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prey risk for an animal al a depth of 4 to 6 cm. though within the reach of the bird's bill, is much less than for an individual living near the surface (Fig. 1 IB). Optimal foraging theory (Krebs et al. 1983) offers the explanation: deep-living prey are ignored because handling time increases with depth, which makes them less profitable (Wanink & Zwarts 1985). Scrobicularia > 30 mm live the greater part of ihe year below a danger line of 4 to 6 cm (Fig. 5) and in fact individuals must have a poor condition as well as a low siphon weight before they expose themselves to a higher predation risk (Zwarts 19861. The conclusion is that in summer ihe 'average' Scrobicularia remain just below the danger line, but that from a point of view of minimization of predation risk there is no need lo nearly double the burying depth in winter. This suggests that other factors may be involved in determining burying depth. The Oystercatcher is also an important predator for large Mya, but it is the Curlew Numenius arquata which causes a high mortality among individuals living in the upper 14 to 15 cm (Fig. IIC). Oystercatchers select Mya > c. 17 mm from the upper 5 to 6 cm, while Curlews lake Mya > 30 mm present in the upper 14-15 cm (Zwarts & Wanink. 1984). There is thus a small overlap in size and depth classes preyed upon by both bird species. When the pari of Mya which is vulnerable to predation by Oystercatchers or Curlews is compared to the depth distribution (Fig. 2), it is clear that risk to be taken by an Oystercatcher or a Curlew is maximal for Mya of c. 25 mm and 35 to 45 mm, respectively, which closely resembles the observed size selection (Zwarts & Wanink 1984). It could also be shown that Oystercatchers and Curlews exert a heavy predation pressure on Mya > 2 cm. which makes it reasonable to assume that the depth/si/e curve is determined partly by Oystercatchers and Curlews creaming off the individuals living in the upper 6 and 14 cm respectively. Most animals > 50 mm live out of reach of all predators present on the intertidal flats. Cerastoderma, unlike Scrobicularia and Mya, remain within reach of most of their predators during iheu life-time, but by growing, the) gel rid of many of them. Knot Calidris canutiis prey upon Cerastoderma < 10 mm (Goss-Custard ei al. 1977b. Ens unpubl. data), because the profitability of the prey increases wilh si/e I Sutherland 1982c). SIPHON SIZE AND DEPTH IN BENTHIC BIVALVES 108 Macoma have, like Cerastoderma and Mya, many different enemies during the first stage of their life. Five wader species, which are abundant on ihe intertidal flats from late summer till spring, are the chief predators for Macoma which have survived their first summer (Table 4). In winter most Macoma > 10 mm live out of reach of Dunlin Calidris alpina, Redshank Tringa totanus and Knot I Reading & McGrorty 1978). but a small number of the larger animals remain preseni in the upper 3 cm (Fig. 4) and hence available for these short-billed waders. They are. however, ignored even by the Bar-tailed Godwit Limosa lapponica with its longer bill. It is very likely thai gape width determines the upper limit of acceptable prey size. Contrary to the oilier waders, which swallow Macoma whole, the Oystercatcher takes the flesh out of the shell after hammering the shell or prising the bill between the valves (Hulscher 1982). This species selects Macoma > 10 mm (Goss-Custard et al. 1977b. Hulscher 1982). Large Macoma have thus to reckon with Oystercatcher and Bar-tailed Godw it as ihe only predators. It is still unknown from which depth layer the prey are actually taken by these waders, but if the profitability of the prey decreases with depth as shown for Oystercatcher feeding on Scrobicularia I Wanink & Zwarts 1985). il is also likely thai wilh large Macoma the predation risk is maximal for those animals living in the upper 3 to 5 cm. In winter, the depth of Macoma levels off at 4 to 5 cm (Fig. 4) and animals with a heavy siphon enlarge their depdi till diey reach a depth of c. 4.5 cm (Fig. 8). This suggests that also in summer the depth refuge is 4-5 cm, but that the reduced siphon size and the need to feed force most Macoma to move nearer to the surface. The general conclusion is that benthic bivalves which attain a safe depth refuge do not increase their burying depth anymore. The great winter depth of Scrobicularia deserves another explanation. On European flats, large Mya have to deal with Curlews as their only predator and large Macoma with Oystercatchers and Bar-tailed Godwits only. That is why the upper limit of ihe depth refuge is set for both species al c. 14 and c. 4 cm. respectively. The Blue Crab Callinecles sapidus is an important predator for both species in Chesapeake Bay (e.g. Virnstein 1977. Blundon & Kennedy 1982b). Large Blue Crabs are able to crack
10 size (mm) SIPHON SIZE AND DEPTH IN BENTHIC BIVALVES 20 30 size (mm) Fig. 12. Siphon weight i'.< ± SF.i relative to total body weight in winter (•) and in summer (O) as a luiiciion ol shell size of A, Mya arenaria B. ('trastoderma edule C. Mai oma balihica I). Scrobictdaria plana. Table 5 gives staiisiical analysis and number of cases. TableS. Results of4 two-way analyses ol variance (ANOVA) to test effect ol shell si/eon relative siphon weighl in winter and summer (same data as in Fig. 12). Spei ies Site Size x Season II P P R r Mya arenaria 8.1 0.001 9.4 0.001 1 9 0.001 1816 Cerastoderma edule 55.8 0.001 V5 0.001 IJ 0.23.' 237 Mai oma Indlhica 50.6 0.001 4.1 0.001 0.8 0.001 2700 , ularia plana 13.8 0.001 III 0.001 0.7 0.001 6178 109
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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
Page 57 and 58: I Fig. 3). Peak condition was reach
Page 59 and 60: total biomass since most of those t
Page 61 and 62: 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 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 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
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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 ! •
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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
Page 367 and 368:
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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