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This paper describes an experiment in which E. h and X for the different prey types were quantified. This allowed us to compare the observed behaviour of a predator lo the decisions it has to make to maximize ils intake rate al different prey densities, and to describe the functional response of a predator feeding on several prey types. We selected ihe Oystercatcher Haematopus oslralegus L. as the predator and the bivalve Scrobicularia plana (da Costa) as the prey. Oystercatchers locale buried bivalves like Macoma balihica L. and Scrobicularia entirely by touch. The number of prey encountered can be estimated by a simple random search model (Hulscher 1976, 1982). The prey types were bivalves of the same si/e and thus ol the same energy content, but they were buried in the substrate al different depths. Handling time increases when the prey are buried at greater depth (Hulscher unpubl.) so varying prey depth also varies prey profitability. We selected a bird that showed the greatest increase in handling time for the less accessible prey so prey profitability could be manipulated most readily. This paper attempts lo answer the question whether the Oystercatcher obeys (he functional response as predicted by Eq (3) and thus ignores less profitable prey as predicted by the optimal diet model. However, before the model can be tested, we have to show that the bird really searched at random for all prey densities. Methods The predator We worked with an adult male Oystercatcher (bill lengih 70 mm: weight 460 g) caught near the Dutch Wadden Sea in April 1981. Before we conducted our experiments which look during 30 July-18 August 1981, the Oystercatcher had been foraging on Scrobicularia and Mya arenaria L. in pilot studies conducted by Hulscher in the same experimental set-up as we used. The prey Scrobicularia 35-36 mm long were dug out from the intertidal mudflats nearby. Their caloric value was 6.1 kJ (274 mg ash free dry weight: 22.2 J mg 1 ). The prey OPTIMAL FORAGING AND THE FUNCTIONAL RESPONSE 140 were stored in fresh sea water al 4 C for a period ol up to 5 days, during which time they remained in perfect condition. The experimental set-up We used two cages connected by a small passage (hat we could open or close from our hide. A hide was placed next lo ihe cage in which we had constructed an artificial mud-fiat. The Oystercatcher was allowed to enter this cage only during the experiments. The mudllal consisted of three plastic boxes in a row. filled up with mud taken from the intertidal Hats and sieved to remove potential prey. The surface area was 0.663 m 2 . The depth of the boxes was 12 em. but bj distributing at random 72 wooden blocks with a thickness of 1,2,...9 cm over the bottom, we made the deplh of the mud-layer in the boxes irregular. The bivalves were placed on the bottom of the substrate in a vertical position. Prey depth was distance between the mud surface and the upper tip of the shell: since Si mbicularia ol lengih 35-36 mm have a height of 3 cm, when placed vertical, we created 10 classes of prey depth (0. 1.... 9 cm). A grid system numbered in relation to the side and backside of the plastic containers allowed us to retrieve every buried prey afterwards. The prey were buried in the substrate just before each experiment. All traces were erased from the surface to prevent the bird from using visual clues to locate the prey. The buried Scrobicularia quickly Started feeding in their normal way hy scraping their syphon over the mud surface. We never noticed the Oystercatcher responding to the moving syphons, thus reinforcing our opinion that the bird searched solely by touch. If necessary we added a little sea water to the mud surface so [he bird could wash the llesh before eating as do Oystercatchers in the field. We offered ten different prey densities. At the start of the experiment the Oystercatcher fed on an intermediate density (88 in' 2 ), after which we successively lowered the density to a level where the bird refused to search (Fig. I). After that we increased the density progressively lo the maximal value al the end of ihe experiment. This and not a random order was chosen lo minimize the time the bird had to spend in adjusting its feeding behaviour lo Ihe changed prey densities. The bird was not able to learn the location of the profitable prey because we changed the depth distribu-
100 200 300 prey density (n-m-2) Fig. I. Relationship between the time spent probing by the Oystercatcher (as a percentage ol available foraging time minus total handling lime) ami the densit) I D)oi Scrobicularia plana Means ±SE (sample sizes indicated) arc shown for all offered densities ir»l. During three successive sessions al l>= 2 in ' the bird found nn prey al all and the amount of probim; simngly decreased. Mean percentage probing lime ± SE tO) per 5 min obscivaiion nine is given lor (a) lirsi session (40 mini. Ibi second session after40 nun 130 mini: le) third session after 19 h (20 min). lion completely before any session, and in most sessions the bird was only allowed to take a few prey. We controlled hunger level by allowing the bird the same amounl of food each day; viz. 35 g (AFDW): this is the mean food requirement of an Oystetcaichei in captivity withabodv weight of 460 g (Hulscher 1974). The extra food needed to make up this amount was given lo the bird al ihe end of the daily experimental sessions in the form of opened Mussels Myiilus editlis L. and Scrobicularia in ihe non-experimental cage. Any remaining fcxxl was taken away every evening at 23.30 h. The experiments started each morning at (WOO h. so the bird was deprived of fcxxl for 9.5 h. I his is about 3 I) longer than usually occurs in ihe field. so our captive bird was always motivated to feed The experimental sessions The following were measured during a session: (I) Available foraging time (duration of a session). The Oystercatcher always entered the experimental cage immediately after the passage was opened. After OPTIMAL FORAGING AND THE FUNCTIONAL RESPONSE 141 it had eaten the allowed number of prey, the passage was opened again and the bird driven out. (2) Searching lime. Since we assumed searching was bj touch, only the period in which the bill was beneath the mud surface and no handling could be observed was scored as searching lime. This probing time' amounted to 30-40% of the total lime minus handling time (Fig. 1). but was lower at the lowest prey density offered (2 prey per nr). In this case ihe bird did not find a prey after probing for 15 min. so this densit] was not used in the analysts (3) Depth of located prey. We noted ihe coordinates of prey that were found so dial after the experiment we could determine its depth, 14i Handling time. As with Oystercatchers feeding on Macoma balihica (Hulscher 1982) our bird handled prey in two ways: ui) opening the shell in situ, or (b) lifting the shell from the mud before opening. We always noted which method was used and measured total handling time by stopwatch. We also measured the lifting lime (time needed to pull the shell to the surface and to put it down); cutting lime (time needed to remove the flesh from the shell), and eating time (time needed to wash and swallow the lleshi. (5) Number of prey taken. The number of prey the bird was allowed to take in one session depended on prey density: one prey at D = 2 or 6 nr 2 ; two prey at Da 12 or 24 nr 2 : three prey at D = 44 nr 2 ; five prey at D=88m" a and ten prey (3-7%) from density 144 nr 2 onwards. Because of the very low prey depletion, there was no need to correct for decreasing prey density during the course of a session (Rogers 1972). However, the fact that the number of prey the bird was allowed to take, increased with prey density, might have affected the decision making process of the Oystercatcher. As shown by Lucas t 1983) the bird might lower its selection criterion when the remaining time available for feeding decreases Since not time itself was limited, but the number of prey allowed, time constraints are unlikely to have affected ihe optimal diet choice in these experiments. The analysis would have been complicated if the bird had increased iis handling of searching lime during long sessions because of decreasing motivation (Holling 1966|. However, this appeared not to happen: in 21 of the 24 sessions in which the bird tcxik 5 or more prey, there were no significant trends in ihe han-
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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
Page 85 and 86: where they switch to surface-living
Page 87 and 88: Chapter 3 BURYING DEPTH OF THE BENT
Page 89 and 90: DEPTH AND SIPHON CROPPING IN SCROBI
Page 91 and 92: I DEPTH AND SIPHON CROPPING IN SCRO
Page 93 and 94: DEPTH AND SIPHON CROPPING IN SCROBI
Page 95 and 96: Table 3. Three-Way analysis Of vari
Page 97: Chapter 4 SIPHON SIZE AND BURYING D
Page 100 and 101: 15 20 size (mm) Fig. 3. Cerasioderm
Page 102 and 103: 10 size (mm) SIPHON SIZE AND DEPTH
Page 104 and 105: o, 7 01 5 | * CL 5- SIPHON SIZE AND
Page 106 and 107: 4 Q) Si •a a. 16- 20- A predator:
Page 108 and 109: prey risk for an animal al a depth
Page 110 and 111: Table 6. Size al which there is a m
Page 112 and 113: * ' 1 /-/ "».-^*«C
Page 114 and 115: face and so expose themselves to a
Page 116 and 117: surface in die containers was varie
Page 118 and 119: 50 OOF 310.00 I 5.00 ^ 1.00 3=" 0.5
Page 120 and 121: Tahle 1. Macoma and Scrobicularia.
Page 122 and 123: siphon would not have to reach the
Page 124 and 125: known siphon weight and burying dep
Page 127 and 128: ACCESSIBLE PREY ARE OFTEN IN POOR C
Page 129 and 130: 1 2 3 4 burying depth (cm) Kin. I.
Page 131 and 132: I—1 • • : : : : ! - ! -|-r 0
Page 133 and 134: Chapter 7 DOES AN OPTIMALLY FORAGIN
Page 135: OPTIMAL FORAGING AND THE FUNCTIONAL
Page 139 and 140: Table I. Results of five one way an
Page 141 and 142: Table 2. Results of eight iwo-wav a
Page 143 and 144: OPTIMAL FORAGING AND THE FUNCTIONAL
Page 145 and 146: prey density II III Fig. 12. Freque
Page 147 and 148: OPTIMAL FORAGING AND THE FUNCTIONAL
Page 149: PREY SIZE SELECTION AND INTAKE RATE
Page 152 and 153: Predicted 'passive size selection'
Page 154 and 155: lig. 2. Larger Mussels are less ava
Page 156 and 157: Fig. 5. Scrobicularia plana. Size c
Page 158 and 159: and maiiv are too thick-shelled to
Page 160 and 161: 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
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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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PREY PROFITABILITY AND INTAKE RATE
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• ' • 14 PREY PROFITABILITY AND
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Notes lo appendix: PREY PROFITABILI
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Chapter 10 WHY OYSTERCATCHERS HAEMA
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2 4 6 8 10 time on feeding area (h)
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80- 60- o 80 60- 40- 20- INTAKE RAT
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est of bod\ behind: an estimated 22
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INTAKE RATE AND PROCESSING RATE IN
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Wanink 1993). The energy content of
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(7) Age All studies dealt with adul
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irds over long periods. As an examp
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Discussion There is no difference i
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comparison between the weight of ih
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. 1', L ! •
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Introduction PREDICTING SEASONAL AN
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PREDICTING SEASONAL AND ANNUAL FLUC
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1000 r 1 II" 1 - r^Jan'80 PREDICTIN
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IOO BO 60 40 20 0 PREDICTING SEASON
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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
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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