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gon and fish (e.g. Potamoschistus microps and Plaiichthysflesus) do not dig deeply and musl capture Nereis from near or al the surface. The larger the shrimp the larger ihe worm taken, to a maximum of 3 cm (Pihl & Rosenberg 19S41. /' microps tire able to kill and eat an adult Nereis, but mainly take the smallesi worms in the field (Muus 1967). There is no information available on si/e seleciion by Flounder /' flesus. but since they make feeding holes of only a few cm (Summers 1980). a restriction to the smaller si/e classes is to be expected. Nereis, however, are an unimportant prey for P. flesus or Plaice Pleuroneclus platessa in ihe Wadden Sea (de Vlas 1979a). in contrasi io the situation elsewhere (Muus 1967. Pihl 1982). Baird & Milne (19811 calculated thai in the Ythan estuary (Scotland), for example. 60% of the total production of Nereis was consumed by predators. of w hich 1/3 was taken by flatfish and the other 2/3 by three bird species: Shelduck Tadoma ladorna. Redshank Tringa totanus and Oystercatcher Haematopus oslralegus. Xci; is present at the surface during low water retreat quickly when disturbed (Linke 1939. Vader 1964. own obs.). The frequency distribution of burrow depth. .is given for the different size classes (Fig. 21 can therefore be used to calculate accessibility to predators which dig or probe to a certain depth. The maximum probing depth of waders, as determined by bill length, varies from 3 cm (Dunlin Calidris ulpina. Grey Plover Pluvialis squatarola) to 15 or 16 cm (female Curlew Numenius aiyitaia). In winter all burrows of Nereis > 5 cm in body length are deeper than 4 cm. thus greater than the bill lengih of Dunlin or Grey Plover (Fig. 12A). The first species indeed selects Nereis < 4 cm in winter (Worrall 1984). Kersten & Piersma (1984) found that Grey Plover took 2.5 and 5 worm min ' in May and September, respectively, and calculated that Grey Plovers had to take worms of 5 to 7 cm, the most common size class present in the mud. to meet their energy demands. Evans a al. (1979) obsened that most Grey Plovers wintering in the Tees estuary (NE. England) took the larger Nereis. Grey Plovers must, therefore, rely on worms present in or at the burrow enlranee. This must also be the case with most other short-billed wader and gull species Observations on the feeding behaviour of the Black-headed Gull Lams lidilnindus (Curtis et al. BURROWING AND FEEDING IN NEREIS 312 J'F'M'A'M'J'J'A'S'O'N'D' Fig. 12. Nereis diversicolor. Annual variation in ihe accessibility ol 4 size classes ol Sereis to predators with different bill lengths ,;. calculated from ihe frequency distribution of ihe bumnv depths in mud (Figs. 2 and 3). The panels show from top to hotioni the accessible fraction in the range 0 lo 4 cm (within reach of e.g. Dunlin. Grey Plover and Black-headed Gull). 0 to 8 cm (within reach of e.g. Redshank and Oystercatcher). 0 lo 12 cm (wilhin reach of 9 9BartaUed Godwit and id Curlew) aadO to I6cm(wiihin reach of 99 Curlew I.
1985) and Curlew (/wails ,v, Lsselink 1989) show thai these birds take most Nereis from ihe surface. Avocets Rccurvirosira avosetta sweep their bill through the upper !: ,v f the surface and tievertheless lake large Nereis (Tjallingii 1969, Engelmoer & Blomert 19831. I he iniake rale of Avocets in the course ol" the emersion pericxl closely resembles the lidal trends in surface feeding of Nereis as shown in Fig. 9 (own obs.i. In winter Redshank (bill length 4 cm) prev mi wi s largei than 4 cm (Goss-Cuslard 1969. 1977a. 1977b). which would be unavailable if they remained deep in their burrows (Fig. I2B). Most worms caught by Curlews are > 9 cm. The fraction ol this size class accessible to a probing male Curlew (bill lengih 12 cm) is Ml', in summer against 20' i in winter (Fig. 12C). In summer Curlews leed on worms which are extracted from the burrows, while in winter nearly all worms are taken from the surface. Female Curlews are the only predators which are able to probe to a depth of 13 to 16 cm (Fig. 12D). Worms below this danger line are out of reach of all their predators. Apparently, the majority of the waders take worms from the surface. Most of these predators have no choice, since the burrow depth exceeds their bill length. Figure 3 shows that burrow depth increases with worm size but that die depth levels off as soon as worms reach a critical depth below the maximal probing depth. The same has also been found in estuarine bivalves (Zwarts & Wanink 1989). A simple explanation for this might be excessive predaiion pressure on all accessible worms. The ereaming-off effect of predators taking worms from shallow burrows was studied with an exclosure experiment set up in July: after 2 months a difference in average depth was apparent (worms inside the exclosure burrowed on average 0.5 cm shallower). This difference (although not siaiistically significant on account of small sample si/esi suggesis that ihe critical depth of 12 to 15 cm is partly caused by. a selective predation on shallow-living worms. Seasonal variation in burrow depth There are 3 possible explanations for the seasonal variation in burrow deplh (Fig. 7): predaiion pressure, temperature and feeding method. Assuming that the selection of burrow depth has BURROWING AND FEEDING IN NEREIS J13 evolved so as to minimize predation risk, ii is to be expeeled that a seasonal variation in predation pressure will correspond with a similar trend in burrow depth, ll is indeed true thai worms live al a minimal depth from May to mid July when most deep-probing predators (Curlew. Oysieicalchei and Bar-tailed Oodvt it i aie absent, but die burrows remain shallow after these waders return in Julv and are present in maximum numbers in August/September. Seasonal depth variation is thus not governed by predation pressure alone. Temperature seems to be of overriding importance In summer, mud temperature decreases with depth but the reverse is true when the surface leinperature is c. 0 °C (de Wilde & Berghuis 1979a, Zwarts unpubl.). A deeper burrow in winter mighl therefore be an adaptation lo escape low temperatures. Worms do indeed increase their depth at an average rate of 0.6 cm for a sea temperature drop of I "C, at least below e. 15 °C (Fig. 8). The digging activity of the worms after a frost in October, mentioned earlier in the text, can be interpreted as a direct effect of temperature on burrow depth. This observation also shows that the worms' adjustment of burrow deplh is not continuous, but erratic. A third seasonal effect might be prompted by the use of different feeding techniques. If die worm obtains all its food from the surface, the burrow is solely a refuge from predators and an adverse climate. During filter feeding, however, the burrow also serves as an irrigation channel. Irrigation is an energy-demanding feeding technique (Kristensen 1981), but these COStS are minimized if the burrow depth is reduced, ll is therefore likely that, oilier things being equal, filter feeders burrow less deep than surface-feeding worms. This effect is likely to be found only if filler feeding occurs predominantly at high sea water temperature, given the close relationship found between temperature and burrow depth. Though detailed information is lacking, il is true that filler feeding is rare in winter and very common in summer (Goerke 19711. One can thus conclude that temperature is an important factor determining the seasonal variation in burrow depth. The possibility thai filter feeding is more profitable if the burrow depth is reduced and that seasonal variation in the significance of filter feeding might effect the observed trends in burrow depth deserves further research.
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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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Page 258 and 259: PREDICTING SEASONAL AND ANNUAL FLUC
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
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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 •
Page 305 and 306: 8 10 I 12 JZ 5. -g 1*» •e o i 16
Page 307: 6 - n=!080 5 • g 4 CP > i 3 CO CD
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 and 336: to locate the clams after they had
Page 337 and 338: Curlew, by bury ing some hundreds o
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
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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
Page 371 and 372:
huch der Vogel Milteleuropas, Band
Page 373 and 374:
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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