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PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY classes were selected: (1) Herring Gulls took Mussels in 1985 that were still too small to be taken by Oystercatchers (Zwarts unpubl.): (2) Knot Calidris canutus selected medium-sized Macoma hardly taken by Oystercatchers (Zwarts & Blomen 1992): (3) Oystercalchers look second year Mya still ignored by Curlews (Zwarts & Wanink 1984). Hence, although different bird predators successively coniribule lo the decline ol the prey cohorts, there was no reason to take into account the predation pressure of the other bird species when we compared oystercatcher predation to the elimination of the prey biomass harvestable by Oystercatchers, Seasonal variation in intake rate and prey selection The intake rate of Oystercalchers varied seasonally, being high in summer and low in winter. This trend was more pronounced in the burying prey species, Scrobicularia and Macoma, than in the surface prey, the Cockle (Fig. ISA). The seasonal variation in intake rate was even smaller in Mussel-eating Oystercatchers (Goss-Custard & Durell 1987: Fig. 17 in Zwarts et al. 1996b). The explanation is that, due to the variation in burying depth, the encounter rate with burying prey was reduced in winter. There was no such a difference in surface prey, although birds stabbing the bill betvveen Ihe valves may more often encounter closed bivalves in winter than in Ihe summer, when Cockles and Mussels (ccd more often themselves. This may explain why the seasonal variation in intake rate in Mussel-eating Oystercatchers was larger in slabbers than in hammerers (Goss-Custard & Durell 1987). According to the predictions (Fig. I5B) and direct nbservaiions (see text), burying prey were selected in summer and only laken in winter when there were no surface prey. Several other studies provide similar evidence for such a seasonal shift in the diet of the Oystercatcher. Bunskoeke et al. (1996) and Hulscher ct al, (1996) show that Macoma was the main prey on Schiermonnikoog in spring and completely disappeared from the Oystercatchers diet in late summer. That Macoma is indeed a summer prey, hardly taken by Oystercatchers in winter, is also evident from the work of Beukema (1993a) on the Balgzand in the western part of the Wadden Sea. He found that the monthly mortality of Macoma between mid-March and mid-August (five months) was. on average, three 258 limes as large as in the remaining seven winter months (Beukema 1993a: Fig. 6). Bird counts indicated thai ihe monthly predation pressure by Oystercatchers was only 1/4 the level in the five summer months compared to the seven winter months (Beukema 1993a: Fig. 4). Assuming that the mortality of adult Macoma was completely due to oysieicalchei predation. ii follows J F M A M J J A S O N D Kig. 19. Seasonal variation in ihe number of
PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY TiuHisamis oi 0*r*stercatchers roost in the grassland polder on west Schicminiiiiikooii. but most continue to iced during the iiigfi water period, .il least in autumn and winter, when ihe feeding lime during the preceding low water period was shorl. thai an individual Oystercatcher took Macoma in spring and summer 12 times as often as in autumn and w inter. The significance of Macoma as autumn + winter prey for Oystercatchers must be still lower, because large numbers of Knot winter on the Bale/and (Zegers & Kwint 1992). and this Species must be responsible lor a major pari of the mortality in autumn and winter. Another source of in torn union on ihe seasonal variation in prey selection may be derived from ihe many field sludies assembled from different areas m NW. Europe (Zwarts el al. 1996b. 1996c). Not one of the 276 studies from September to March refer lo Ma- (•''.wi-eating Ov steraichers. but 77 of 311 sludies during ihe four summer months. This suggests again lhal Macoma disappears from the diet in late summer. Winter predation by Oystercatchers of Scrobicularia must be impossible during the majority of the w inters because the prey live tix> deep. There are, however, two winter studies of Oystercatchers feeding on Scrobicularia. In one case, this concerned birds on 259 Schiermonnikcxig feeding on relatively small prey 20 mm long, of which the majority lived just within reach of the bill (Habekotte 1987). The other study was done by Boates & doss-Custard (1989) in the Exe estuary. Although ihe majority of the birds in the Exe are found on the mussel beds, some birds feed each vv inter on Scrobicularia. Perhaps this prey on ihe Exe in winter do not live as deeply buried as in the Wadden Sea. II so. ihe seasonal variation in iniake rale would not be expected to be as large as in the more northern tidal flats where the accessible traction of Scrobicularia is much lower in winter lhan in summer (Zwarts & Wanink 1993). Seasonal variation in bird density Despite the large variation in the number of Oystercalchers feeding in the area from year to year, the trends were similar each year (Fig. 6A). The seasonal course of the change in numbers in the study area I9A) deviated from those elsewhere in the eastern part
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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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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
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 and 278: 10 15 lower size threshold (mm) Fig
Page 279 and 280: lime (Hughes 1979). This would furt
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 •
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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
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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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