Knots, White-rumped Sandpipers and other wader species performing bat-like aerial acrobatics on staging sites

TheunisPiersma, PatriciaM. GonzAlez,Petra de Goeij, GudmundurA. o Gudmundsson & Ake Lindstrdm

Piersma,T., GonzAlez,P.M, de Goeij,P., Gudmundsson, G.A. & Lindstr0m,.•. 1996. Knots, White-rumpedSandpipers and otherwader speciespedorming bat-like aerial acrobaticson stagingsites. Wader Study GroupBull. 79: 69-70. On severaloccasions, Knots Calidriscanutus having just arrivedon stagingareas in Argentina and Iceland,and White-rumpedSandpipers C. fuscicollisunder similar circumstances in Argentina,were observedto make very peculiarmovements in flightwhilst and after takingoff over the sea from I•igh tide roostsat recedingtides. On such occasionsmany individualbirds independently"shot out" of the departingflocks in all directionsand with quickturning movementsimmediately retreated back in the flock. These weirdlyflying waders briefly resembledbats (Chiroptera)or Swallows(Hirundo) hunting aerial insects. Functional explanationsfor this peculiarphenomenon have to take intoaccount that it was only observed in roost-leavingflocks just after they had arrivedat stopoversites. T. Piersma& P. de Goeij,Netherlands Institute for Sea Research(NIOZ), P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands. P.M. Gonzalez,Direccidn de Fauna,Ministerio de Econotufa,C.C. 84, PedroMordn 385, (8520) San AntonioOeste, Rfo Negro, Argentina. G.A. Gudmundsson,Icelandic Institute of NaturalHistory, P.O. Box 5320, IS-125 Reykjavik, Ice/and. ,•. LindstrSm,Department ofAnimal Ecology, Lund University, Ecology Building, S-223 62 Lund, Sweden.

Knots Calidris canutus and other shorebirds are well and.•L onother occasions whilst working with Knots on knownfor the co-ordinatedaerial manoeuvresmade by stopoverin Iceland later in May. their large flocks (Davis 1980; Potts 1984), sometimes in responseto aerial attacksby predatorssuch as On 6 March 1995 in the late afternoon, PMG, PdG and TP PeregrinesFalco peregrinus (Harrington 1996). were observing3,500 Knotsand several thousandWhite- Completelyindependently, and at stagingsites very far rumpedSandpipers Calidris fuscicollis at their high-tide apart, we have witnesseda very differentand strikingkind rooston the beach at Los Alamosfacing the Golfo San of flightby waders that brieflyleave a synchronizedflock Matias, just southwestof San AntonioOeste, Rio Negro, for shortbursts of unsynchronizedflight manoeuvres Argentina.The Knotshad arrivedat this site duringthe beforere-entering ito This phenomenonappears to be preceding24 hrs;the week beforeonly a handfulof birds undescribed in the literature. If this note serves to alert were presentat maximum. White-rumpedSandpipers othersto similarphenomena at othersites and times,and were also much more numerous than on earlier dates. in otherspecies, a futurecompilation of observations Between 17.00 and 17.22 hr local time the Knots left the mightperhaps suggest causal and/or adaptive hypotheses roostin flocksof 200-400 birds. These flocksdeparted to accountfor these funny and intriguingflights. low over the water towardsthe feeding areas at Banco Lobosin a south-easterlydirection. Immediatelyafter In northwestIceland on 10 May 1988 in the afternoon, take off from the beach roost, the first individual Knots GAGand .•L were observing 3,300 Knots on a roostin startedtheir "bat-flights".Such birdsflew almostvertically Vatnsfj0rdur.Many of the birdsmust have just arrived out of the onwardflying flock in an erraticfashion, here;2,400 were presenton 7 May and 5,200 were immediately"falling back" in the flock. There were tens of countedon both 11 and 12 May. At about 15.00 hr local individualsdoing this simultaneouslyout of differentparts time the birds started to leave the roost and it was noticed of the flock, and the behaviour continued as the flocks that they did so in a very strangeway. Parts of the flocks disappearedlow over the sea. In flocksof White-rumped flyingoff overthe waterwere temporarily broken up, the Sandpipersroosting close by on the same beach, some individuals"throwing themselves wildly in all directions,a birds"jumped up" with opened wings to a heightof 10-20 bit like swallowsor bats in foragingflights" (translated cm. It gave the impressionthat the beach was too hot to quotefrom notebookin Swedish). Suchflights were standon (but it was a cool day!). When the White- observed under similar conditions on a few other rumpedSandpipers started to leavethe roostin small occasionsthe two followingdays. They are referredto as flocks,individual birds jumped out of and intothe flying "bat-flights"but were not describedin detail again. The flocks,just as withthe Knots. The type of flightdisplayed peculiarflight behaviour has not been observedby GAG by these Knotsand White-rumpedSandpipers reminded

69 us (independentlyof the observersin Iceland)of aerially observationsin Icelandand Argentina,the godwitshad foragingswallows and bats. just arrived, but in contrastto the earlier observationstheir bat-flightsoccurred during south-bound, post-breeding, PMG had seen this behaviourseveral times previously migration. duringperiods of northwardmigration, although never with the same intensityas in March 1995. On 19 January Whatare bat-flightsfor? Do theyrepresent the stretching 1995 at Rio Grande,Tierra del Fuego,2-3 Knotswere of tiredbreast muscles after longand sustainedflights? seen in bat-flightas their flock left a beach-roost. Do theysignal aspects of the birds'quality, for example Jumpingup from the groundto a greater heightthan on their proficiencyin aerial acrobatics?Do bat-flights the roost,perhaps as highas 1 m, was seen on 10 March enablebirds to quicklyoversee larger areas than would 1989 in a groupof White-rumpedSandpipers foraging on otherwisebe possiblewhen flyinglow overthe water? intertidalflats near San AntonioOeste. On 7 April 1995 Giventhe paucityof descriptivematerial on thispeculiar vertical,up and down, bat-flightswere two times observed flightphenomenon, we refrainfrom picking our choice or in a flyingflock of about 100 HudsonianGodwits Lirnosa expandingthe listof causaland functional explanations haemastica. The behaviourwas never seen during for the flights. Rather, we like to hear from other southward,postbreeding, migration or duringthe austral observerswhether they have witnessedsomething similar, summer. and if so, in which speciesand underwhich conditions thesebat-flights occurred. We lookforward to hearing Our observationsof bat-flightsin Icelandand Argentina your experiences. showstriking similarities in the behaviourpatterns of two speciesof long-distancemigratory waders. Not onlywas the behavioursimilar if not identical,the conditionsduring ACKNOWLEDGEMENTS whichthe bat-flightswere seen were also remarkablyalike at the two sites. All three areas were used as staging This is publicationnumber 3023 of NIOZ, Texel. We sitesduring northward,migration by the two speciesof thank In•s do Nascimentoand Guy Morrisonfor long-distancemigrating waders. At both sites the .• preliminaryfeedback on this note. behaviouroccurred on days just after the arrival of large numbersof migrants,and it happenedas roostingflocks broke up into smallerunits that flew off overthe sea to low-waterfeeding groundsø REFERENCES

All of us have spentmuch time lookingat waders, Davis, J.M. 1980. The coordinatedaerobatics of Dudin flocks. Anita. Behav. 28: 668-673. especiallyduring the migrationseasons, but these bat- flightobservations stand entirely on their own. Alertedby Harrington,B.A. 1996. Flightagainst time. Theendless spring and the Argentinianexperience earlier in the year, in late July summer of the Red Knot. WGBH, Boston. 1995 TP noticedsimilar vertical erratic flights of Bar-tailed Potts,W.Ko 1984. The chorus-linehypothesis of manoeuvre GodwitsLirnosa lapponica, jumping out of smallflocks as coordinalJon in arian flocks. Nature 309: 344-345. they left the high-tideroost of Richel(in the westernDutch Wadden Sea) low over the water. In agreementwith the

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