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

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2 3 4 5 6 7 8 size of Mya arenaria in cm Kij». 4. Deplh of clams on offer in relation to their size, and depth of 84 clams taken by the colour-banded Curlew 20Y (dots). The grej field shows ihe depth range at which 9Sfl of Ihe clams are living (based on the same dala as in Fig 31 shows, large clams appear io be more profitable than small ones. Mosl clams taken by the bird were indeed large (Fig. 2A). The risk to a clam of being taken was maximal if it measured 4-5 cm (Fig. 2B). but the risk to the most common clam size present (2-3 cm) was 130 times less. From Fig. IC it can be seen thai clams below 23.6 mm length would be unprofitable for this bird. In March 1981, when large clams were very rare, we observed that the marked bird look some small prey but since we could not locate the clams which were taken. their exact size was unknown. Assuming these were solely second-year clams, which averaged 25 mm in si/e at that time, the bird would have obtained 4.14 mg/s handling time (indicted with an open circle in Fig. IC). which is just above the rejection threshold. Certain other colour-banded Curlews also took clams of about 25 mm. or even smaller, but in those PREY DEPLETION BY OYSTERCATCHER AND CURLEW 10 340 cases ihe birds swallowed ihe ekim whole, including ihe shell. The handling time was very short: 3.41 s (n = 252) for two Curlews observed in November 1980; hence they obtained quite a high yield of*).: mg/s handling lime. Eating clams in this way may have disadvantages, however, for is was observed predominantly in periods when big clams were very rare (the winter of 1977-78 and of 1980-81). and even in those lean w inters musl o| the colour-banded Curlews for which clams were the main prey never swallowed the small ones. Perhaps they would have caused a "digestive bottleneck' (Kenward & Sibly 1978). The accessible fraction of the profitable elams The data on the yields of food from different sizes explain why Oystercalchers take smaller sizes than Curlews, but cannot help us to understand why the Oystercatchers on Ihe mudflats do not attack successfully those clams of over 4 cm and why Curlew 20Y took relatively few clams above 6 em (Fig. 2). These upper limits are in fact determined by the proportion of the different size classes within the reach of the hills of the two waders. To investigate the depth distribution of the elams in each size class, we used a circular corer (of surface area 176 cnr'i pushed 4(1 cm into ihe substrate. After breaking open the core, it is possible to measure accurately ihe distance between the mud surface and ihc upper tip of die bivalves. From these measurements it appeared (Fig. 3) that an Oystercatcher cannot find clams of more than 4 cm in length in the upper 7 cm of the substrule (the bill length of an Oystercatcher) and thai the greater part of the shell above 7 em length arc out of the reach of the female Curlew (bill length 14 cm). Even fewer are accessible toamalelbill length 12 cm) (Fig. 3, inseti. Most clams taken by the marked Curlew lived, given their size, al remarkably shallow depths (Rg. 4). Nearly all were buried above the mean depth of their respective size classes. The bird managed to find ihevery rare clams of more than 6 cm length which lived at atypically shallow depths, only 8-14 cm below the mud surface. In the summer of 1979 we manipulated the food supply in the former feeding territory ol" the marked
Curlew, by bury ing some hundreds of big clams just below the surface of the intertidal Hats. As an unexpected but interesting result, an Oystercatcher started to prey on the big clams and vigorously defended the site, with success, not onlv againsi congeners but also against Curlews, li did so I'm a couple of days, during PREY DEPLETION BY OYSTERCATCHER AND CURLEW which this single bird completely depleted the planted population of prey. This kind of behaviour was never observed in "natural' situations. Interactions between Oystercatchers and Curlews an* normal!*, quite me In the natural situation, il is verj unlikel) that there is diicci competition between Oystercatcher and 1980 1981 Fig. 5. A. Average size of two cohorts ol clams ol which Ihe spatlall look place in I976 (open circles) and 1979(solidcircles) in the Nes area (Frisian coast); rejection threshold for both bod species is indicated. B. Avenge depth of t*w cohorts of which the spatfall usik place in 1976 (open Circles) and 1979 (solid Circles); hill lengih (and thus maximal prey depth) for both bird species is given. C. Density of clams/m-' since 1977 in Ihe Nes area (number of two cohorts together). Upper panel shows total density and density ol clams vulnerable lo predation by Oystercalchers (clam sizc7> 15 mm. depth < 7 cm. see Fig. 3). Lower panel gives density oi potential prey for Curlew s < si/e > 30 mm. depth < 14 cm. set .te the different scale used for both hird species Mam cvploitalion periods las derived Irom bird counts and observations on pre] Selection) are indicated hy black bars along the K-a*ds 341
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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 ! •
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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
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
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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
Page 328 and 329: 2 4 . • 2.2 • 30 .-.2.0 to Q. 1
Page 331 and 332: PREY DEPLETION BY OYSTERCATCHER AND
Page 333 and 334: the rule: the observed lower limit
Page 335: to locate the clams after they had
Page 339 and 340: PREY DEPLETION BY OYSTERCATCHER AND
Page 341 and 342: SAMENVATTING 345
Page 343 and 344: Voorgeschiedenis In 1960 verscheen
Page 345 and 346: maken. Als gebied A een grotere voe
Page 347 and 348: omdat ze nog vrijwel niets wegen. l
Page 349 and 350: kunnen opzuigen. of diep leven en z
Page 351 and 352: van de uitgerekte sifo over het wad
Page 353 and 354: het toen niet gemakkelijk hebben ge
Page 355 and 356: doende nonnetjes van 10 tot 15 mm l
Page 357 and 358: aangevoerde voedsel en de wulp past
Page 359 and 360: zijn dan twee jaar en wulpen geen s
Page 361 and 362: was dan ook dank/ij de inzet van al
Page 363 and 364: REFERENCES 367
Page 365 and 366: Allen RL. 1983. Feeding behaviour o
Page 367 and 368: BreyT. 1989. Der Einfluss physikali
Page 369 and 370: latie tol chironoiiiiilen en de wai
Page 371 and 372: huch der Vogel Milteleuropas, Band
Page 373 and 374: llolmann II. & H. Huerschelmann 196
Page 375 and 376: 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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