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face and so expose themselves to a greater predation risk. Ihese observations led Zwarts (1986) lo hypothesise thai the burying depth of deposit-feeding bivalves represents a compromise between foraging demands and predator avoidance. The hypothesis could not be tested because the feeding radius had not been simultaneously measured with burying depth and siphon weight. The aim of this paper is to present some measurements on the elongation of ihe siphon that are essential to answer the question about the trade-off between food and predation. We show that feeding radius, burying depth, and thus also siphon length, increase with shell size. We relate individual differences in feeding radius and burying depth in Scrobicularia and Macoma of different si/e classes to variations in siphon weight. We cut off siphons to determine the effect of siphon cropping on feeding radius and burying depth. The paper also describes techniques to measure the length of the inhalant siphon and gives equations lo predict siphon length from siphon weight. These data will be used to estimate the feeding radius of a large sample of Scrobicularia in which burying depth and siphon weight has been measured. Materials and methods Burying depth, feeding radius and siphon weight: field data The field data were collected on a tidal mudflal in ihe eastern pan of the Dutch Wadden Sea along the mainland coast of the province Friesland (53° 25' N, 6° 04' E). We determined ihe maximal feeding radius of 75 Scrobicularia in June 1992 using two methods. First, we observed for 20 min the siphon activity of an individual bivalve, noted its maximum extension over the mud. using surface cues such as shell fragments as markers, and later measured the distance to the siphon hole. The second method was simply to measure the distance between the siphon hole and the maximum extent of the traces in ihe mud: like other deposit-feeding bivalves. Scrobicularia leave starlikc tracks on the surface around the burrow caused by ihe siphonal activity (Hughes 1969. Hulscher 1973. Wikander 1980. Levinton 1971. 1991). Since both methods gave identical resulis. we subsequently only used the second FEEDING RADIUS, BURYING DEPTH AND SIPHON SIZE 114 method, selecting stars that were so clear-cut ihal the maximal feeding radius could be measured precisely. After that, the burying depth of the selected bivalve was determined using a circular corer pushed into the mud. Alter retraction, the core was laid horizontally and the burying depth of the focal bivalve measured to the nearest 0.5 cm. Burying depth is defined as ihe distance between surface of the mud and the upper edge of the shell; for a detailed description of the methods, see Zwarts (1986) and Zwarts & Wanink (1989). The animals were stored in sea water and taken to the laboratory where shell length was measured and the inhalant siphon separated from the bcxly, as described elsewhere (de Vlas 1985. Zwarts & Wanink 1989). The siphons were dried for 3 days at 70 °C and burned at 550 "C for 2 h to determine the ash-free dry weight (AFDW). To study the relation between burying depth anil siphon weight alter cropping, we collected 60 Macoma 16 to 17 mm long in May 1986. The animals were placed in dishes with shallow water, after which die extended part of the siphon was cut off in 44 individuals. Using adhesive, a thin nylon thread of known length was attached to the shell and the animals were reburied at a depth of 3 cm in the same mudflat from which they had been collected 8 hours before. The depth to which the animals subsequently buried themselves was determined each low water period over five days, by subtraction, from the length of the thread remaining above the surface. After that Ihe animals were collected to determine the ash-free dry weight of the bcxly weight and the remaining siphon, as well as of the weight of the part of the siphon that had been removed. Experiments in the laboratory Further experiments were conducted on the relationship between siphon weight, feeding radius and burying depth in the laboratory. Twenty-six Scrobicularia. 13 lo 37 mm long, and 77 Macoma. II to 20 mm long, were collected in October 1992 and taken to the laboratory, where shell length was measured and thin nylon threads of known length were attached to the shell with super glue. The bivalves were then allowed to bury themselves in 20 cm of mud taken from the area where ihe bivalves had been collected. The mud temperature was kept constant at 15 °C. The waler table above the
FEEDING RADIUS, 8URYING DEPTH AND SIPHON SIZE We took hjviilvcs home tn measure the siphon length of bivalves Nylon datadi HOC used io determine ihe depth of bivalve- We could determine Ihe exiensmn ol the siphon by measuring ihe burying deplh (using the threads) and Ihc feeding radius on the mud surface 115
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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
Page 63 and 64: However, for obvious reasons, we ma
Page 65 and 66: prey. A decrease in the prey densit
Page 67 and 68: Tahle 2. The intake rate ol < lyste
Page 69 and 70: * ' % -. . ; a X - . - * • ^ •
Page 71 and 72: (Zwarts & Wanink 1989). In quiet we
Page 73 and 74: general law relating handling time
Page 75 and 76: nearly twice as much time (0.79 s).
Page 77 and 78: 4 5 6 7 8 9 size class of Corophium
Page 79 and 80: and annually. Second, the lower siz
Page 81 and 82: Prey switching Waders feeding on ti
Page 83 and 84: 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 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 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
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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 ! •
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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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