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CanutoOermi 5 10 15 20 25 profitability (mg s-' handling) Fig, IS. The intake rate (mg s ' (coding) as a function ot profitability (mgs' 1 handling) woeOynercatdtera to feed on Mytilus, I eras toderma and Nereis of differenl weights. The graphs are based upon the relationships between profitability and prey weight (Table I) and between intake rate and pre) weight (Figs, 12, \s& 15). Pot Mytilus a Selection is made of birds using ilic slabbing technique. ity (Fig. 18). All else being equal, a doubling of profitability will have little impact on the intake rate when the birds spend most of their time searching, but will nearly double intake rate when the birds spend most of their time handling. It is therefore not surprising that in Fig. 18 profitability differences strongly affect intake rate in Cockles and Mussels, in which handling lime is very long, but have much less effect when the birds search for Nereis, in which most time is spent in searching. To investigate the relationship between profitability and intake rate between prey species, we plotted the average intake rate per species (Table 3) againsi ihe average profitability (Fig. 19A). This comparison shows lhat. even when profitability increases sixfold, the intake rate remains the same. This implies that when the prey species are compared, the average lime spent handling as proportion of total feeding time, i.e. handling and search time combined, decreases with profitability (Fig. I9B). Soft-bodied prey are so profitable that. even when 959?- of the feeding time is spent in searching, the intake rate remains at 2 mg s'. If Oystercatchers were also to search for 95% of the feeding lime when they take Mussels, their intake rate would reach PREY PROFITABILITY AND INTAKE RATE 30 200 the extremely low level of 0.1 mg s '. and they would starve as shown in the next section. Since average prey weighl differed so much be- Ivveen species, we standardized the profitability and intake rate to prey of similar weight choosing 2(X) mg (Fig. 9). We calculated ihe intake rate for each species with the same weight, using the multiple regression equations for soft-bodied and armoured prey with different intercepts but parallel slopes. When prey species with similar body weight are compared, the intake rate doubles within the range of profitabilities observed (Fig. 19C) and the handling time as proportion of the total feeding time halves (Fig. 19D). Thus, when prey species of similar weight are compared, iniake rale strong I v depends on the profitability, such as was also found within the species. Profitability and minimal intake rate What is the minimal intake rate of food on which Oystercatchers can sustain themselves? Oystercatchers in the wild need at thermoneutmlity 36 g a day to keep their body weighl constant (Zwarts et al. 1996b). If they feed for all the five to six hours per low water period during which the feeding areas are usually exposed, the intake rate must be at least about 1 mg s '. If it is less than this, they have to collect extra food either on the upper shore during (he incoming and receding tides and/or on inland grasslands al high tide. The iniake raie on ihe high shore is low. however (Sutherland 1982, Ens et at. 1996c, Meire 1996b). because the huge prey taken by Oystercatchers do not occur there or only at very low densities (e.g. Zwarts et al. 1996c) and their condition is often poor (Goss-Custard 1977, Sutherland 1982a). Inland grasslands may provide some compensation, but these opportunities are only available locally. Thus, an intake rale of 1 mg s ' may be considered as a limit below which food consumpiion will in the long term usually be too low. If the average weight and handling time of the prey taken are known, it is possible to calculate the lengih of the searching time at which the intake rate reaches the lower acceptance level of 1 mg s~*. Since the relationships between prey weight and handling time are known (Figs. 2-6. Table 1). we can easily calculate the search time per prev needed to achieve an intake rate of I mg s '. The resulting Fig. 20 shows that if Oystercatchers feed on prey containing more flesh, they may
5.0 - f-4.0 S 3.0 in I 20 a 2 a | 1.0 0.8 V average prey - ® PREY PROFITABILITY AND INTAKE RATE * s Ai 10 " ! • • • ii % - 9 •7, 13 i • prey 200mg N ,© 5 • • J • 11 12 • 4 4 5 10 20 30 40 3 4 5 10 20 30 40 profitability (mg s-' handling) . 1 1 a Anadara 2 = Arenicola 3 - Cerastoderma 4 m earthworm 5 = Macoma 6 = Mya 7 = rVfyWus- dorsal 8 = Mytilus- slat) 9 = Mytilus- venlral 10 = iVerers 11 = Scnxxularia 12 = ripufa 13 = Uca Fig. 19. A. Intake rate and B. handling lime relaiive to local feeding time, both averaged per prev species, as a function of the average prey profitability. Percent handling time, shown in panel B. was calculated Irom the averages shown in panel A. Similar relationships are shown in C. and D., with the profitability and intake rate standardized to prev of 2(X) nig. The standardized profitability of the different prey species was laken from Fig. 8. The intake rate was calculated for prey weighing 200 mg, such as predicted by two multiple regression analyses on the effect ol prev weight on the iniake rale wilh soft-bodied or armoured prey species as dummy variables isee text). The value;, are extrapolation'. for some small prev always weighing less than 200 mg. 1000 g 100 sv E 10 Nereis MyWusstab 10 100 prey weight (mg) ••:.'••,- * / - 1000 201 Fig. 20. The lime Oystercatchers are allowed to search for pre-. certain weight to achieve an intake rate of I mg v' when ihey feed ilus. Cerastoderma and Nereis. The searching time is derived from the functions describing the relationship between handling time and prey weighl iTable 11. For Mytilus a selection is made of birds using ihe stabbing technique.
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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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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
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: winter. Similarly, handling time in
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 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
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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
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PREDICTING SEASONAL AND ANNUAL FLUC
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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
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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