50 | Triennial Scientific Report

Monitoring bird migration by weather radar

Adriaan Dokter and Iwan Holleman

IntroductionWeatherandbirdmigrationareintimatelyrelated.Evolutionhasshapedthemigrationstrategiesofbirdssuchthattheyoptimallyrespondtoandmakeuseofweatherconditionsduringtheirlongdistancetravelsbetweenbreedingandwinteringgrounds.Bothwindandprecipitationstronglydeterminetheday-to-daytimingofmigrationandaltitudeusebybirds.

Spatiotemporalinformationonbirdmigrationisofinvaluableusetoscientistsandsocietyalike,butsofarnosensornetworkshavebeenestablishedthatcanmonitorbirdmovementcontinuouslyoverlargeareas.Inaviationbirdmigrationinformationisimportantforimprovingflightsafety.Especiallymilitarylowlevelflyinghasahighriskofen-routebirdstrikesandspatialbirdmigrationinformationisessentialforgeneratingreliableflightwarningstopilots.Comprehensivemonitoringofbirdmigrationatcontinentalscalescanalsoprovideinsightintomigrationpatternsandtheimpactonmigratoryflightofsynopticscalefactorslikeweatherandorography.

AspartofaninternationalprojectbytheEuropeanSpaceAgency(ESA)aimedatreducingcollisionsbetweenaircraftsandbirds,wehaveexploredthepotentialofoperationalDopplerweatherradarasabirdmigrationsensor.

Observing bird migration by Doppler weather radarOperationalweatherradarnetworksexistine.g.EuropeandtheUnitedStatesformeteorologicalapplications.ThesenetworkshavealargespatialcoverageasillustratedinFigure1,showingpartoftheEuropean

networkOPERA(OperationalProgrammefortheExchangeofweatherRadarinformation)1).Althoughweatherradarsareusedprimarilyforprecipitationmonitoring,alsobiologicalscattererscanbeobservedbythesesystems.Boundarylayerclear-airweatherradarechoesarecausednearlyexclusivelybyarthropods(most-lyinsects)andflyingbirds.Weatherradarsarethereforeapromisingsensorforprovidingbirdmigrationinformation.

Abirdmigrationquantificationalgorithmneedstoauto-maticallydistinguishbird-scatteredsignalsfromallotherechoesdetectedbyweatherradar.Birdechoesandotherclear-airsignalstendtobeconsiderablyweakerthanmeteorologicallyrelevantsignalsfromhydrometeors.Birds,insectsandhydrometeorsgiverisetosignalsinan

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Figure 1. Map of operational weather radars for part of Western Europe.

Radar sites are indicated by bullets, the weather radars used in this study

are labelled and coloured red. The OPERA reflectivit y composite is overlaid

for 19 April 2008 19.30 UTC.

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overlappingreflectivityregime,whichmakesitchallengingtodistinguishthem.

Dopplerweatherradarsprovideinformationontheradialvelocityofscatterers,whichweusetoselectoutechoesrelatedtobirdmigration.Figure2showsweatherradarimagesduringintensebirdmigration(toprow)

andduringacasewithconvectiveshowers(bottomrow).Birdmigrationgivesrisetoacharacteristicallyhighspatialvariabilityoftheradialvelocityscandata,whichisnotobservedforechoesfromprecipitationorinsects.Unlikeprecipitationandinsects,birdsperformactiveflightwhichvariesinspeedanddirectionperindividual,causingahighervariabilityintheDopplervelocity.

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Figure 2. Radar displays for reflectivity factor (left) and radial velocity (right) during a bird migration event (top rectangular box in green) and an event

with weak convective showers (bottom rectangular box in blue). The displays show the 1.2 degree elevation scan for a circular area of 25 km radius

around the weather radar in De Bilt. Contiguous areas of echoes identified by the algorithm are indicated within the reflectivity displays by red borders.

For the precipitation case (bottom) these areas removed from the data based on a low spatial variance of the radial velocities, while for the bird migration

case (top) these areas are retained based on a high spatial variance of radial velocities. After removing precipitation echoes average bird densities are

calculated from the remaining reflectivity data.

52 | Triennial Scientific Report

Wedevelopedabirddetectionalgorithm3,4)basedonexistingwind-profilingalgorithmsforDopplerweatherradars,usingtheVolumeVelocityProfiling(VVP)tech-nique5).Atargetidentificationschemewasdevelopedtofilteroutnon-birdechoesfromtheradarvolumedata,basedonananalysisofthelocalvarianceinradialvelocity.Thefilteredreflectivitydataisusedtoconstructabirddensityaltitudeprofile,andananalysisoftheradialvelocitydatayieldstheaveragebirdspeedanddirection.Reflectivitywasconvertedtobirddensitybyassumingaconstantbirdradarcrosssectionof11cm2,whichwefoundtobeanadequateapproximationduringnocturnalmigrationoverWesternEurope,whichisstronglydominatedbypasserines(smallsong-birds).

Bird radar field campaigns for validationIncorporationwiththeSwissOrnithologicalInstituteweusedadedicatedbirdradarofthetypeSuperfle-dermaus(seeFigure3)tovalidatetheweatherradarobservationsofbirdmigration.Thededicatedradariscapableofdetectingthewingbeatpatternforindividualradartargets6)(seeFigure4).Basedontheseechosigna-tures,insects,birdsandhydrometeorscanbedistinguis-hedwithahighselectivity.Themobiletrackingradaristhereforeastate-of-the-artreferenceforvalidatingtheweatherradarbirdobservations7).

Threefieldcampaignswereorganizedtovalidatetheweatherradarbirdobservations.Thebirdradarhas

beenstationedwithinthemeasurementvolumeoftheweatherradarinDeBilt,theNetherlandsfrom19Aug-16Sep2007,inWideumont,Belgiumfrom18Sep-22Oct2007andinTrappes,Francefrom10Mar-9May2008,thuscoveringafullautumnandspringmigrationseason.InFigure5thebirddensitiesaltitudeprofilesdetectedbybirdradar(toppanel)andweatherradar(middlepanel)aredisplayedfortheperiodof11-16October2007,whenthebirdradarwasstationedinTrappes.Wefindaremarkablecorrespondenceinthedetectedbirddensitiesbythetwosensorsduringallfieldcampaigns.Thealtitudedistributionsandabsolutenumberofdetectedbirdsmatchquantitatively.Weatherradarcanthusbeusedasareliablesensorforbirdmigrationquantification.

The effect of wind on bird migrationTheeffectoftheenvironmentalwindonflightaltitudesbecomesevidentbycomparingtheweatherradarbirddensityprofile(Figure5middlepanel)withthewindprofilescalculatedbytheHIRLAMnumericalweatherpre-dictionmodel(Figure5bottompanel).Wefindthatbirdsadjusttheirflightaltitudetomakeoptimaluseoftailwindsalongthepredominant(south-westerly)migratorydirection.Forexample,theHIRLAMwindprofile(bottompanelFigure5)showsthatonOctober4windconditi-onsathighaltitudewereunfavourable,wherestrongwesterlywindscausenegativewindeffects.Migrationthereforedoesnotextendabove1km(seemiddlepanelFigure5).Ontheotherhand,onOctober6tailwindsaremostfavourableabove1kmandasaresultalargefractionofmigrationtakesplaceathighaltitude.

Birdmigrationpatternsobservedatsinglesitescanstronglydependonweatherconditionselsewhere.Thisappliesparticularlytonortherntemperateclimate,wherefrequentpassagesofhighandlowpressuresystemscausealargespatialvariabilityinweather.Usingfourweatherradars(seeFigure1)we

Figure 3. The bird radar Superfledermaus equipped with a camera moun-

ted parallel to the radar antenna.

Figure 4. Wingbeat pattern of a small songbird with regular phases

of wingbeats (frequency around 15 Hz) and pauses as recorded by the

Superfledermaus bird radar. The single wingbeats are clearly visible. The

envelope of increasing and decreasing amplitude (vertical axis) of the

signal along the time axis (horizontal axis) reflects the birds flight entering

the radar beam at one edge, flying through the centre and leaving at the

other edge.

Weather radar can be used as a reliable sensor for bird migration quantification

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Height-integrated bird density and wind profile

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Figure 5. Comparison of the bird densities altitude profiles determined by bird radar (top panel) and weather radar (middle panel). Integrated bird densi-

ties over all height layers are displayed in the lower panel for both weather radar (red) and bird radar (blue) (left vertical axis). The period between sunset

and sunrise is shaded in grey. Wind barbs in the lower panel show the wind profile from the HIRLAM numerical weather prediction model (right vertical

axis). Wind barbs in the middle panel indicate the bird speed and direction as retrieved by the weather radar algorithm. Each half flag represents 10 km/h

and each full flag 20 km/h.

monitoredthebirdmigrationflywayatseveralsitessimultaneously.Weatherradarbirdobservationsforthenightof19-20April2008aredepictedinFigure6.ForthisnighttheOPERAreflectivitycompositeisoverlaidonFigure1togiveanimpressionofthelargescaleweatherconditions.

Ateachsite,thetimingandaltitudeprofileofbirdmigra-tionisobservedtobeinfluencedbyweatherconditionsatlocationspassedearlierduringamigratoryflight.Asanexample,migrationatthemostsouthernsite(Trap-

pes)ischaracterizedbystrongdepartureandascenttohighaltitudesaround2.5km,wherebirdsexperiencefavourabletailwindsalongthepreferredmigratorydirection(NEinspring).Lowaltitudemigrationisavoidedbecauseofunfavourableeasterlylowlevelwinds.Atoneradarsitenorth-eastofWideumont,weobservetheappearanceofamigrationlayercentredaround2kmheightafter22UTC.Thismigrationlayerappearsafter4hoursofflighttime,whichcorrespondswithameasuredbirdgroundspeedof70km/htobirdshavingtravelledoveradistanceofabout280kmtowards

54 | Triennial Scientific Report

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north-east.ThesebirdsmustthereforehavedepartedinthevicinityofTrappesinnorthernFrance,wherebirdschoseaflightaltitudeof2.5km.SinceWideumontislocated500mabovemeansealevel,the2kmaltitude

bandcloselymatchesthecruisingaltitudeafterdepar-turenearTrappes.Birdshavethusmaintainedaconstantflightaltitudeduringtheirmigratoryflightwithrespecttosealevel.ThealtitudeprofileobservedinWideumontisclearlyaffectedbytheparticularwindconditionsatthemoresouthernsiteofTrappes.InDeBilt,northofWideumont,nobirdsdepartintheearlynight.ThisisexplainedbytheweakocclusionfrontslightlysouthofDeBilt,whichblocksmostmigration.Withthisfrontweakeninginactivity,migrationcondi-tionsbecamemorefavourableoverthecourseofthenight,andafter00UTCwedoobservethepassageandarrivalofmigratingbirds.NomigrationisobservedonthemostnortherlylocatedweatherradarinDenHelder.Theextenttowhichbirdsareabletoadapttheirflightaltitudesinresponsetochangingmeteorologicalcondi-tionswillbetopicoffutureresearch.

ConclusionWefindthatDopplerweatherradarishighlysuccessfulindeterminingquantitativebirddensitiesandaverageflightspeedsanddirectionsasafunctionofaltitude.Wefindthatweatherradarreflectivitycanbequantitativelycorrelatedtothebird-densitiesdeterminedindependentlybybirdradar.Thedevelopedmethodsforbirddetectionandquantificationcanbeeasilyextendedtofulloperationalweatherradarnetworks.WiththeestablishmentoftheOPERAdatacentreforradardatawithinthecomingtwoyears1)theestablishmentofacontinent-widebirdmigrationsensornetworkinEuropeiswithinreach.Suchanetworkcanenableimportantapplicationsbothinflightsafety,health(aviandiseasespread)andenvironmentalimpactassessments.

Figure 6. Bird densities as a function of time and altitude at different

weather radar sites (see Figure 1) for the night of 19 April 2008. The wind

barbs represent the measured bird ground speed and direction. The lower

panel shows the height integrated bird densities over 0.2-4 km altitude.

The period between sunset and sunrise is shaded in grey and civil twilight

is shaded in light grey. The pink boxed inset on the right hand side of each

panel shows the HIRLAM wind profile at 00 UTC. Around 00 UTC a double

layered bird density profile is observed in Wideumont. We attribute the top

band to birds that departed in the vicinity of Trappes, while the lower band

results from birds departed at closer distance.

The establishment of a continent-wide bird migration sensor network in Europe is within reach

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References1) Holleman I., L. Delobbe and A. Zgonc, 2008. Update on the european weather radar network (OPERA). Proc. Fifth European Conf. on Radar in Meteorology and Hydrology, June-July 2008, Helsinki, Finland, 5pp2) Holleman, I., H. van Gasteren and W. Bouten, 2008. Quality assessment of weather radar wind profiles during bird migration. J. Atm. Oceanic Technol., 25, 2188-2198, htpp://journals.ametsoc.org/doi/pdf/10.1175/2008JTECHA1067.13) Dokter, A.M., F. Liechti, H. Stark, L. Delobbe, P. Tabary and I. Holleman, 2010. Bird migration flight altitudes studied by a network of operational weather radars. J. R. Soc. Interface 7, doi:10-1098/rsif.2010.01164) Gasteren, H. van, I. Holleman, W. Bouten, E. van Loon, and J. Shamoun-Baranes, 2008. Extracting bird migration information from C-band weather radars. Ibis, 150, 674-686.5) Waldteufel, P. and H. Corbin, 1979. On the analysis of single doppler radar data. J. Appl. Meteor., 18, 532-542.6) Zaugg, S., G. Saporta, E. van Loon, H. Schmaljohann and F. Liechti, 2008. Automatic identification of bird targets with radar via patterns produced by wing flapping. J. Royal Soc. Interface, 5, 1041-1053.7) Schmaljohann, H., F. Liechti, E. Bchler, T. Steuri, and B. Bruderer, 2008. Quantification of bird migration by radar - a detection probability problem. Ibis, 150, 342-355.