Abstract Wind directions measured at two differentheights (850 hPa and 700 hPa) and at different hours ofthe night were analysed during the spring migration pas-sage at a bird stopover site located in the western Mediter-ranean, in order to evaluate the importance of wind com-ponents for a stopover decision. From a huge ringing cam-paign of bird migration in north-east Spain, data from thereed warbler Acrocephalus scirpaceus have been used forthe analysis. From a total of 2,478 reed warblers capturedbetween 1993 and 1997 data recording significant days,with a high number of captures, and decrease days, withfew captures, have been selected to develop an analysis ofwind direction in relation to stopover and flight resump-tion. On days with a high capture the winds had mainly afourth-quadrant flow (from the north, north-west andwest), these being mainly head winds. Winds with wester-ly component (from the north-west, west and south-west),which enhance the flight, account for the majority of thedays when there was a low capture of reed warblers. Winddirection therefore appears to be a determining factor forstopover decisions and resumption of flight for the reedwarblers at an intermediate stage of their spring migrationwhere topographical characteristics govern the winds.

Keywords Acrocephalus scirpaceus · Stopover decision ·Wind flow · Spring migration · Western Mediterranean

Introduction

Migration occurs twice during the annual cycle for manybirds moving from breeding grounds to wintering areas.In the course of migration several selective factors affectthe success of these movements between territories oftensituated widely apart. Time, energy and safety have beenconsidered key factors for a successful bird migration(Alerstam and Lindström 1990). However, some authors

have suggested that some climatic variables could influ-ence the timing of migration (Lack 1960; Richardson1978, 1990) are of these being wind. Wind affects deci-sions about the travel routes, flight speeds and day of de-parture for a migratory flight (Able 1973; Richardson1978, 1990; Alerstam 1979; Butler et al. 1997).

Birds migrating at night use several stopover siteswhere they stay for one or several days. Decisionswhether to depart from or remain at a stopover site arebased on several criteria involving physiological (fat re-serves and rate of accumulation) and ecological (thepresence of ecological barriers) factors or even weatherconditions (Liechti and Bruderer 1998). Some authorshave even suggested that wind direction and intensityhave a major effect on decisions for departure (Akessonand Hedenstrom 2000) but there has been little systemat-ic evidence for what influences landing decisions. More-over, birds select flight altitudes in order to maximizewind support, as revealed by Bruderer et al. (1995).Many studies dealing with the influence of wind on mi-gration have even pointed out that a tail wind is a prereq-uisite for successful migration, at least for shorebirds(Piersma and Jukema 1990; Butler et al. 1997, but seeHedenstrom and Weber 1999), as taking advantage of fa-vourable winds minimizes the time for migration.

In order to evaluate the influence of wind on stopoverdecisions involving either landing or departure, a fieldstudy combining data from the birds and climatic param-eters is needed, although a theoretical model has alreadybeen developed (Weber et al. 1998).

In this study, the influence of wind direction on thestopover and departure decisions of migratory reed war-blers, Acrocephalus scirpaceus, was investigated at astopover site in north-east Spain during spring ringingcampaigns. The reed warbler is a species with a highcapture frequency in these campaigns, providing enoughdata for an analysis of the importance of wind on thegeneral migratory patterns.

The reed warbler is a night-migrating passerine bird thatbreeds in the middle latitudes of the west Palearctic andwinters in Africa south of the Sahara desert (Cramp 1992).

C. Barriocanal (✉) · D. Montserrat · D. RobsonDepartamento de Geografia Física, Facultad de Geografia, Universidad de Barcelona, 08028 Barcelona, Spaine-mail: barrioca@wanadoo.es

Int J Biometeorol (2002) 46:192–196DOI 10.1007/s00484-002-0133-3

O R I G I N A L A RT I C L E

C. Barriocanal · D. Montserrat · D. Robson

Influences of wind flow on stopover decisions: the case of the reedwarbler Acrocephalus scirpaceus in the Western Mediterranean

Received: 24 July 2001 / Revised: 4 February 2002 / Accepted: 2 April 2002 / Published online: 4 June 2002© ISB 2002

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Study area and methods

The study was carried out at the Aiguamolls de l’Emp-ordà Natural Park, a coastal reed area located in north-east Spain. This marshy area is a very suitable habitat forbird stopovers, being mainly dominated by the reedPhragmites australis. The site is located at an intermedi-ate stage of spring and autumn bird migration where thetopographical characteristics frequently cause strongwinds. The data form part of a huge project studyingspring bird migration through the Mediterranean Sea(Piccole Isole project, see Spina et al. 1993 for details)and were collected during the course of 30 days, from 16April to 15 May, the total time of the campaign eachyear, during the period 1993–1997. Nets were set atfixed sites throughout the whole study period. Each cap-tured bird was ringed and several biometrical measure-ments taken (Robson et al. 2001). The number of birdscaptured was documented and a table was drawn up todetermine the evolution of captures along the 30 days ofthe study period each year. From this table we selectedimportant peak days on which large number of captureswere recorded, and those with a minimum number ofcaptures. As a rule, days selected for a high capture fre-quency show values representing at least 10% of the to-tal captures for a given year.

Wind calculations

Wind data were calculated from the nightly readings dur-ing the whole study period, a total of 600 readings. Fromthese we calculated the geostrophic wind speed (vg) at700 hPa and 850 hPa (an approximate height of 3,000 mand 1,500 m) by the method of Cuadrat and Pita (1997),

where g is the acceleration due to gravity, f is the Corio-lis parameter, a value of 9.5898 × 10–5 s–1 for the studyarea, z – z′ is the difference between two consecutive iso-hipses and a is the horizontal distance between the iso-hipses.

Data of the NOAA-CIRES Climate Diagnostics Cen-tre were used to calculate the wind direction using thenumerical data for the 700-hPa and 850-hPa geopoten-tials where x is the zonal component and y the meridion-al component. The differences between 40°N 0°E and42.5°N 0°E for the zonal component and 42.5°N 0°E and42.5°N 2.5°E for the meridional component were used.These differences result in an (x, y) vector of which theresultant (hypotenuse) is the wind direction. The arctan-gent obtained from the (x, y) vector (in degrees) is thewind direction relative to the co-ordinate axis. These da-ta are drawn on a co-ordinate axis where the 0° valuecorresponds to a north wind, 90° to the east wind and soon to 360°. Finally a direction-of-flow graphic is ob-tained.

The geostrophic flow at 850 hPa and 700 hPa givesthe wind conditions calculated on the basis of a pressuregradient and the deviation due to the earth’s rotation. The

values obtained represent a true wind but without takingaccount of the earth’s friction. For a point far from thesurface the geostrophic wind becomes the true wind be-cause the earth’s friction decreases, so at 700 hPa valuesare considered to represent a true wind. The data at850 hPa, however, will be slightly different from data at700 hPa because of the earth’s friction. These heights areselected to the altitudinal distribution of nocturnal mi-grant birds observed on their spring migration at severalareas in the desert (Bruderer et al. 1995).

Statistical tests were performed, using Rayleigh’s uni-formity test to calculate the probability of the null hy-pothesis that the wind directions are distributed in a uni-form manner. This and other circular statistical parame-ters (mean vector) were calculated using Oriana v 1.06.

Results

General aspects

The number of reed warblers caught during the study pe-riod was 2,478, annual values ranging from 189 for 1993to 928 in 1996. From the 600 days of field work, 22 dayswere selected as having a large number of captures, cor-responding to at least at 10% of the total captures foreach study year. Over the same period those days withfew captures (values close to 0) were also selected. A to-tal of 18 days were selected for the analysis.

Wind conditions on days with high capture values

From the 22 days selected, an analysis of wind directiontaken at 0000 hours GMT and 0600 hours GMT and at700 hPa and 850 hPa gave detailed information about theflight conditions for the night-migrating birds.

At 0000 hours GMT at a height represented by850 hPa, a flow from the fourth quadrant (winds fromthe north, north-west and west) was the main wind on77.2% of the 22 days selected (Rayleigh test: P<0.001;mean vector, µ=313°). At 700 hPa winds from this quad-rant were identified on 77.3% of the days selected (Ray-leigh test: P<0.001; µ=305°). At 0600 hours GMT flowsfrom the fourth quadrant were also the main winds on77.3% of the days selected at 850 hPa (Rayleigh test:P<0.001; µ=344°) and on the 78.5% of the days at700 hPa (Rayleigh test: P<0.001; µ=312°) (Fig. 1).These significant resulting winds are considered headwinds, unfavourable for migratory birds.

Wind conditions on days with few captures

On the basis of the 18 days selected for their low captureof reed warblers, (values close to 0) the winds, calculat-ed at the same times as for days of high capture (0000hours GMT and 0600 hours GMT), proved less signifi-cant at 0000 hours and 850 hPa, through a clear tendencywas shown (Rayleigh test: P=0.01271; µ=276°). At

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Fig. 1 Wind directions on dayswhen there was a high reedwarbler capture at 0000 hoursand 0600 GMT at 850 hPaand 700 hPa (n=22)

Fig. 2 Wind directions on dayswith low capture at 0000 hoursand 0600 hours GMT at850 hPa and 700 hPa (n=18)

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700 hPa, at the same hours, 77.8% of the selected dayshad westerly winds with north-west, west and south-westcomponents (Rayleigh test: P<0.001; µ=295°) (Fig. 2).

The analysis at 0600 hours GMT at 850 hPa alsoshowed less significant results for the main wind direction,although there was a clear tendency towards westerly com-ponents (Rayleigh test: P=0.03747; µ=278°). On 77.8% ofthe days selected for their low capture value, days had awesterly component at 700 hPa at 0600 hours (Rayleightest: P<0.005; µ=289°) (Fig. 2). The main interpretation ofthese results supports the idea of tail winds enhancingflight. Although some of the analyses performed showedresults that are statistically less significant, the discussionoutlined in study area and methods explains how the truewind takes account of the earth’s friction.

Discussion

The analysis of spring wind components at two heights,measured twice during the night at a stopover site innorth-east Spain, showed that at western Mediterraneanlatitudes, a stopover decision could be influenced bythese meteorological patterns. Wind is a factor affectingflying during the migration period (Richardson 1978,1990). Birds select a flight altitude where the wind flowmost accords with the main direction of the migrationflight (Bruderer et al. 1995). In general, migration flyingwith optimal winds reduces time and energy consump-tion. Movements aligned with tail winds can then be ex-pected to influence the migratory pattern, which in turnaffects landing decisions. According to our analysis, onthose days selected for their low capture rate, windsfrom the south west (tail winds) predominated, indicat-ing longer flights to the breeding territories.

There is an intense variability of weather in territoriescrossed by Palearctic migrants in western Europe. Aftercrossing the Sahara, birds face the Mediterranean Sea, aminor ecological barrier but available habitats on whichto land are rare (Liechti and Bruderer 1998). Once theIberian Peninsula is passed migrating birds find the Py-renees. As indicated by radar studies, these mountainsare not a barrier for trans-Saharan migrants in the spring(Hilgerloh et al. 1992). Migrants passing east of the Py-renees must face a windy area that could influence anystopover decision. In our study area situated at that point,westerly and southerly wind components do not develophigh-velocity flows. Winds with a clear northerly com-ponent do become high-velocity flows that can persistfor several days because of western Mediterranean cy-clones (Jansà 1986) and the presence of the Pyrenees.

Reed warblers arriving at Aiguamolls de l’Empordàare at an intermediate stage of their spring migrationfrom a general south–north passerine flight pattern ashas been demonstrated by radar studies (Hilgerloh et al.1992). Therefore, in order to conserve energy when en-countering of unfavourable winds, the birds stop and restuntil more favourable flight conditions prevail (Alerstamand Lindström 1990).

On the days selected for their high reed warblers cap-ture there had been opposing wind components relative tothe general migration directions in this area (head winds)on the previous night. A clear north wind component isnot suitable for migration once nocturnal migrants arriveat Aiguamolls de l’Empordà, an area where the birds decide to stop until better wind conditions prevail.

Our results agree with the idea that weather condi-tions affect the decision of about where to make a stop-over and resume migration to a suitable habitat (Pyle etal. 1993; Akesson and Hedenström 2000), especiallyduring the spring passage in a western Mediterranean ar-ea where wind flows are enforced. Winds with clearnortherly and easterly components are not favourableand birds make a stopover. When the wind situation isfavourable, having south and southwesterly components,migrating birds make a more uninterrupted flight. A sim-ilar pattern is described by Pyle et al. (1993) for Califor-nia although the wind components had no effect on de-parture.

Despite of the reported by Hilgerloh et al. (1992) fol-lowing radar studies, the Pyrenees could be consideredas a minor ecological barrier because of the weather con-ditions they generate.

Weber et al. (1998) presented a model that analysesstopover decisions on the basis of two different wind sit-uations: no wind and tail-wind assistance, concludingthat wind assistance cannot explain the differences be-tween predicted and observed field data. Despite thesemodels, further field studies are needed using data ofwinds at several heights and analysing their directionalcomponents and velocities along avian migration routesfor a better understanding of how the wind influences thegeneral programme of nocturnal bird migration.

Acknowledgements Mariano Barriendos helped us with the firstphase of this paper. We are grateful to Charles D. Kaufman for hishelp in correcting the English of an earlier draft and to an anony-mous reviewer who improved the manuscript with his comments.The Departament de Medi Ambient de la Generalitat de Catalunyasupported the Piccole Isole project at Aiguamolls de l’Empordà.D. Bigas, J.M. Bas, A. Bonan, O. Clarabuch, G. Gargallo and P.Pons were responsible of the collection of data in the study period.

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