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80- 60- o 80 60- 40- 20- INTAKE RATE AND PROCESSING RATE IN OYSTERCATCHER & ^ - - ^ \ 1 e*> •ZD'D • / •G .0 • H .ZW 'O • • •• .0 • # • ^ • • ^—- 1 6 8 10 12 14 time on feeding area (h) CAPTIVE BIRDS sStSfi- FREE-LIVING INDIVIDUALS FREE-LIVING BIRDS (population averages) s ^ • ••_^-o'l. 2. Total food consumption (g AFDW) as a function nl the time spent on the feeling .irca. jiiven separately tor A. captive birds (n = 53), B. free-living birds (long-term observations of individuals: n = 244) and C. free-living birds (population average-: ll =83). I he grey field indicates the predicted maximal consumplion delermmed by ihe digestive bottleneck (Kersten &. Visser 1996a). The solid lines give ihc consuni|ilion wilh Ihc crude intake rate (during feeding and non-feeding periods combined) sel at 0.5. 1. 2 or 4 mg •>"'. The sources of the data shown in panels A and Care listed in ihe appendix of Zwarts el al. (1996a). In addiiion. panel A gives the 34 measurements ol Swennen
ates. Across all data, most crude intake rates fall below the predicted maximum, but a few do exceed this level. In contrast, there are several outlying points for the population averages, Le. calculated for free-living birds feeding on a certain prey species (Fig. 2C). Some ot these estimates are even three times higher than the predicted maximum consuinpiion. There are three possible explanations when the food intake exceeds the predicted maximum: (I) the bottleneck hypothesis is raise; (2) the hypothesis is Hue. but the actual level of the digestive constraint is not constant but varies between conditions and (3) the lood consumption is estimated incorrectly. If I?) is true, we would expect the frequent**, ol excessively high food intakes to increase with the potential lor errors being made. Our next step is therefore to assess the possible sources of error in the estimation of food intake and the mosi likely direction ol these errors. Food consumption in the field is often overestimated To estimate the total food consumption over a feeding period it was necessary in most studies to measure the length of prey taken, their weight, the feeding rate (the number of prey taken per unit time feeding), the feeding activity (percentage of time spent feeding) and the duration of the feeding period. Since the estimation errors are multiplicative, even a few small errors in the component estimates may easily lead to a large overall error in the estimated total consumption. Estimating prey size When prey size is estimated from emptied prey found on the substrate surface, small prev mav be overlooked more often than large ones. This error is possibly small when prey are collected on bare sand and mudflats, or from "anvils'. where each shell may be easily located. Small prey are possibly more easily being hammered under the surface, so one always needs in search vv uh care. The error may be more serious on mussel beds (Ens 1982, Speakman 1984b. 1990, Cayford 1988). However, even if the sizes of prey found on the surface are representative of those eaten at the surface, the estimates may be biased if Oystercatchers eat certain si/e classes of burying bivalves in situ, and so beneath the surlace. Although this possible error has not yet been investigated, we might expect them to be errors of overesiimation for two reasons, first, prev in situ are eaten in INTAKE RATE AND PROCESSING RATE IN OYSTERCATCHER 217 less lime than lifted prey (Wanink & Zwarts 1985. 1996. Hulscher et al. 1996), allowing small prey eaten in situ still to be profitable. Moreover, captive Oystercatchers lifted il.'.-n-liv fag nr.-x m,.n- nil,MI lli.in lliev lifted shallow prey (Wanink & Zwarts 1985. Hulscher et al. L996), winch would generally be smaller than those ai greater depths (Zwarts & Wanink 1993). Thus. collecting samples of emptied prey at the surface presumably causes large prey to be over-represented in the diet. Converting prey size into prey weight Prey si/e is usually converted to prey weight from allometric relationships based on random samples of prev. thus estimating ihe average weight of each size class. Ov stercatchers, however, do not take 'average' prey. They select thin-shelled Mussels Mytilus ediilis when hammering: they search for bivalves thai are slightly gaping when stabbing: and when they probe for buried prey, they lake those living closest io the surface. Such accessible prey are often in a relatively poor condition (Esselink & Zwarts 1989. Zwarts & Wanink 1991. Goss-Custard et at. 1993). The greatest overestimates arising this way probably occur in Oystercatchers feeding in winter on Scrobicularia plana. 1 lie majoi it) of these live out of reach of the bill and the few that are still accessible are in very poor body condition. Prey not completely eaten It is usually assumed that Oystercatchers clean a bivalve completely, whereas some flesh often remains along the mantle edge and where the adductors are attached to the valves. When in the laboratory, the bivalves are briefly immersed in boiling water so that all flesh can be easily removed from the shell, the stub of the adductor muscle remains attached to the shell, just as when Oystercatchers clean a shell. However, it is not clear in how main studies the adductors are entirely cut free from the valves instead. Obviously, ihe amounl of llesh extracted in ihe laboratory is never below, but always exceeds the amounl eaten by Oystercatchers. although the differences may usually be small. Occasionally, however, the error may be large: for example. Oystercatchers left behind 11.7*91 s'i the AFDW of Cockles Cerastoderma edule (Hulscher unpubl.): 7.6% of Mussels i Speakman 1984a) and some 501 in Giant Bloody Cockles Anadara senilis I Swennen 1990). The error is also serious when Oystercatchers pull out the siphon of the Soft-bodied Clam Mya arenaria onlv and leave the
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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
Page 162 and 163: 1 2 3 4 5 6 7 probing depth (cm PRE
Page 164 and 165: valves may vary by a factor of two
Page 166 and 167: optimal foraging model, the minimum
Page 168 and 169: their gut is full? Do they reduce t
Page 171 and 172: PREY PROFITABILITY AND INTAKE RATE
Page 173 and 174: catchers to take only certain prey
Page 175 and 176: licult error of estimate arose if p
Page 177 and 178: Counts of feeding and non-feeding b
Page 181 and 182: 20 30 40 50 shell length (mm) PREY
Page 185 and 186: Nereis Arenicola tipuia earthwo'm M
Page 187 and 188: take rate. We might therefore expec
Page 189 and 190: prey species, using parallel slopes
Page 191 and 192: 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 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 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
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PREDICTING SEASONAL AND ANNUAL FLUC
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WHY KNOT TAKE MEDIUM-SIZED MACOMA W
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in Zwarts & Esselink 1989). The len
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shell length (mm) FIR. 3. Peringia.
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fused, specimens larger than this w
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50 100- s s I — f = S. plana, n=2
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area is twice as large as the touch
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10 15 lower size threshold (mm) Fig
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lime (Hughes 1979). This would furt
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WHY KNOT TAKE MEDIUM-SIZED MACOMA T
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Chapter 13 ANNUAL AND SEASONAL VARI
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VARIATION IN FOOD SUPPLY OF KNOT AN
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Two sites (N and M in Fig. 2) were
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20 30 VARIATION IN FOOD SUPPLY OF K
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20 l i r'l ' i •o j^y^T n ^ ' " ^
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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
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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
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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