Koolhaas et al: Disturbanceof foraging Knots by aircraft

Disturbance of foraging Knots by aircraft in the Dutch in August-October 1992

Anita Koolhaas, Anne Dekinga & Theunis Piersma

Koolhaas, A., Dekinga, A. & Piersma, T. 1993. Disturbanceof foraging Knots by aircraft in the Dutch Wadden Sea in August-October 1992. Wader Study Group Buff. 68: 20-22.

KnotsCalidris canutus were observedduring low-water periods on Hengst,a mudflatcomplex south of the islandof Vileland in the westernmostpart of the Dutch Wadden Sea. In the period 19 August to 12 October 1992 we registeredtheir numbers and behaviour on 13 days with low- flying (c. 50 m above surface level) aircraft (usuallyjet fighter) activityand on 20 days without such aircraft activity. Large numbers of Knots were rarely present on days with aircraft activity. On aircraft-days,Knot flocksshowed a greater tendencyto fly up at large distancesupon the approachof 2-3 human observers.They also more frequentlytook to the air withoutapparent reason on such days. Under foggy conditionsthe alarm response to jet fighters appeared more .severe. In the course of the study periodthere was perhaps a limiteddegree of habituationto jet fighteractivity. Light airplanes caused very strongdisturbance even when flyingat altitudes above 100 m.

NetherlandsInstitute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, , and ZoologicalLaboratory, University of Groningen,P.O. Box 14, 9750 AA Haren, The .

The Wadden Sea is of huge internationalimportance to the Wadden Sea and a National Nature Reserve. m•gratingwaterbirds. It is also intensivelyused as a Despite all these designations,the area is used also as militaryaircraft training ground, but there has been little an approachand turningspace for low-flyingNATO studyon the effectsof militaryand other aircrafton the jet-fightersattacking practice targets in a bombing numbers and behaviour of waders during low water (see range. The jet fighters usually overflew the area at summary in Smit & Visser 1993). Our recent incidental estimated heightsof >50 m, althoughsometimes as low observationson Knots Calidris canutusforaging on an as 10 m (S. Boonpers. comm.),firing repeatedly at intertidalarea close to the jet fighter target area on targets on the western half of Vileland, 6 km north of Vileland in the western Dutch Wadden Sea, are thus of Hengst. On target-practicedays we registeredup to interest.The day-to-dayvariability in the degree of three passingjet fighters per minute, but the intensity militaryactivity, ranging from none to high-intensity was more usually in the order of 3 thunderinglyloud flying by jet fighters,allowed us to compare the passages per 5 min. Flight activity lasted usuallyfor 2-5 numbers and behaviour of Knots in the presence and hr per day, but sometimescontinued for much longer absence of low-flyingjet fighters.Since we were often in into the night. the field duringweekends, when there was no military flying,we obtainedmany 'control'-days.On two Between 19 Augustand 12 October 1992 we spent 39 Saturdays this calm was, however, replaced by the days in the field. A field day consisted of a visit to the activityof low-flyinglight 'tourist'airplanes. intertidalstudy area by inflatableZodiac-boat startingas soon as part of the mudflats became drained. Visits Our work was carried out on a sandflat complex called were always in daylightand usually lasted 4-5 hrs, but 'Hengst'.This lies east of the entranceof the sometimes only 2 h and rarely 7 h, the period into the Wadden Sea (EyerlandseGat) betweenthe dependinglargely on the vagaries of tide and weather. islands of Texel to the south-west and Vileland to the When on the sandflatswe were occupiedchiefly either north (The Netherlands, 53ø10'N, 04 ø 56'E). Hengst, by benthicsampling or by intensive observationson the consistingof area called Ballastplaatalong its southern foraging behaviourof Knots. In addition,we always edge and Steenplaatin the northand coveringc. 1650 made an estimate of the numbersof Knotson Hengst in ha, is surroundedby deep gulliesand can be reached the course of each field day. only by boat. Part of the northernshore of Hengst has the status of Harbour Seal Reservation, and no people On most observationdays we recorded the presence or are permittedthere between 15 May and 1 September. absence of jet fightersand other aircraftand we tried to The entire area is officiallydesignated as a 'quiet area' describe their activity.We also took notes on three (Stiltegebied)and is part of the Ramsar designationfor aspects of the behaviourof Knots. Firstlywe scoredthe

2O Koolhaas et al: Disturbanceof foraging Knots by a•rcraft

Table 1. Observedeffects of disturbanceby two categoriesof aircraft(light airplanes and jet fighters)on the numbersand behaviourof Knotson Hengstduring low-water periods. Incidences of approachabilityand restlessnessare scoredas x out of y fielddays.

Condition n (days) Average Incidenceof Incidenceof no. of Knots low approachability restlessness

No. aircraft 20 6,392 1/9 1/9

Lightairplanes 2 9,000 2/2 2/2

Jet fighters 11 2,691 1/5 4/7

extent to which they were approachableby human The maximum daily number of Knots present on Hengst observers. In areas withoutjet fighter activity such as fluctuatedgreatly between 19 August and 12 October. near Griend, farther east in the Wadden Sea, Knot Numbers varied from no birds to a maximum of 40,000 flocksare usuallyapproachable to distancesof 60 m or birds (Figure 1). During the study period, the Knots less (2 or 3 observersslowly walking towards the flock present in the Wadden Sea are consideredto be mostly withoutit takingflight) (Piersma et al. in prep.). Such of the islandica-subspeciesand are undergoing a approachabilitywas also observedon Hengst, but on complete wing-, tail- and contour-feathermoult (Boere & some days the birds'approachability was much lower, Smit 1980, Davidson & Wilson 1992, pers. obs.). This with flocks flyingaway at distancesof 120 m or more period is well before many birds have started the from observers. The difference between the categories post-moultmovement to their early winter areas such as of high (< 60 m) and low (> 120 m) approachabilitywas the Wash in Britain. Hence, we would not expect quite distinct.We notedalso the frequencyat which temporal trends in the size of the local Knot population flockswere flyingup duringlow water. The resulting in the course of our study period, and indeed no such flightsusually lasted only about 30 s beforebirds landed trend occurred(Figure 1). On jet fighter-daysthe close to their startingpoint. On some days, when there average numbers of Knots on Hengst are much lower were many of these round-flights,the Knotsgave an than on field days without aircraft (Table 1). Indeed, in impressionof strongrestlessness. We use this degree five of the six cases when Knots numbered more than of restlessnessto give a field day-specificrestlessness 10,000 birds, there was no aircraft activity. (This score. Finallywe notedwhether or not duringa low- difference is not statisticallysignificant from the 'null'- water period (many) Knotsleft Hengstfor other areas, expectationbased on the ratio of negative and positive usuallylocated to the east and north-east.These areas aircraftdays when numbersof Knots are lower than are less harried by low-flyingmilitary aircraft since they 10,000, exact p-value is 0.37 for a two-sided likelihood lie farther from the target area on . Chi-square ratio test based on the Chi-square distribution.)Another statisticswere carried out with StatXact (Cytel 1991). way to look at the count data is to compare successive days with and withoutaircraft activity in order to avoid a possibleseasonal bias (thoughadmittedly introducing 4O another one: carry-over effects between days). In 3

35 pairs of days the numbersof Knotswere higher on the aircraft-freeday, and in 5 pairs of days the numbers 3O were lower on the aircraft-free day. Longer-term studies are requiredto establishif such effects are statistically significant. x 2O '• •5 During days when either jet fightersor light airplanes were active, the Knot-flockson Hengst were less approachable than on days without aircraft (Table 1; the exact p-value accordingto a likelihoodratio test on the 5 basis of a Chi-square distributionis 0.06, i.e. there is only a 6% chance of mistakenlyattributing significance). 18 28 7 17 27 7 17 Knots showed also a statisticallysignificant greater August September October restlessness(exact p=0.02). It should be noted that on Figure1. Variationin the maximumnumbers of Knotscounted on field all days that the Knots were difficultto approach they days on Hengstfrom 19 Augustto 12 October 1992. Open were also independentlyscored as restless. On days circles indicatefield days withoutaircraft activity, big dots indicatedays withjet fighteror small airplaneactivity and small when they were easy to approach they were 'calm'. dots days when no informationon aircraftactivity was collected. There was, however, one day without aircraft when

21 Koolhaas et al: Disturbanceof foraging Knots by aircraft birds were difficultto approach. This was after a heavy small multipliereffects can do great damage given the night-timemilitary shooting/training session close to right (negative) conditions.The possiblysevere effect is their roost on south-central Vileland. We suspect that that the increasein the numbersof unproductiveflights this resulted in a heavy disturbance on the high-tide during low-water periods decreases the overall time roost during darkness, and that the birds' restlessness spentfeeding and increasesthe energyexpenditure may have been a lastingeffect of this great disturbance. (see also Belanger& Bedard1989, 1990). Such oppositeeffects are proneto lead to a negativeenergy F•nallythere are a few more qualitative phenomena to balance. There is no doubt in our minds that Knots find report. At the start of the study period in late August, a it very difficultto adjustto lightairplanes in particular. bme when the majority of Knots must just have arrived, there was a more marked responseto jet fighterflights than later on. The response to overflyingjet fighters ACKNOWLEDG EM ENTS •n•t•allyconsisted of very short flights of part of the flock while other individualsremained on the ground but We are mostgrateful to Mr J. Groen for keepinga close showed a light crouchingresponse, stoppingfeeding for watch on us when we were out 'in the wild' and to Sil a t•me and lookingup. With heavy jet fighter activity(>3 Boon for offering harbourspace for the Zodiac and passages per min.) this crouching behaviour was shown many other niceties.Jaap van der Meer helped with even at the end of the study period. We had the definite statistics. Nick Davidson, Jan Beukema and Cor Smit impression that under foggy conditionsthe Knot flocks commented on a draft. showed a more severe response to jet fighter overflights than under clear conditionsbut we could not properly quantifythis effect. When light airplanes played low over REFERENCES the mudflats on 12 and 26 September the response was, however, clearly severe: Knot flocks left the area Belanger,L. & Bedard,J. 1989. Responsesto stagingGreater Snow directly after the disturbance began. Geese to human disturbance.J. WildlifeManagement53: 713- 719. Although there is thus some reason to believe that Belanger, L. & Bedard, J. 1990. Energeticcosts of man-induceddis- turbanceto stagingSnow Geese. J. WildlifeManagement 54: Knots become a little behaviourally 'adjusted' (i.e. 36-41. habituated:less disturbed)once they have gained Boere, G.C. & Smit, C.J. 1980. Knot (Calidriscanutus L.). Pp. 136-145 experience of the jet fighter-phenomenonthis holds only in: Smit, C.J. & Wolff, W.J. (eds.), The birdsof the Wadden for the individualsthat are persistent in using the area Sea. Balkema, Rotterdam. Cytel Software Corporation. 1991. StatXact User Manual Version2. throughoutthe season. The presence of low-flyingjet Cambridge, Mass. fightersis likelyat least subtly, and possiblyseverely, to Davidson,N.C. & Wilson,J.R. 1992. The migrationsystem of decrease the carryingcapacity of the area for Knots. European-winteringKnots Calidris canutus islandica. Wader Study Group Buff.64, Suppl.:39-51. The subtle effect is that a decrease in approachabilityis Smit, C.J. & Visser, G.J.M. 1993. Effects of disturbance on shorebirds: indicativeof a much greater susceptibilityto light a summaryof existingknowledge from the Dutch Wadden Sea disturbances. It is therefore a multipliereffect, and even and Delta area. Wader Study Group Bull. 68:6-19.

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