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-^_>-> * * & • WHY KNOT TAKE MEDIUM-SIZED MACOMA Knot have to probe their bill into the substrate to locale their bivalve prey. 282
lime (Hughes 1979). This would further reduce the profitability of all si/e classes, and would cause an upward shift in the lower acceptance threshold. Thirdly, when only prey similar in size to the Ion er size thresholds are available. Knot cannot lose time searching for them; they must handle prey continuously. This situation is rarely found. However. Peringia occur al densities of several individuals cm'', one of the rare situations where it seems possible to feed on prey without spending lime searching. The Knot on Vlieland took Peringia 2 to 4 mm long (Fig. 3), as was also the case in Wash Bay. England (Goss Custard et al. 1977b). Peringia of 2. 3 and 4 mm contain 0.12, 0.30 and 0.55 mg dry flesh, respectively (Fig. 2). Assuming these snails are handled in 0.4 s. the highest possible intake rate would be 0.5,0.8 or 1.4 mg s" 1 , according to whether all the prey were 2.3 or 4 mm long. In other words. Knot should be able to reach the observed 0.6 mg s' 1 only if snails 3 mm and larger were taken. However, in the study area, most Peringia were less than 2 mm long (Fig. 6). This may explain why ihese extremel*. abundant and readily available prey were completely ignored. The Knot on Vlieland took Peringia c. 3 mm long, weighing 0.3 mg on average (Fig. 3). Two of these prey would have to be taken per second to achieve an intake rate similar to that of Knot eating Macoma. The rate at which Knot lake Peringia has been measured previously: 980 snails were taken in a feeding period of 20 min (Swennen un- Table 3. Total biomass of six potential prev specie! for Knot ig AFDW in i in ihe study area (corresponding to data given in Fig. 6) and lhat of prey individuals too large, too deep or too small for Knot 1 ig. I'll. The remaining hionia-s is harvestable for Knot. tpecies Willi too not loo too harvest- Im nihi ss large large, but small able too deep for Knot tor Km n Peringia 3.7 ll 0 32 0.5 Cerastoderma 87.1 77.0 0 0.1 10.0 Macoma 28.2 19.6 lis 0.3 3.7 Scrobicularia 12.7 i:.4 02 0 0.1 Mya 6.5 4.1 i: 11 i.: Mytilus 04 0 0 0 0.4 TOTAL 13H.6 113.1 5.0 3.6 15.9 WHY KNOT TAKE MEDIUM-SIZED MACOMA 283 P. ulvae M balihica C edule M arenaria S. plana M edulis S j 1 L INGESTIBLE 10 15 shell length (mm) ' i( , "- , V i -:i PROFITABLE HARVESTABLE 20 2b I- iii- 16. Size classes of the six hard-shelled prev species (Peringia. Macoma. Cerastoderma. Shu S, mhi< iiluriu and Mytilus) which are ingestible by. accessible and presumably profitable to Knot (light shaded sizes are defined .is harvestable when (I) they can be swallowed. 12) they lay within reach ot the bill, and i 31 iheir handling ettk icncv is higher than ihe average feeding rale (mg s ' handling > mg s ' feeding). As explained in the text, the thresholds are not to be considered as fixed hums. publ.. cited in Glutz ei al. 1975). or one prey per 1.2 s. meaning that an intake rate of 0.6 mg s ' might have been achieved if the snails were 4 mm long. Size classes of prey which are harvestable by Knot Figure 16 summarizes, for each of the six prey species, the sizTe classes that are most likely to be taken. The biomass harvestable by Knot is only a fraction of the total biomass, because the majority of the flesh is found in prey too large to be swallowed (Table 3). The upper threshold is set by gape width (si/e threshold) and, to a lesser degree, by bill length (depth threshold i. and the lower si/e threshold is set by the speed at which prey may be handled (profitability threshold).
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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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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: 10 15 lower size threshold (mm) Fig
Page 281 and 282: WHY KNOT TAKE MEDIUM-SIZED MACOMA T
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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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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