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PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY 50 40 - A 77«. 85* 30 - 20 84 /Co /83 10 50 40 n i 30|- 20- 10- 62 • i •'.' : , / y=-42*0 88x r.0.74 81 p=0.02 79 • • ( ) 20 40 60 80 100 total biomass (gm-2) C 20 40 60 80 harvestable biomass (gm z ) 0 0.0 0.4 0.8 1.2 1.6 intake rate (trigs-' feeding) y = -3.0.81K r=0.95 p=0.0001 _L- 100 Fig. 20. \verage feeding density in Deotanber (Oystercatcherha"') ;is ii function ol A. total bkxnass, B. harvestable biomass and C predicted intake rale (mg s' feeding); same data us Fig, 16. Since no bird counts were available lor December 1985. we compared lor lhat year Food supply ami hinl clensiiy at the end ol October, Bird densit) in December was, on average, 1.5 times as high as in Octo- v I'll: hence Ibis [imliiplymy factor WHS used to estimate the density in l°85. ol the Wadden Sea. where the Oystercatchers decreased from October onwards. /\s an example, lio. 19B shows the monthly averages for nearby Schiermonnikoog. given separately for the eastern and western pan of the island. 260 How can we explain why our Stud) area attracted so many Oystercalchers in mid-winter? 'Flic* simples! explanation is that birds leave a feeding site if the) can achieve a higher intake rale elsewhere. II so. buds feeding in summer on mudflats with buried bivalves and Ragworms as the only prev would leave these areas in late summer to move to mussel and cockle beds (Fig. 15A). Since cockle and mussel beds are usually found on the lower part of the shore, and Macoma and Ragworms occur on mudflats often situated above mean sea level, the high shore is in summer a relatively more important feeding area than in winter. However, it is unlikely lhat this explains why our study area attracted so many Oystercatchers in winter. The tidal Hats in our study area consisted of mud and the greater pan were situated above mean sea level, so fewer buds would be expected to remain to feed in winter. In fact, the reverse was found I lie. 19). Possibly, the tendency of Oystercatchers lo concentrate from October onwards on tidal Hals adjacent to inland feeding areas (Fig. 19) explains this unexpected finding. This would explain Ihe relatively low numbers remaining to vv unci on Ihe eastern part of Schiermonnikoog. where there is no grassland, and the relatively high numbers wintering along the Frisian coast and on western Schiermonnikoog where grassland is available. The bird census data of ihe Dutch Wadden Sea (Zegers & Kwint 1992) also revealed that the number of Oystercatchers on Vlieland. an island without inland feeding areas, is in vv inlet 47'r lower than in late summer, whereas ihe winter numbers along the mainland coast of the provinces Noord-Holland, Friesland and Groningen. w ith extensive grasslands next to die sea wall, arc 28* I higher, on average, than in late summer; ibis calculation is based upon a comparison between the January counls from the four mild winters 1981. 1983. 1989 and 1991, and preceding counts from August OT September. As shown elsewhere (Zwarts et al. 1996d). the daily variation in exposure lime of the low water feeding areas in the Wadden Sea is much larger in winler than in summer. Consequently, the ability to compensate at high tide for short feeding periods is more important in the w inter half of the year lli-.ni in the summer half. This opportunity is apparently important enough for Oysiercatchers to move to parts of the Wadden Sea where compensatory feeding on grassland is available.
PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY total biomass io.ee , harvestable harvestable biomass biomass +0.68 _ •0.74 _ intake rate •0.93 bird density . +0.96 +0.95 Fig. 21. Dependence of bird densit] on food supply and predicted intake rate. The correlations are shown along the arrows: same data as I ig 20. Response of Oystercatchers to variation in harvestable prey biomass The Nes area was certainly not a marginal feeding area. The biomass of the food supply and the feeding density of Oystercatchers were, on average, respectively, 4 and 10 times as high as Ihe average for Ihe tidal flats of the Dutch Wadden Sea as a whole (Beukema 1976, Zwarts & Wanink 1993). The bird density fluctuated in accordance with the food supply, as shown in Fig. 20A. using the December data from Fig. lb. There is a good correlation between bird density and "total biomass". r = 0.74. which is actually surprising, because total biomass is an inaccurate measure of the fcxxl supply, due to the highly variable fraction that is unharvestable. Because of this, we expect a better correlation between bird densit) and the biomass ..| ihe prey that is both profitable as well as available, and this indeed appeared to be the case, r = 0.95 t lig. 20B). Very likely, this response is caused by the birds acting on the intake rales that they achieve i lie. HOC). Figure 21 summarizes the causal relationships, as we see them Two relationships are essential: bird density depends on intake rale, which in turn rate depends on the harvestable biomass. The high correlation between intake rale and harvestable biomass is not surprising, because the intake rate has been predicted from the density, si/e and burying depth of the prey, three variables which all contribute to harvestable biomass. It is al 261 ready obvious, however, that many wintering Oystercalchers only occur in the study area when older Cockles or second year Mya are abundant and ihe iniake rale is > 1.2 mg s*'. In contrast, when ihe wintering birds have to feed on Scrobicularia or Macoma. hardly any birds remain to Iced in the .uca because of the extremely low iniake rale. The corrcialions show n alongside the arrow s in Fig. 21, are based upon linear regressions, However, instead of a linear relationship between bird density and intake rate (Fig. 20C). we rather expect a J- orS-curve. No birds should ever feed in the area when the intakeis depend on what the birds can obtain elsewhere. In other words. even at a rather high intake rate, few birds may be concentrated in the area il "the situation elsewhere is even better. Once intake rales are so high dial the area ranks among the best in the region, the bird density will increase until all available" birds have been attracted to the area, after which the density will level off al still higher intake rales because the supplv of recruits dries up. How man) birds are "available' will depend on the total number of birds in the region and the size ol the region, which will depend on the size of the home range of ihe birds. With extreme site fidelity, home ranges and therefore the region will be very small, so that the pool of 'available' birds is quicklv depleted and very little relationship between bird density and food supply is to be expected when years are com pared. If. on the other hand, home ranges are large and variation in loud supplv between years quite extreme. a high correlation between bird densit) and intake rate can occur, as was probably the case in our study. The sightings of the colour-banded birds certainly proved thai movements of I to 3 km regularly occurred, although individuals could stav for inonihs. and some even for years, 00 the same spot of less than 0> ha. The sile fidelity of the Oystercalchers in our study area seems intermediate between that in the Exe estuary. where the mussel beds are extremely stable and home ranges often less than I ha (Goss-C'iistard et al. 1982), and the estuary of the Ribble. where an exceptional]) large spatfall of Cockles caused a massive influx of Oystercatchers < Suiherland I982d).
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WADERS AND THEIR ESTUARINE FOOD SUP
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plia^ohi. WADERS AND THEIR ESTUARI
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Waders and their estuarine food sup
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WADERS AND THEIR ESTUARINE FOOD SUP
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figuren: Dick Visser omslagfoto: Ja
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15 Versatility of male curlews (Num
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That science is making progress, ma
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INTRODUCTION The remnants of brushw
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When hcnlhic bivalves extend llien
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the burrow depths and on the fracti
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INTRODUCTION Ms,i invest 409 of (he
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able lo sw itch 10 oiher prey which
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Long-term, broadly-based research c
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Chapter 1 SEASONAL VARIATION IN BOD
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SEASONAL VARIATION IN BODY WEIGHT O
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Tahle 1 Weight loss ('•- ± SE) m
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i 60 50 40 30 20 10 0 • INFESTED
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240- SEASONAL VARIATION IN BODY WEI
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20 10 0 -10 -20h 2 3 4 5 6 7 8 9 se
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a E 400 - S. plana 35 mm 320 240 16
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Chapter 2 HOW THE FOOD SUPPLY HARVE
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FOOD SUPPLY HARVESTABLE BY WADERS H
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Methods The study sites were situat
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less than that of bivalves, partly
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I Fig. 3). Peak condition was reach
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total biomass since most of those t
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on the basis of the preceding and t
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However, for obvious reasons, we ma
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prey. A decrease in the prey densit
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Tahle 2. The intake rate ol < lyste
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* ' % -. . ; a X - . - * • ^ •
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(Zwarts & Wanink 1989). In quiet we
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general law relating handling time
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nearly twice as much time (0.79 s).
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4 5 6 7 8 9 size class of Corophium
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and annually. Second, the lower siz
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Prey switching Waders feeding on ti
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autumn and spring, and the contrary
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where they switch to surface-living
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Chapter 3 BURYING DEPTH OF THE BENT
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DEPTH AND SIPHON CROPPING IN SCROBI
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I DEPTH AND SIPHON CROPPING IN SCRO
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DEPTH AND SIPHON CROPPING IN SCROBI
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Table 3. Three-Way analysis Of vari
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Chapter 4 SIPHON SIZE AND BURYING D
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15 20 size (mm) Fig. 3. Cerasioderm
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10 size (mm) SIPHON SIZE AND DEPTH
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o, 7 01 5 | * CL 5- SIPHON SIZE AND
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4 Q) Si •a a. 16- 20- A predator:
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prey risk for an animal al a depth
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Table 6. Size al which there is a m
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* ' 1 /-/ "».-^*«C
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face and so expose themselves to a
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surface in die containers was varie
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50 OOF 310.00 I 5.00 ^ 1.00 3=" 0.5
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Tahle 1. Macoma and Scrobicularia.
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siphon would not have to reach the
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known siphon weight and burying dep
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ACCESSIBLE PREY ARE OFTEN IN POOR C
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1 2 3 4 burying depth (cm) Kin. I.
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I—1 • • : : : : ! - ! -|-r 0
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Chapter 7 DOES AN OPTIMALLY FORAGIN
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OPTIMAL FORAGING AND THE FUNCTIONAL
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100 200 300 prey density (n-m-2) Fi
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Table I. Results of five one way an
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Table 2. Results of eight iwo-wav a
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prey density II III Fig. 12. Freque
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PREY SIZE SELECTION AND INTAKE RATE
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Predicted 'passive size selection'
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lig. 2. Larger Mussels are less ava
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Fig. 5. Scrobicularia plana. Size c
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and maiiv are too thick-shelled to
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The measurements of handling time i
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1 2 3 4 5 6 7 probing depth (cm PRE
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valves may vary by a factor of two
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optimal foraging model, the minimum
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their gut is full? Do they reduce t
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PREY PROFITABILITY AND INTAKE RATE
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catchers to take only certain prey
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licult error of estimate arose if p
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Counts of feeding and non-feeding b
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20 30 40 50 shell length (mm) PREY
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Nereis Arenicola tipuia earthwo'm M
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take rate. We might therefore expec
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prey species, using parallel slopes
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5.0 4.0 1-3.0 a & • % * • * •
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time during the feeding period. As
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winter. Similarly, handling time in
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5.0 - f-4.0 S 3.0 in I 20 a 2 a | 1
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situation arrived in the western pa
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• ' • 14 PREY PROFITABILITY AND
Page 205 and 206: Notes lo appendix: PREY PROFITABILI
Page 207: Chapter 10 WHY OYSTERCATCHERS HAEMA
Page 210 and 211: 2 4 6 8 10 time on feeding area (h)
Page 212 and 213: 80- 60- o 80 60- 40- 20- INTAKE RAT
Page 214 and 215: est of bod\ behind: an estimated 22
Page 216 and 217: INTAKE RATE AND PROCESSING RATE IN
Page 218 and 219: Wanink 1993). The energy content of
Page 220 and 221: (7) Age All studies dealt with adul
Page 222 and 223: irds over long periods. As an examp
Page 224 and 225: Discussion There is no difference i
Page 226 and 227: comparison between the weight of ih
Page 228 and 229: . 1', L ! •
Page 230 and 231: Introduction PREDICTING SEASONAL AN
Page 232 and 233: PREDICTING SEASONAL AND ANNUAL FLUC
Page 240 and 241: 1000 r 1 II" 1 - r^Jan'80 PREDICTIN
Page 252 and 253: IOO BO 60 40 20 0 PREDICTING SEASON
Page 265 and 266: WHY KNOT TAKE MEDIUM-SIZED MACOMA W
Page 267 and 268: in Zwarts & Esselink 1989). The len
Page 269 and 270: shell length (mm) FIR. 3. Peringia.
Page 271 and 272: fused, specimens larger than this w
Page 273 and 274: 50 100- s s I — f = S. plana, n=2
Page 275 and 276: area is twice as large as the touch
Page 277 and 278: 10 15 lower size threshold (mm) Fig
Page 279 and 280: lime (Hughes 1979). This would furt
Page 281 and 282: WHY KNOT TAKE MEDIUM-SIZED MACOMA T
Page 283 and 284: Chapter 13 ANNUAL AND SEASONAL VARI
Page 285 and 286: VARIATION IN FOOD SUPPLY OF KNOT AN
Page 287 and 288: Two sites (N and M in Fig. 2) were
Page 289 and 290: 20 30 VARIATION IN FOOD SUPPLY OF K
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Page 293 and 294: July ' Aug ' Sept Fig. 9. Proportio
Page 295 and 296: available. There must, however, be
Page 297 and 298: Chapter 14 SEASONAL TREND IN BURROW
Page 299 and 300: BURROWING AND FEEDING IN NEREIS SEA
Page 301 and 302: Table 2. Results of a 2-way analysi
Page 303 and 304: June 1981 June 1982 is * N.MUD •
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6 - n=!080 5 • g 4 CP > i 3 CO CD
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1985) and Curlew (/wails ,v, Lsseli
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Chapter 15 VERSATILITY OF MALE CURL
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Smid! 1951. Muus 1967, Wolff 1973,
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0.6 0.8 1.0 1.2 1.4 1.6 jaw lengih
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Na oi both Nrieck Fig. 7. Numenius
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6 8 10 12 14 16 18 worm length (cm)
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too (Fig. 11 A). Indeed, the averag
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VERSATILITY OF CURLEWS FEEDING ON N
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total time spent al the surface. Th
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2 4 . • 2.2 • 30 .-.2.0 to Q. 1
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PREY DEPLETION BY OYSTERCATCHER AND
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the rule: the observed lower limit
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to locate the clams after they had
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Curlew, by bury ing some hundreds o
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PREY DEPLETION BY OYSTERCATCHER AND
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SAMENVATTING 345
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Voorgeschiedenis In 1960 verscheen
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maken. Als gebied A een grotere voe
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omdat ze nog vrijwel niets wegen. l
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kunnen opzuigen. of diep leven en z
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van de uitgerekte sifo over het wad
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het toen niet gemakkelijk hebben ge
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doende nonnetjes van 10 tot 15 mm l
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aangevoerde voedsel en de wulp past
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zijn dan twee jaar en wulpen geen s
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was dan ook dank/ij de inzet van al
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REFERENCES 367
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Allen RL. 1983. Feeding behaviour o
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BreyT. 1989. Der Einfluss physikali
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latie tol chironoiiiiilen en de wai
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huch der Vogel Milteleuropas, Band
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llolmann II. & H. Huerschelmann 196
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l-cndrem D.W. 1984. Flocking, feedi
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Pekkarinen M. 1984. Regeneration of
Page 379 and 380:
lion of Mussels Mytilus edulis hy O
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lulal Dal esiuanes: a comparison of
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