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andomly probing their bill into the mud. so prev detectabihty is determined by the surface 'touch area' Of the shell (Hulscher 1976. 1982. Zwarts & Blomert 1992). Oystercatchers either open the prey in situ beneath the surface or lift them out of the substrate. The bivalves are opened by stabbing, or forcing, the bill between the valves, after which the llesh is eaten out of the shell (Hulscher 1982. Wanink & Zwarts 1985). Because no size limit is set by gape size and only the llesh is consumed, all prey are ingestible and digestible. Hence the harvestable fraction depends solely on the limits determined by the accessibility and profitability of the prey (Fig. 14A). Oystercatchers lift deep-living prey more often than shallow ones (Wanink & Zwarts 1985). The lifting of prey prolongs the handling time and so makes them less profitable. As discussed above, deepliving, but accessible, pre) may thus be ignored due to their low profitability (Wanink & Zwarts 1985). We therefore assume that ihe Oystercatchers (bill length 6.5 to 8.5 cm) take the majority of Macoma and Scrobicularia from ihe upper 4 cm of the substrate, and perhaps from the upper 6 cm when either the intake rate and/or the density of easily accessible prey is low. Oystercatchers take large bivalves (e.g. Hughes 1970a. Goss-Custard et al. 1977a. Hulscher 1982, Bunskoeke et al. 1996. Zwarts et al. (1996a). Macoma smaller than 10 mm and Scrobicularia smaller than 13 mm are always ignored because they are unprofitable (Hulscher 1982. Zwarts et al. 1996a). Intake rate varies greatly and primarily depends on the presence of large bivalves. When, due to the high density of the large size classes. Oystercatchers achieve a high intake rate, they are more selective and ignore Macoma less than 15 mm long and Scrobicularia smaller than 25 mm long (Zwarts et al. 1996a). The harvestable fraction is thus highly variable for two reasons. First, the depth distribution and prey biomass fluctuates seasonally Fin. I-I- The harvestable traction of benthic prey depends on the fraction of prey being A. accessible and profitable in touch-feeding Oystercatcher: B. accessible, ingestible and profitable in louclifeeding Knot, and C accessible, detectable and profitable in Curlew feeding by sight lor siphon holes. FOOD SUPPLY HARVESTABLE BY WADERS 7.: DETECTABLE V opth ACCESSIBLE AVAILABLE INGESTIBLE ' PROFITABLE *o N HARVESTABLE ^ 1 f size K. Oystercatcher (A) Knot
and annually. Second, the lower size acceptance threshold as well as the depth selection varies being a function of the intake rate. Knot feed by touch when they search for benthic bivalves, but in contrast to Oystercatchers. they ingest then pre) whole. This makes ihc lun veslablc lids lion of prey for Knot much smaller than for Oystercatchers (Fig. 14B). Macoma is a preferred prey for Knot, but they ignore the size classes smaller than 10 mm long and reject Macoma larger than 16 mm long: the small prey are unprofitable, while the large ones are too wide to be swallowed (Zwarts & Blomert 1992). Knot visited our Stud) site only in August when the total biomass of Macoma over ten years varied between 6 and 35 g nr 2 , with an average of 17.7 g nv 2 (Fig. 15). On average. 44',{ of this biomass belonged to the suitable size classes, whereas only 31% was harvestable. i.e. both of suitable size and accessible (living in the upper 2 cm). The annual variation of 6 to 35 g m - in the total biomass of Macoma was small compared to those of other benthic species (Beukema et al. 1993). the standard deviation of 8.6 g nr- being only 49% of the mean (17.7 g in 2 ). The relative standard deviation (RSD) lot suitable biomass (6 to 16 mm long) was 44' i. and thus lower than that for the total biomass. The annual variation in the fraction of Macoma living within reach of the bill was much larger still: for instance. 98% of the prey was found in the upper 2 cm of the substrate in August 1984. against only 15% in August 1986 (Fig. 15). On average. 54% was accessible and the RSD was 60%. As a consequence of this large variation in prey accessibility, the RSD of the harvestable biomass increased still further to 77%. It was therefore the variation in depth distribution that was a major contribution to year-toyear fluctuations in the biomass of Macoma actually harvestable by Knot staging in our study area in August. Piersma et al. (1993b) arrive at the same conclusion in their study of Knot staging on Griend. western Dutch Wadden Sea. Knot did not stage in our study area when the biomass of the harvestable Macoma was low (Fig. 15). The harvestable prev fraction in touch-feeding waders, such as the Knot, is less complex to measure than in most waders that feed by sight. Figure 14C illustrates the relatively simple situation of a Curlew searching for the siphon holes of Mya. This prev is FOOD SUPPLY HARVESTABLE BY WADERS n 1977 78 79 80 81 82 83 84 85 86 Fig. 15. The biomass of Macoma balihica in August 1977 to 1986, given for all size classes i 'total' i. lor onlv the specimens III ihe range 6 lo 16 mm I 'suitable si/e' i and lor animals of 6 lo 16 mm living in Ihe upper 2 cm of the substrate I 'harvestable' I. The grey field gives the averages* SD. The lower panel slums the response of the Knot (peak numbers in the study area). Dala from Zwarts el ul 11992). harvestable if it lives within reach of the bill (13 to 16 cm), if il is profitable (size > 3 cm) and. at least for Curlews feeding by sight, if the siphon hole is v isible at the surface (Zwarts & Wanink 1984). Only a small part of the profitable fraction is actually accessible. This is probably the main reason whv short-billed male Curlews (bill length 10 to 13 cm) never feed on Mya, while it is the main prey for the females (bill length 13 to 16 cm) in areas where ihe prev specieoccurs (Zwarts & Wanink 1984). The detectable fraction also varies considerable. Siphon holes arise when Mya extend their siphon lo the surface for suspension feeding. Mya cannot feed at low tide as no water lies on the surface, but the siphon holes may remain visible until they decay over the low water
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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
Page 27 and 28: Long-term, broadly-based research c
Page 29 and 30: Chapter 1 SEASONAL VARIATION IN BOD
Page 31 and 32: SEASONAL VARIATION IN BODY WEIGHT O
Page 33 and 34: Tahle 1 Weight loss ('•- ± SE) m
Page 39 and 40: i 60 50 40 30 20 10 0 • INFESTED
Page 41 and 42: 240- SEASONAL VARIATION IN BODY WEI
Page 43 and 44: 20 10 0 -10 -20h 2 3 4 5 6 7 8 9 se
Page 45 and 46: a E 400 - S. plana 35 mm 320 240 16
Page 49 and 50: Chapter 2 HOW THE FOOD SUPPLY HARVE
Page 51 and 52: FOOD SUPPLY HARVESTABLE BY WADERS H
Page 53 and 54: Methods The study sites were situat
Page 55 and 56: 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: 4 5 6 7 8 9 size class of Corophium
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 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
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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 ! •
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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
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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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