waders and their estuarine food supplies - Vlaams Instituut voor de ...

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The upper size threshold has been investigated in relation to gape width, but il is also conceivable that there is ;i si/e limit related lo the ahilitv in crush prej in the gizzard, as suggested for the Purple Sandpiper ('alidris maritima (Summers et al. 1990). Work in progress (T. Piersma unpubl.) will show whether in Knot the digestibility of prey decreases with size. The upper si/e threshold is probably fixed for the must part, ll should be noted, however, that ihe relation between shell circumference and shell length (Fig. 7C) may vary locally and geographically. Variation in shape has been attributed to age and density in Mytilus (Seed 1968) and to growth rate and ambient salinity in Cerastoderma (Fisma 1963. Hancock 1967). Macoma in ihe southern part of their range are more slender than those in the north (Beukema & Meehan 1985). The effect of this is that the critical length for Knot varies by 2 to 3 mm. Though Knot do not lake bivalves that live too deep, ttiey can vary the depth to which they probe. Oystercalchers probed more deeply when ihe density of prey within reach was low (Wanink & /.wans 1985). The fraction of prey living in the upper two and three cm is given in Fig. 8 for our study site in late summer. Macoma and Scrobicularia live much deeper in winter than in summer, with the proportion accessible to Knot varying between 0 and 10098 (Reading & McGrorty 1978. Zwarts & Wanink 1989, Zwarts ei at. 1992). There is no seasonal variation in the burying deplh of Mya and Cerastoderma (Zwarts & Wanink 1989). In addition, according to the profitability rule, the lower size threshold should be variable, because it is closely linked lo the current intake rate. When only the less profitable prey are available, the intake rate will inevitably decrease, so that the low-ranking prey then become profitable (e.g. Zwarts & Drent 1981). Profitability will also depend on die condition of ihe prey. Cayford & Goss-Custard (1990) have shown the implications of variations in the condition of Mytilus for size selection by Oystercatchers. Similar variations in AFDW occur in Macoma: individuals ol similar size contain about 1.5 times as much flesh in May and June as in February and March. This seasonal variation is even larger in Cerastoderma (Zwarts 1991). Thus, the lower acceptance level should increase in winter as the condition of prev decreases. This is only true, however. WHY KNOT TAKE MEDIUM-SIZED MACOMA 284 if Knol are able to maintain their intake rale at a constant level. The lower size threshold would not change if the intake rate decreased at the same rate as did preycondition. Whether Knot can survive on a lower intake rate depends on the available feeding time and on their energv requirements. The available feeding period does not vary much between intertidal areas used by Knot, but ihe energy requirements do differ: when wintering in Africa, for example, energy requirements may be half ihose in NW. Europe during a cold spell (Kersten & Piersma 1987. Piersma et al. 1991) or during the pre-migration period when Knot are fattening up (Klaassen el al. 1990). The Knot in the study area in late summer were on migration, so the high intake rate observed was probably required to enable them to accumulate body reserves. This would imply that the small prey ignored during this pre-migration period might be profitable under other circumstances, and so might be taken when Knot require a lower daily food consumption. The average intake rate of Knot wintering in Africa was estimated lo be about 0.37 mg s ' (Zwarts el al. 1990b). 40% below the 0.63 mg s" 1 found in this study. This may be one of the reasons why Knot in Mauritania accept very small prey (Zwarts el al. 1990a). Does a digestive bottleneck explain prey selection? The rate of food processing in Oystercalchers is constrained by ihe capacity ol the gut to digest food, because, although they can ingest I(X) mg wet weight s '. the gut processing rate is only 4.4 mg wet weighl s i Kersten & Visser 1996). Thus, when the ingestion rate over a long pericxl exceeds 4.4 mg s ' digestive pauses are necessary to empty ihe digestive tract. Are there any reasons to believe lhat the rate of food intake of Knot was also constrained by such a digestive bottleneck? When the preening periods arc included as possible digestive pauses, the overall iniake rate was oneprey item per 82 s. Knot produced one faecal packet per 80 s and the faeces contained, on average. 0.98 prey, which means lhat the defecation rale was one prey item per 78 s. This suggests that the observed ingestion rate was equal to the processing rate, as derived from the defecation rate, and that higher intake rales while feeding would nol be possible, unless the birds were to lake more digestive pauses (or increase their
WHY KNOT TAKE MEDIUM-SIZED MACOMA Table 4. Rale at which three waders process their food in terms of mg total wet weight s '. mg dry weight s ' and mg dry flesh s '. The food composition is also given. 'Non-flesh' refers to inorganic dr) matter (shell, sand. salt). Knot fed on Macoma including the shell ipresem paper). Whimbrel took crabs of the genus Uca (the maximal processing rate is laken from Zwarts & Dirksen (1990) and the lood composition Irom Zwarts & Blomen (l°*X)i); M)'< of the organic matter, found in the skeleton of the crab, was indigestible and therefore added to the non-flesh component Oystercatchers ate only the llesh of Mussels t Kersten & Visser 19%). Species Body weight, g Processing rale, mg I ' in-/ dry dry flesh Food composition, ''• sealer non-fie sh flesh Knot 119 9.0 4.: 0.51 54 40 6 Whimbrel 413 11.1 2.8 1.0 75 If. 9 Oystcrc.itclici 525 4.4 0.9 0.7 80 4 Id processing rate and probably decrease Ihe digesiihilnv of the food). Knot ingested the wet flesh and shells along with Ihe water included in the shells. Knot eating Macoma. ingested 54
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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 & • % * • * •
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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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• ' • 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 ! •
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: lime (Hughes 1979). This would furt
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
Page 295 and 296: available. There must, however, be
Page 297 and 298: Chapter 14 SEASONAL TREND IN BURROW
Page 299 and 300: BURROWING AND FEEDING IN NEREIS SEA
Page 301 and 302: Table 2. Results of a 2-way analysi
Page 303 and 304: June 1981 June 1982 is * N.MUD •
Page 305 and 306: 8 10 I 12 JZ 5. -g 1*» •e o i 16
Page 307 and 308: 6 - n=!080 5 • g 4 CP > i 3 CO CD
Page 309 and 310: 1985) and Curlew (/wails ,v, Lsseli
Page 311: Chapter 15 VERSATILITY OF MALE CURL
Page 314 and 315: Smid! 1951. Muus 1967, Wolff 1973,
Page 316 and 317: 0.6 0.8 1.0 1.2 1.4 1.6 jaw lengih
Page 318 and 319: Na oi both Nrieck Fig. 7. Numenius
Page 320 and 321: 6 8 10 12 14 16 18 worm length (cm)
Page 322 and 323: too (Fig. 11 A). Indeed, the averag
Page 324 and 325: VERSATILITY OF CURLEWS FEEDING ON N
Page 326 and 327: total time spent al the surface. Th
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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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