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PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY growing season in 1977 to 35 mm in 1979. From then on the length increment was 1 mm per year. Over the same period, the body weight increased from 50 mg in 1977 to about 300 mg from 1981 onwards (Fig. 9C). The seasonal variation in the body weight was large, however, being in March 40% below the level reached nine months before in June (Fig. 9C; see also Fig. 8 in Zwarts 1991). There was a rather low mortality between 1977 and 1982. but the population collapsed in 1983 when the animals were seven years old (Fig. 9B). In the three years before, the biomass varied seasonally between 40 and 60 g ta" 2 (Fig. 9A). However, the seasonal variation in biomass accessible to Oystercatchers was much larger, between 0 and 60 g nr 2 , because Scrobicularia lived in winter out of reach of the bill, except during Ihe winter of 1982/1983 (Fig. 9A). The predicted intake rate of Oystercatchers feeding on Scrobicularia depended mainly on the burying depth of their prey (Fig. 9D). being high in June when the burying depth was minimal and close to zero in winter when the majority of the prey lived out of reach of the bill. Again, the exception was the winter 1982/1983 when the intake rate was predicted to have been 0.8 mg s' 1 . The intake rate before 1980 was estimated by assuming thai (he seasonal variation in burying depth did not deviate from the average depth in 1980-1982. The high intake rate in 1979 was due to the very good prey condition of the individual Scrobicularia (Fig. 9C). Scrobicularia were still small before ihe growing season of 1979 (Fig. 9C). which explains the low intake rate in 1978. Although they were still smaller, and thus less profitable, in 1977 and 1978, this was compensated by a higher fraction of the prey being accessible. Whereas Scrobicularia > 25 mm buried in winter out of reach of the Oystercatcher's bill, specimens < 25 mm remained accessible (Zwarts & Wanink 1989). and were indeed taken by Oystercalchers in winter (Habekotte 1987). Oystercatchers and Macoma There was spatfall of Macoma during six of the eleven years, but only the 1979 recruitment was extremely large (Fig. 10B). Oystercatchers always ignore Macoma < 11 mm (Hulscher 1982) and take above this lower size limit relatively much more of the largest si/e classes i/warts et al. 1996a). Macoma > 15 mm were rare between 1977 and 1982. but 2-3 times more 248 common in 1983-1985 (Fig. I0B). Macoma grew slowly. The year class 1979 measured 6 mm after one growing season, reached the length of 12 mm in 1980, after which the growth in length was 1 mm per year (Zwarts et al. 1992). The first heavy spatfall after 1979 took place in 1985. In the years between, the population density remained remarkably stable, which would suggest there was no mortality. However, a closer look on Fig. 10B shows there was a seasonal variation in the numbers. The population increased each year in March-May and decreased over the rest of the year. The increase in the Nes area in early spring must have been due to a change in distribution pattern of Macoma by which animals living further upshore resettled in the Nes area, such as has been documented by Beukema (1993b) elsewhere in the Wadden Sea. Careful inspection of the monthly size-frequency distribution revealed that the immigrating animals were small compared to those from the Nes area. This difference was to be expected since growth is retarded on high-level mudflats (Wanink & Zwarts 1993). The immigrating specimens were each year larger than in the preceeding year, which suggests that the immigration was of clams of the strong year class 1979 and thus the migration from high to low flats continued over several years. The biomass varied seasonally (Fig. 10A). This was mainly due to the variation in the average weight of the larger prey (Fig. IOC; see also Fig. 8 in Zwarts 1991). Clearly, the winter would be a very difficult period for Oystercatchers if they were to depend solely on Macoma. Not only was the body condition of the prey 40% below the summer level, but me prey also had to be located at a larger depth (Fig. 10A). Consequently, the predicted intake rate was much lower in winter than in summer (Fig. I0D). Since no depth measurements were made before 1980. the predicted intake rate could not be calculated, but on the assumption that the seasonal variation in burying depth did not deviate from average, the intake could be estimated. The intake rate was expected lo have been low in the first years of observation due to the low density of large Macoma (Fig. 10B); the exception was in the summer of 1979 when the bivalves had an exceptionally good body condition (Fig. 10C; Zwarts 1991: Fig. 8). Oystercatchers and Mya There was recruitment of Mya in 1976 and in 1979 but
PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY not in the other nine years. The relationship between density and prey weight is shown for both cohorts in Fig. 11. Oystercatchers did not feed on Mya < 17 mm. and thus ignored prey containing less than 10-15 mg. Hence, the decline in the population during the first year of life was not due to Oystercatcher predation. y E 1000 r T0 ° SMALL 100 e cr) C -o 10t- 1 - 1 1 100 - CD -o 10 COHORT 1976 Jaii'SoJ^ Jan 78 Jan '81 -*i Jan 132 -. TOO DEEP Issnta-rm^ Jan 84 -r,^" COHORT 1979 jan 85-/ 1 ~ - 10 100 prey weight (mg) 1000 Fig. 11. Density of Mya belonging lo two different cohorts as a function of flesh weight during four years. The lines conned the course of the change in the density and average prey weight as long as the cohort existed; based on monthly samples in the Nes area. In each winter there was a decrease of the prey weight due lo a decline in the body condilion. Prey < 10 mg are too small lo be harvested h\ Oystercalchers, whereas prey > 200 mg live ioo deeply buried to be .iccessihle. 249 1977 78 79 80 81 82 83 84 85 86 year Fig, 12. Mya. Total biomass and biomass of the specimens living in the upper 6 cm of the substrate (g m ') during IO years. Periods during which Oystercatchers heavily exploit this food resource are in- .Ik.iu-.l During the second year, however, Oystercatchers exerted a heavy predation pressure on the remainder. After the next growing season. Mya measured 45 mm and weighed 700 to 1000 mg. At this size, they all lived out of reach of the Oystercatcher's bill (Zwarts & Wanink 1984. 1993). There was a large annual and seasonal variation in the total biomass of Mya (Fig. 12). The year-to-year variation was still larger for the biomass accessible to Oystercatchers (Fig. 12). In fact, only in two years were Oystercatchers able to feed on this prey. From the observations in October 1980, we estimated the intake rate that the birds achieved (see Methods'). The Oystercatchers also foraged on Mya in the winter of 1977/78, when they took prey of about die same size as in autumn 1980 but the prey density was higher. Since the intake rate was not measured, we assumed that it did not differ from that three years later. Oystercatchers and Mussels Mussels might have been an alternative prey for Oyslercatehers in 1985 and 1986. when the biomass nf this prey reached a level of 10 g nr** (Zwarts & Wanink 1993: Fig. 4E). In both years, the population consisted of spat. There was a huge mortality: 99% of the popu-
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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 & • % * • * •
Page 193 and 194: time during the feeding period. As
Page 195 and 196: winter. Similarly, handling time in
Page 197 and 198: 5.0 - f-4.0 S 3.0 in I 20 a 2 a | 1
Page 199 and 200: situation arrived in the western pa
Page 201 and 202: PREY PROFITABILITY AND INTAKE RATE
Page 203 and 204: • ' • 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
Page 291 and 292: 20 l i r'l ' i •o j^y^T n ^ ' " ^
Page 293 and 294: July ' Aug ' Sept Fig. 9. Proportio
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available. There must, however, be
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Chapter 14 SEASONAL TREND IN BURROW
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BURROWING AND FEEDING IN NEREIS SEA
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Table 2. Results of a 2-way analysi
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June 1981 June 1982 is * N.MUD •
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8 10 I 12 JZ 5. -g 1*» •e o i 16
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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
Page 371 and 372:
huch der Vogel Milteleuropas, Band
Page 373 and 374:
llolmann II. & H. Huerschelmann 196
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l-cndrem D.W. 1984. Flocking, feedi
Page 377 and 378:
Pekkarinen M. 1984. Regeneration of
Page 379 and 380:
lion of Mussels Mytilus edulis hy O
Page 381 and 382:
lulal Dal esiuanes: a comparison of
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