm eye of the storm. After making land-fall in the early hours of the morning, settlingout took place, during which the birds wereovertaken by the hurricane-force southwesterlywinds adjacent to the eye, and swept downwind.Their subsequent distribution across southernEngland corresponded almost exactly to thehorizontal trajectory of the airflow. A similarlandfall of Sabine’s Gulls occurred in westernFrance in September 1993, when the remnantsof Hurricane ‘Floyd’ crossed the north of thecountry. Severe westerly gales drove many birdsinland, where more than 2,000 Sabine’s Gullswere reported. In both the 1987 and 1993storms, the landfall of birds occurred at nightwhen light levels were extremely low. A noc-turnal landfall has also been attributed to thedeep inland penetration of seabirds during hur-



133. Waxwings Bombycilla garrulus, Liverpool, January 2005.Waxwings are perhaps the most familiar eruptivemigrant to reach Britain and Ireland, and the irruption in winter 2004/05 has been one of the largest for many

years.A combination of high population levels and low food supplies is responsible for triggering these irregularmovements away from the breeding grounds, although weather conditions en route play a role in the birds’

ultimate destination.

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ricanes along the eastern seaboard of the USA,and it appears that the birds may not be suffi-ciently aware of their position overland toattempt an immediate return to the sea (Curry1996; Elkins & Yésou 1998).

EruptionsSome species that would not normally be con-sidered as migrants can, nevertheless, oftenmove considerable distances and in largenumbers. The trigger for such outward move-ments, or eruptions, is a combination of highpopulation levels (following a successfulbreeding season) and a lack of available food.The majority of eruptive species feed on fruitsor tree seeds, many of which are borne in anirregular cyclical pattern, with lean years inter-spersed by productive seasons. These cycles are,however, not normally synchronised across thevegetation zone. The distance birds will movefrom the breeding range (normally westwardsfrom the forests of northern Eurasia, butkeeping within the habitat zone) depends ontheir ability to find alternative food sources.Should these be unable to satisfy the eruptingpopulation, varying numbers will continuewestwards, often coming under the influence ofeasterly winds that carry them towards Britain

and Ireland. Those irruptive species reachingBritain regularly comprise well-known speciessuch as Waxwing Bombycilla garrulus and cross-bills Loxia sp., but also other species as diverseas Rough-legged Buzzard Buteo lagopus (depen-dent upon small-mammal population cycles),Great Spotted Woodpecker Dendrocopos major,Fieldfare, Coal Tit Parus ater, Eurasian Jay Gar-rulus glandarius and Bullfinch Pyrrhulapyrrhula. Irruptions by rarer species, such as the1968 invasion by Nutcrackers Nucifraga cary-ocatactes, and the late-nineteenth-centuryappearances of Pallas’s Sandgrouse Syrrhaptesparadoxus in large numbers, demonstrate thatirruptive species can sometimes originate fromareas as distant as Siberia and Central Asia.

Spring migrationDeparture of winter visitorsDeparting geese and swans are conspicuous,particularly northward-moving skeins of theformer in April. While awaiting suitable weatherconditions, feeding flocks often resort to newlygrowing vegetation that will allow them toreach peak body condition. A typical departuresituation for geese and swans bound for Icelandrequires anticyclonic conditions with lightwinds. Although weather conditions at the time



134. Whooper Swans Cygnus cygnus in a blizzard, Northumberland, March 2005. Snow showers would nottrouble this party of wintering birds unduly (unless conditions were severe enough to freeze large waterbodies),but encountering heavy rain or snow and unfavourable winds on migration between Britain and breeding grounds

in Iceland may severely disrupt their passage, as described in the text.

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of departure may be eminently suitable, suchweather can often be short-lived with anunforeseen deterioration changing an idealmigration situation into one that can be haz-ardous, particularly for landbirds over the sea.While this is a familiar scenario for autumnmigrants over the North Sea, it also brings par-ticular difficulties for northbound migrantsreturning to Iceland and Greenland in thespring. Those leaving north and west Scotlandand encountering deteriorating weather overthe sea have been recorded turning back to landto await an improvement (Williamson 1968).Unfortunately, this is often not an option. Forexample, in late March 1995, Whooper Swansleft the Solway Firth in fine anticyclonicweather (Pennycuick et al. 1996). One bird wastracked by satellite moving northwest along thewest coast of Scotland. Once over the ocean,this bird (doubtless with its companions) flewinto a gale between the Faeroe Islands andIceland and was forced off course, landing onthe sea east of Iceland on 1st April. A trawlerskipper near that location watched six swansskimming the waves in a snow storm andsouthwesterly gale. On the following day, anobserver on the Iceland coast reportedexhausted Whooper Swans lying on the beach.

The tracked swan recommenced its migrationas the gale abated, reorienting to reach southernIceland on 2nd April.

Arrival of summer visitors and retromigrationSimilarly, for migrants returning to theirbreeding grounds, there is often much toendure. Short-distance migrants can takeadvantage of local changes in weather that willallow them to establish territories rapidly, theearliest generally being more successful. Con-versely, long-distance migrants cannot foreseeconditions on their remote breeding grounds,and are subject to vagaries of weather whenreaching more northern latitudes. If they arrivetoo early, food resources may be limited, butarrive too late and the best breeding territorieswill be occupied. Those crossing westernEurope frequently experience rapid weatherchanges owing to North Atlantic depressions,and early migrants in particular may be delayedby adverse conditions. Away from the oceanseaboard, arrivals of spring migrants are gener-ally more constant and, on the whole, latermigrants are also more regular in their appear-ance. Inbound migrants in spring arrive rapidlyand relatively unnoticed, with few pausing onthe coast. For many, the urgency to reach the



135. Common Cuckoo Cuculus canorus, Dungeness, Kent, April 2004.The arrival of spring migrants in goodweather is almost always unseen and is often registered only when a familiar song is heard for the first time since

the previous breeding season.

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breeding territories is paramount, and it is onlywhen the first Willow Warbler or CommonCuckoo Cuculus canorus is heard singing thatwe realise spring has arrived (see fig. 8).

Just as some migrants appear to move in the‘wrong’ direction in autumn, spring migrantsmay also show similar behaviour, often knownas ‘retromigration’. In spring, such a reversedmovement normally occurs in response to coldweather over the breeding grounds, and is morefrequent in northern Europe. Here,unfavourable weather, such as cold winds, frostand snow, affects food supplies and encouragesnewly arrived migrants to return southwards toseek more reliable food sources. This can alsobe a problem to montane species, wherebreeding habitats are subject to later snowfallsthan on lower ground. This ‘retromigration’(and reverse altitudinal movement for montanespecies) has great survival value, and can occurin Britain during late-spring snowfall. Such



behaviour is not always well doc-umented, but poor conditionsduring the early breeding seasonmay affect the breeding successof many species. Once nestinghas begun, the shortage of fooddue to cold, wet or snowyweather can affect the survival ofboth adults and their young,although the consequences varyaccording to species. Oneexample concerns the chilling ofyoung birds, whether newlyhatched (such as wader young)or older nestlings that are toolarge to be brooded successfully(such as raptor young). In both

cases, thermoregulation has not developed suf-ficiently to maintain body temperature in wetweather.

Adverse weather conditions en route maycause delays and erratic arrivals of long-dis-tance migrants in spring. This is especiallynoticeable when northbound trans-Saharanmigrants encounter rain and strong windsacross North Africa and the Iberian Peninsula.April is notorious for two events which canhave a catastrophic impact on migrating birds.First, depressions crossing southwest Europecan produce snowfall over mountains andheavy rain over low ground. Second, the inci-dence of ‘blocking’ anticyclones west of Britainand Ireland is relatively high, introducing cold,northerly winds and delaying northwardpassage.

The difference in arrivals between years hasbeen highlighted by the BTO MigrationWatch/BirdTrack projects (www.birdtrack.net).

Using Sand Martin Ripariariparia as a typical example, thenumber of records in spring2002 showed a steady rise inMarch to reach a peak at thebeginning of April, during aperiod of relatively favourablemigration weather over thespecies’ route. Reports droppedmarkedly in the second week ofApril as low pressure becameestablished over southern Europeand cold east or northeast windspredominated over central andwestern Europe (fig. 9). Warmersouth or southwest winds

Fig. 8. Synoptic chart, 00.00 hrs, 15th April 2003. A typical warm,southeasterly airflow used by returning spring migrants.

Fig. 9. Synoptic chart, 00.00 hrs, 13th April 2002.

returned on 21st, coinciding withanother pulse of records. Inspring 2003, sightings peaked inlate March, after which a succes-sion of fronts crossed westernEurope, followed by cold northor northwest winds at the begin-ning of April. Sightings of SandMartins dipped markedly untilthe second week of April, whenwarmer south or southeast windsushered in another influx whichpeaked in mid month. Finally,records in spring 2004 peaked asearly as mid March, with a subse-quent decline owing to a spell ofnorth or northeast winds overwestern Europe on 22nd-26th. A second, largerpeak occurred in early April and coincided withwarm south or southwest winds over Britainand France, followed by a further decline asfresh to strong cold north or northwest windsset in after 4th April.

Conditions for spring fallsEven after arrival in the country, the movementof migrants northwards through Britain can beasymmetrical if easterly winds, cooled by a coldNorth Sea, steer birds up the milder westernside of the country. For example, a substantialfall of inbound breeding species was recorded atPortland Bill, in Dorset, on 5th and 6th May2000. Fronts moving slowly northwest acrossthe English Channel in a fresh easterly windclearly interrupted passage from France andIberia, resulting in counts of, among others, 750Common Swifts Apus apus and 500 WillowWarblers; 5,000 Barn Swallows and 1,500 HouseMartins were recorded the following day.

Easterly winds usually have a greater impactupon Scandinavian-bound migrants movingnortheast across the North Sea, however, sincepoor weather and strong crosswinds can deflectthem from their routes. One such fall occurredalong the North Sea coast in May 1997. Over 90Red-backed Shrikes Lanius collurio arrived from16th May onwards in the Northern Isles, coin-ciding with an easterly wind flow that hadmoved slowly north up the east coast between15th and 18th, finally becoming confined tonorthern Scotland and the Northern Isles on19th-21st. A warm front embedded within thisairflow also moved north and became sta-tionary over northern Scotland by 18th. Other

arrivals during this period comprisedBluethroats, Subalpine Warblers and GoldenOrioles Oriolus oriolus, a significant influx ofHouse Martins, Common Whitethroats andSpotted Flycatchers Muscicapa striata plus a latepulse of Barn Swallows. Fig. 10 illustrates theweather conditions associated with a typicalspring fall situation.

Another major fall occurred in May 1993,towards the end of a period (during 4th-11th)of fresh east or northeast winds to the south ofan anticyclone centred north of Scotland. Theairflow was relatively cloud-free, but migrantswere evidently influenced by the fresh windsover the North Sea. The main fall began on 10thwith up to 100 Common Redstarts on the Isleof May and many elsewhere down the wholeeast coast. Also on the Isle of May were 15Bluethroats on 11th with more in the next fewdays as a depression moved NNE across thecountry bringing heavy rain to the south. Thefall included substantial numbers of Whinchats,Northern Wheatears, Lesser Whitethroats Sylviacurruca, Common Whitethroats, Willow War-blers and Pied Flycatchers with several Red-backed Shrikes, Tree Pipits and Wrynecks Jynxtorquilla.

A west-coast event on 30th April 2001 high-lighted what was probably a surge of springmigrants delayed by a run of westerlies andchangeable weather over Britain and France.Light winds and clearing weather from north-west Spain to western and northern Britain overthe night of 29th/30th April gave the impetus tonorthward-bound nocturnal migrants. OnBardsey Island, Caernarfonshire, 98Grasshopper Warblers Locustella naevia, 85



Fig. 10. Synoptic chart, 00.00 hrs, 18th May 2002, showing typical springfall weather, with southeasterly winds over the North Sea and weak

fronts moving north across Britain.

Sedge Warblers Acrocephalus schoenobaenus and465 Willow Warblers were recorded the next day,together with numerous other warbler speciesincluding a claimed Iberian Chiffchaff Phyllo-scopus ibericus (the latter, a non-singing bird,was not accepted by BBRC, but only after muchdeliberation: see Brit. Birds 97: 614). Nonethe-less, the evidence suggests that the fall almostcertainly involved oversea migrants that, havingdeparted Spain at the first break in the weatheron 29th April, made first landfall on Bardsey.

OvershootingOne aspect of migration peculiar to spring isthe phenomenon of overshooting. This occurswhen southern European species overfly theirdestination during unusually fine anticyclonicweather, particularly if temperatures are wellabove normal. These may also be exploratorymovements that extend the birds’ breeding

range on occasion. Such birds often arrive withtypical inbound migrants. In March 2003, 165Black Redstarts Phoenicurus ochruros, normallya scarce breeding bird in southeast England,arrived in Scilly between 17th and 19th on awarm east or southeast wind flow associatedwith an anticyclone over the east coast ofBritain, presumably having overshot theirbreeding ranges in Spain and France. However,many more Mediterranean species typicallyarrive during southeasterly winds with Scandi-navian migrants (having perhaps originallyovershot into central Europe). Although irrup-tive species are usually associated with autumn,a few appear in spring and summer, with twogood examples being Common Quail Coturnixcoturnix (from the south) and Rose-colouredStarling Sturnus roseus (from the southeast).

Common Quails breed annually in Britain insmall numbers, with occasional ‘quail years’when exceptional numbers arrive. Summer 1989was just such a quail year, perhaps the mostnotable on record, when an estimated 2,600calling males were reported in Britain in Mayand June, of which no fewer than 750 reachedScotland (Murray 1991). The influx began inmid May, associated with an anticyclone acrossFrance and southern Britain producing warmsouthwest winds across the rest of Britain. Thisanticyclone moved northeast, with the warmairflow backing southeast by 20th May andlasting until 25th. Cold winds then intervenedand new arrivals ceased, but this was followed bya more substantial influx of after 10th June,when another surge of warm air moved acrossBritain. The reasons for such influxes are prob-ably a combination of hot, dry weather and successful early breeding across their Mediter-ranean range, inducing many birds to dispersenorthwards and perhaps breed again.

A similar event in 2002 involved a largeinflux of Rose-coloured Starlings. This mayhave resulted partly from an increase in thebreeding population during previous years,while enormous numbers were present on thesteppes of southern Russia in spring 2002.These birds may have been stimulated by warmweather to overshoot the breeding grounds insouthwest Asia and southeast Europe. In bothMay and June, mean temperatures over south-east Europe were 2-3°C above normal. An anti-cyclone built across Europe at the end of May,moving northeast into Scandinavia in earlyJune and bringing winds with an easterly com-



136. Purple Heron Ardea purpurea, Lesvos, Greece,April 2001. In spring, some Mediterranean species,

such as Purple Heron, may overfly their normaldestination on the journey north from southernwintering grounds, the phenomenon known as

‘overshooting’. For species such as this, weatherconditions play an important part in determining

the numbers which reach Britain annually.

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ponent over central Europe and Britain. At first,parties were recorded in northern Italy andsouthern France but, with the changing weatherpattern, the focus of the influx then movednorth into Britain and northwest Europe,although southwest winds after 10th June sug-gested that later sightings were mainly of birdsmoving within the region. Birds reached as farwest as Ireland, while the largest numbers inScotland were in the northwest and the WesternIsles, suggestive of continued onward move-ment through Scotland.

SeabirdsSeabird migration in spring can be notable, inparticular the skua migration off the WesternIsles, which usually occurs in fine weather withlight winds. These birds do not stop to feed

during what appears to be a non-stop passagedirect to the breeding areas. The largest move-ments occur in the veering winds of post-frontal clearances or abating headwinds. In1986, 766 Pomarine Skuas Stercorarius pomar-inus flew north past Balranald, North Uist, infive hours on 21st May in the strong southwestwinds behind an occlusion. During this period,168 Long-tailed Skuas S. longicaudus alsopassed within the space of one hour (Davenport1987). In May 1991, exceptional movements ofLong-tailed Skuas were recorded at the samesite (including 424 on 12th May), in poorweather associated with warm-sector south-westerly winds. In such conditions they often flyhigh and overland. Northward Pomarine Skuamigration in spring off the west coast of Shet-land occurs mainly after a long period of gales


137. Redwing Turdus iliacus, Kent, November 2002. Ice and snow cover are the enemies of many wintering birds,particularly open-ground feeders, and will often trigger ‘cold-weather movements’. Plovers, Sky Larks Alauda

arvensis and thrushes are often the most noticeable species on the move.The normal direction of hard-weathermovements is south and west, and journeys are often of considerable length if the initiating conditions arewidespread. Since 1988, there have been few winters with periods of prolonged severe snow or ice and,

consequently, few significant hard-weather movements. Escape movements occur more often in continentalclimates normally subject to long periods of snow and ice. One Nearctic landbird appears extremely rarely inBritain in midwinter. A large, gregarious population of American Robins Turdus migratorius winters in southeastUSA and any movement, such as might take place in cold weather, may risk a flight out to sea. From this region,where winter formation of depressions is high along the Gulf Stream boundary, transatlantic crossings in warm

sectors may then be initiated (Elkins 1979). David Tipling/Windrush



Norman Elkins, 18 Scotstarvit View, Cupar, Fife KY15 5DX

from the west or northwest and is thought toinvolve birds that have been displaced fromtheir route along the continental shelf to thewest. By far the largest such movement was wit-nessed on 9th May 1992, when a staggering2,093 birds were recorded heading north afterseven days of southwest and westerly gales,while 748 were seen on 14th May 2003.

SummaryThis review of the impact of weather on birdmigration in Britain and Ireland illustrates thediversity of migrant species that fly across aregion where meteorological changes can befrequent and rapid. Certain weather eventsproduce unusual concentrations of migrantsand also convey scarcer species from atypicalsource regions. Migration studies continue toraise awareness of the strategies that migrantsuse to overcome problems caused by weather.The knowledge gained will help any assessmentof the effects that climate change will have onmigration in the future.

Acknowledgments

All weather charts are derived from information kindlysupplied by the Meteorological Office. I wish to thankGunnlaugur Pétursson for preliminary data on Icelandicvagrants in 2003.

References

Alerstam,T. 1990. Bird Migration. CUP, Cambridge.Berthold, P. 2001. Bird Migration: a general survey. 2nd edn.

OUP, Oxford.Curry, B. 1996. Hurricane Fran: September 1996. Birders J.

5: 283-297.Davenport, D. L. 1987. Large passage of skuas off Balranald,

North Uist in May 1986. Scott. Birds 14: 180-181.Davis, P. 1966.The great immigration of early September

1965. Brit. Birds 59: 353-376.Elkins, N. 1979. Nearctic landbirds in Britain and Ireland: a

meteorological analysis. Brit. Birds 72: 417-433.— 1988. Recent transatlantic vagrancy of landbirds and

waders. Brit. Birds 81: 484-491.— 1999. Recent records of Nearctic landbirds in Britain

and Ireland. Brit. Birds 92: 83-95.— 2004. Weather and Bird Behaviour. 3rd edn. Poyser,

London.— & Yésou, P. 1998. Sabine’s Gulls in France and Britain.

Brit. Birds 91: 386-397.Murray, R. D. 1991. Quail in Scotland, 1989. Scot. Bird Report

1989: 45-50.Pennycuick, C. J., Einarson, O., Bradbury,T.A. M., & Owen,

M. 1996. Migrating Whooper Swans Cygnus cygnus:satellite tracks and flight performance calculations.J. Avian Biol. 27: 118-134.

Raven, M. J., Noble, D. G., & Baillie, S. R. 2003. The BreedingBird Survey 2002. BTO Research Report 334. BTO,Thetford.

Thorpe, J. P., & Spencer, R. 1992.A large fall of GreenlandWheatears following exceptional weather conditions.Ringing & Migration 13: 125-126.

Vinicombe, K. E. 1985. Ring-billed Gulls in Britain andIreland. Brit. Birds 78: 327-337.

Wernham, C.V.,Toms, M., Marchant, J., Clark, J., Siriwardena,G., & Baillie, S. (eds.). 2002. The Migration Atlas:Movements of the Birds of Britain and Ireland. Poyser,London.

Williamson, K. 1968. Goose emigration from westernScotland. Scott. Birds 5: 71-89.

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