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$Wetenschap - \Nijverheid- Handel$

showed that handling time increased with prey length and lhat it also took, as expected, more time to handle- Cockles and Mussels containing more flesh. These increases were not significant, however (Table 2). The relationships were even significantly negative in Scrobicularia and Macoma. The explanation is that in the latter two species prey condition and burying depth varied concurrently: they were meagre and buried deep in winter and had a good condition and were close to the surface in summer. As shown in the previous section, handling time increased wilh burying depth, so die negative correlation between handling time and prey condition was most likely due lo the positive correlation between prey condition and burying depth (e.g. Zwarts 1991. Zwarts & Wanink 1993). In conclusion, the increase of handling lime with flesh content, such as shown by Ens et al. (1996b) for his data, is not found hy us when dala Irom different mussel and cockle studies were pooled. Scrobicularia and Macoma are even handled significantly more rapidly if they contain more flesh, but this is because body condition varies seasonally in accordance with burying depth. Handling time of soft-bodied prey in relation to prey weight and burying depth Figure 7 shows the relationship between handling time and prey weight in four soft-bodied prey: Ragworms. Lugwonns, earthworms and Leatherjackets. Handling Table 2. Handling time as a function of prey si/e and prey condition according to multiple regression analyses a is the intercept. b( ± SE is Ihc prey si/e (Infmm)) and b, ± SE is the prey condition (Inf percent deviation from average weight for each mm class)). The average weights are obtained by plotting for each species all weights againsi all • 2-6. The analysis in Mstilus and Cerastoderma is limited to prey opened by stabbing. The prey condition has a non-signilicanl. positive effect on the handling time in Mytilus and Cerastoderma and a significant, negative effect on the handling time of Scrobicularia and Macoma. Species SI- b: SE Mytilus -0.029 +1.081 .075 +0.041 .074 .681 99 Cerastoderma -2.950 +1.851 .112 +0.273 .200 94s ;;,, Scrobicularia -1.255 +1.254 .157 -0.572 .155 .582 61 Macoma -2.452.+1.746 .328 -1.093 .511 .754 28 PREY PROFITABILITY AND INTAKE RATE 186 time quadruples as prey weight increases thirty-fold. The handling times are short in these species for several reasons. First, no lime was spent in opening, or preparing, the prev. since they were eaten whole. Moreover, soft-bodied prey were usually swallowed in one piece and not piecemeal. Further, most of these prey were picked up from, or taken from just beneath. the surface. I kindling times were longer when the pre) were extracted from the substrate. It took Oystercatchers. on average. 4 s to remove Leatherjackets from their burrows. 2-4 cm deep, and only 1.3 S to mandibular them (Blomert & Zwarts unpubl.). Hence, depending on the position of the l.eatherjacket in its burrow, the handling time varied between 2 and 6 s. We might expect an even larger difference in earthworms. When prey are found at, or just beneath, the surface, they can be grasped easily and transported up the bill in only one catch-and-throw movement (see Gerritsen 1988). But when prey are extracted from the turf, they often break and must therefore be eaten piecemeal. Whether the prey are at or beneath the surface presumably also explains the average differences in handling time between species. Lugworms. and the majority of the Ragworms, were grasped while they were close to the surface and at depth in their burrows. In contrast, the Leatherjackets and earthworms were, at least panly. extracted from the turf. We conclude lhat soft-bodied prey are handled rapidly, unless they are extracted from the substrate. We assume that because Oystercatchers usually Iced on estuarine worm species thai appear al the surface, they are handled in less time than Ihe grassland species which are more often extracted from the turf. Profitability of armoured and soft-bodied prey Figures 2-7 show the relation between handling time and prey weight in five armoured and four soft-bodied prey species. The profitability, the amount of llesh consumed per unit time handling, was calculated for these species, and also for Anadara and Uca. and plotted againsi their prey weight (Fig. 8). Worms and Leatherjackets were, on average. 4.43 times more profitable than armoured prey of similar size. Although there was a large scatter in the profitability of the armoured prey, it is clear that the profitability of soft-bodied, as well as armoured prey, increases with prey weight.
PREY PROFITABILITY AND INTAKE RATE Fiddler crabs i upper photo) are the least profitable prey, because ii takes an Oystercatcher a lot ot nine io i>pen the carapace and eat the llesh; by not eaimg the pincers and Ihe jaws, ihey ignore 50** of the flesh. Also Mussels are no profitable prey, even if ihe birds eiicounlei ' which are open and Ihe bill can be slabbed immediately between the valves (lower photo). 187
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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.
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
Page 164 and 165: valves may vary by a factor of two
Page 166 and 167: optimal foraging model, the minimum
Page 168 and 169: their gut is full? Do they reduce t
Page 171 and 172: PREY PROFITABILITY AND INTAKE RATE
Page 173 and 174: catchers to take only certain prey
Page 175 and 176: licult error of estimate arose if p
Page 177 and 178: Counts of feeding and non-feeding b
Page 181: 20 30 40 50 shell length (mm) PREY
Page 185 and 186: Nereis Arenicola tipuia earthwo'm M
Page 187 and 188: take rate. We might therefore expec
Page 189 and 190: prey species, using parallel slopes
Page 191 and 192: 5.0 4.0 1-3.0 a & • % * • * •
Page 193 and 194: time during the feeding period. As
Page 195 and 196: winter. Similarly, handling time in
Page 197 and 198: 5.0 - f-4.0 S 3.0 in I 20 a 2 a | 1
Page 199 and 200: situation arrived in the western pa
Page 203 and 204: • ' • 14 PREY PROFITABILITY AND
Page 205 and 206: Notes lo appendix: PREY PROFITABILI
Page 207: Chapter 10 WHY OYSTERCATCHERS HAEMA
Page 210 and 211: 2 4 6 8 10 time on feeding area (h)
Page 212 and 213: 80- 60- o 80 60- 40- 20- INTAKE RAT
Page 214 and 215: est of bod\ behind: an estimated 22
Page 216 and 217: INTAKE RATE AND PROCESSING RATE IN
Page 218 and 219: Wanink 1993). The energy content of
Page 220 and 221: (7) Age All studies dealt with adul
Page 222 and 223: irds over long periods. As an examp
Page 224 and 225: Discussion There is no difference i
Page 226 and 227: comparison between the weight of ih
Page 228 and 229: . 1', L ! •
Page 230 and 231: 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
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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
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lion of Mussels Mytilus edulis hy O
Page 381 and 382:
lulal Dal esiuanes: a comparison of
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