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Predicted 'passive size selection' If Oystercatchers locale prey beneath the surface of ihe substrate by randomly probing, the probability of a prey being encountered can he calculated Irom prey density provided that two other factors are also taken into account. Firstly, a proportion of the prey mav live out of reach of the bill. Secondly, the probability that a prey is actually nil by ihe bill lip is a function of its size, or more precisely, the surface area the prey presents from above: this is referred to here as the 'touch area'. The observed si/e selection may only be compared with the predictions of passive si/e selection when the calculated encounter rate with prey of different si/e classes takes both these considerations into account (Hulscher 1982). The proportion of prey that lie buried beyond the ieach of the bill depends on bill length and on the depth to which Oystercatchers insert it. Bill length in Oystercatchers varies between 6.5 and 9 cm. When Oystercatchers extract bivalves from the substrate. they can probe so deeply that the base of the bill is pushed 0.5 cm below the surface. Even then, large Mya live out of reach (Zwarts & Wanink 1984, 1989) and the majority of large Scrobicularia are also inaccessible, at least during ihe winter (Zwarts & Wanink 1989). But. in fact, Oystercatchers do not usually probe to the maximum depth. The precise probing depth differs between prey species, but is. on average. always less than ihe bill lengih. Oystercatchers probe to a mean depth of 4 cm when searching for deep-living Scrobicularia (Wanink & Zwarts 1985), 3 to4cm when feeding on the more shallow-living Macoma PREY SIZE SELECTION AND INTAKE RATE (Hulscher 1982). and only 0 to 3 cm when taking Cockles which are found very near to the surface (Hulscher 1976). The fractions of each si/e class oi each clam species liv ing in the upper 4. 6 and 8 cm of the substrate are shown for the w inter and summer period in Fig. 1. The dala show, for example, thai whereas most Macoma are accessible lo Oystercatchers in summer, less than half are in winter. As Cockles live in ihe upper 4 cm of the substrate, with the majority being in the upper 1 or 2 cm. ihey are within reach of the Oysiercatcher's bill throughout the year. For Oystercatchers probing vertically downwards, the 'touch area" ol a bivalve is equivalent to its surface area, measured in the horizontal plane (Hulscher 1982). This sin lace area has been determined by photographing from above the bivalve in iis natural position wilh the substrate removed (Hulscher 1982) or by pressing the bivalve vertically into modelling clav i Wanink & Zwarts [985, ZwaitS & Blomert 1992). The 'touch area' of all bivalve species is elliptical, with the Cockle being the most circular and Scrobicularia the mosi slender, with Macoma and Mya lying between. The first estimates of the 'touch area" in Cockles (Hulscher 1976) and Scrobicularia (Wanink & Zwarts 1985) were given as a function of the squared lengih but. when calculated over a larger range of si/e classes, a better lit was obtained with exponents slightly larger than 2. since small shells are paiticularlj slender (Zwarts & Blomert 1992). The details of this exponential increase of 'touch area' with shell si/e are given in Table I. The real, or 'effective', touch area for a probing Tabic I. The 'Much area' of the shell imm-'i as a function oi us length i mini. The "touch area' is defined as the snftceofa bivalve in us natural position, measured in a horizontal plane: (iron* Zwarts .V. Blomert (1992). The corresponding surface area of Ihe effective touch area' is equivalent Io the surface area lset to a circle: nr) enlarge! vmh the surface area of the bill lip itself (I I mm x I 4 nun = 15.4 mm': HuKchei 1982) and the combined ciicct ot the surface areas of hill tip and bivalve (11 x 2n+ 1.4 x 2jt=24.8*1 ram 2 ); fromllabekotteil l 'X7)and/u.Mis & Blomert 11992). As an example. Ihc surface areas ol the touch area' and 'effective touch area' (mm 1 ) nl two si/e classes i II) and 25 nun long) are given. Spei us Equation Touch area It) mm 25 nun Macoma balthica Si mhii ularia plana Msu arenaria Cerastoderma edule 0.151 mm-' 1 " I) 154 mm '" (I.i:5iiim" ' ! 0.340 mm-" ; 156 22 19 14 -111 87 129 89 266 t touch area 10 mm 25 "»'i 158 80 66 128 287 221 494
PREY SIZE SELECTION AND INTAKE RATE 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 lengih of prey (mm) Fig. I. Proportion of clams of three species living in the upper 4. 6 & 8 cm of Ihe substrate as a function of shell size. The rcsulls are given separate!) for ihc summer months, Inly to Septembei (left), and Ihe winter period, November to March (rightI. The numbers ul clams in the samples are shown I in I Zwarts A: Wanink 1989). Oystercatcher is actually larger than the 'touch area' defined by the equations in Table 1. because the surface area of the bill tip needs also to be laken into account (Hulscher 1982). Probing Oystercatchers leave behind imprints in ihe mud (see photo in Davidson 1967). enabling the hill-lip surface area lo be measured. With ihe small space between the slighilv opened upper and lower mandibles included, the probing surface equals a rectangle measuring II X 1.4 mm (Hulscher 1982). As the touch area depends on the stir lace area of ihe shell and not on its shape, for 'simplicity ti is treaied as a circle. The 'effective touch area' is then equivalent to the touch area" (nr) enlarged by the surface area of the bill lip. Table I shows how lo calculate 'effective touch area' from "touch area' and gives, as an example, the surface areas [i u two size classes. 157 Mussel-eating Oystercatchers occasionally probe by touch, bul usually appear to feed by sight (Hulscher 19961. This makes il impossible lo use ihe method discussed above to measure the probability that prey will be detected at random. Ens (19X2) argues that prev en counter rale in visually hunting Oystercatchers mav also be a function of the surface detection area of the prey, and thus about equivalent to ihe squared lengih of ihe shell. Oilier papers on size selection of mussel-eat mg ()v siercaichers have followed the same asstimpiion i Meire & Ervynck 1986. Sutherland & Ens 1987. Cayford & GoSS-CtJ8tard 1990) and it will also be used here. Studies on mussel size selection have tended to focus on hammering Oystercatchers because it is possible to calculate the available fraction, i.e. those shells that are not covered with barnacles and which are ihin
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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
Page 102 and 103: 10 size (mm) SIPHON SIZE AND DEPTH
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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
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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 and 136: OPTIMAL FORAGING AND THE FUNCTIONAL
Page 137 and 138: 100 200 300 prey density (n-m-2) Fi
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 145 and 146: prey density II III Fig. 12. Freque
Page 149: PREY SIZE SELECTION AND INTAKE RATE
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
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
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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
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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
Page 381 and 382:
lulal Dal esiuanes: a comparison of
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