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spend much more time in searching to attain a sustainable intake rate. For instance. Cockles > 30 mm long in good condition may contain as much as 1000 mg llesh and are handled in 70 s. Even if the birds have to search for 930 s for each prey, their intake rate is still 1 mg S" 1 . However, if they feed on cockle spat 8 mm long containing 3 mg llesh. they have to take one prey each 3 s; with a handling time of 1.7 s. the birds cannot search for longer than 1.3 s which requires the spat to be very dense. Cockle spat are indeed found in high densities of many thousands in ' in their first summer. However, although the density of Cockles is usually reduced lo about 100 and 10 Cockles pet nr after one or two winters respectively (Beukema 1982a. Zwarts et al. I996h). Oystercatchers nonetheless achieve a higher intake rate at a low density of these old Cockles than on the high density of spot Are there alternative prey for Cockles and Mussels in winter? Cockles and Mussels are reported as the main prey of the Oystercatchers wintering in NW. Europe (Hulscher 1996). Since Oystercatcher and man compete for the same food resource (e.g. Goss-Custard et al. 1996a), it is highly relevant to know to what degree Oystercatchers may switch to other food resources if, for instance due to intensive fishing, few Cockles or Mussels are left. The data summarized in this paper show that, especially in winter, these are only limited possibilities of taking alternative prey. Burying bivalve species are, on average, more profitable than bivalves living at, or just beneath, the surface (Fig. 9). However, burying bivalve species, such as Scrobicularia and Macoma. live in winter at twice the depth they do in summer. The increase of handling time with depth makes them, therefore, in winter much less profitable than in summer, by which they become even less profitable than surface-living prey (Fig. 9). Moreover, the search time increases with burying depth (Wanink & Zwarts 1985). Hence, the intake rates would be usually very low were Oystercatchers to feed on Scrobicularia an Macoma in winter (Zwarts etal. I996h). ln some years. Mya is an alternative winter prey, as it does not increase its burying depth during the winter and so many remain accessible all winter. Moreover, this prey is highly profitable, even if taken from deep PREY PROFITABILITY AND INTAKE RATE 202 beneath the surface (Fig. 6). However, only prey of about 20-30 mm are harvestable by Oystercatchers since smaller prey are unprofitable and an increasing proportion of the larger clams burrows out of reach of the Oystercatchers bill (Zwarts & Wanink 1984). Consequently. Oystercatchers can only feed on Mya when they are about 1.5 years old. and since there is no spatlall in most of the years (Beukema 1982b. Beukema el al. 1993). Oystercatchers can harvest tins food supply in only in a minority of the winters (Zwarts et al. 1996h). Two large worm species. Arenicola and Nereis might provide other alternative source of prey. However, since both species live at depth, especially in winter (Zwarts & Wanink 19931. Oystercatchers can only feed on ihein when they come to the surface. Oystercatchers have to wait till Lugworms emerge at the surface to defecate, which means that even actively feeding worms are inaccessible for 99.9% of the time As I .ug worms reduce their feeding activity at lower mud temperatures (Smith 1975. Cadee 1976), they are inaccessible for 1(H)'i of the lime on many winter days. This is also true for Ragworms which leave their deep burrows to graze at the surface or filter food in the mouth of the burrow. They feed actively on the surface on sunny days in late winter and early spring, but are also inactive on many cold winter days (Esselink & Zwarts 1989. Zwarts & Esselink 1989. Vedel et al. 1994). Another option is to switch to inland grassland areas, where Oystercatchers can feed on earthworms and Leatherjackets. The latter prey is still extremely small in winter, and possibly only large enough to feed upon from April to August (Laughin 1967, Blomert & Zwarts unpubl.). Earthworms may be an important prey in winter as long as the fields are not frozen. The 22 studies on the intake rate of Oystercatchers feeding on earthworms and Leatherjackets (Heppleston 1971. Hosper 1978, Ens unpubl., Zwarts & Blomert 1996) all arrive at a rather low intake rate (see appendix). This may explain why grassland is only used as supplemental food resource at high water in winter (Goss-Custard et al. 1984). especially after the proceeding low water feeding period was short (Daan & Koene 1981). What can Oystercatchers do in winter if there are no large Mussels and Cockles? Beukema (1993a) describes the exceptional winter of 1990/91 when tin-
situation arrived in the western part of the Dutch Wadden Sea. The Oj stercatchers did not leave the area. The disappearance rate of the alternative prey (small Cockles, Mac:::::::. M -.. and l>;'.'!jVyi/-.'lvvi|s.*\i-i-nliiinally high (Beukema 1993a). suggesting that the birds started to lake prey that they usually ignored and continued to do so. even when the prey density became very low The iniake rale of the Oystercatchers was not measured, but must have been extremely low. Indeed. il may even have been lower than the 0.5 mg s' 1 ob Appendix PREY PROFITABILITY AND INTAKE RATE served in the Eastern Scheldt during the autumn of 1986 (Meire 1996b) when the only fcxxl source available for Ov stercatchers was 0-year Cockles of 8 mm long Although the winter 1990/1991 was mild, the mortality of Oystercatchers in the Wadden Sea was relatively high iCaniphuysen el ul. 1996). Apparently, the consumption rate in the western Wadden Sea was too low to meet their energy requirements, so that part of the birds starved to death. Overview ol all studies thai measured size and weight of prev taken hv Oystercatchers, as well as their intake rate and total consumption indif ferent areas (indicated in last column w iih a ihree letter code, but sec Fig. I for lull namet and their geographical position in NW. Europe; dak = Baie of Dakhla. S. Morocco and arg = Banc d' Arguin. Mauritania i. Si/e is expressed as prey lengih I mm I. weight as mg AFDW and intake rate as mg AFDW s ' feeding. Column 'Time' gives Ihe time spent on ihc feeding area I h I, "Feed' the proportion of the tunc spent leeding while present on the feeding area. Cons' ihe total consumption (g AFDW) during the time spent feeding; 'B' in column Br' indicates whether the data concern breeding birds; nm ' gives the prey density (only forCockles); 'Mus' -hows whether Mussels were opened by stabbing (S), scn- tral hammering I Vlor dorsal hammering (D). Column 'Cap' shows whether birds were held in captivity (C" i. all other studies were done on bee- living birds. Column Lab' shows whether it was necessary to estimate AFDW Irom prey volume IV). wet weight (W) or dry weight IDI; all other studie- measured AFDW; "I-" refers lo studies using formaline to (tore ihe prev. making a weighl correction necessary. For further gen eral explanation see methods, but for detail- the miles in this appendix. The number ol ihe notes corresponds with ihe digit in first column. v.. Species nun mg mgi 1 Month Year Time Feed Cons «' nm •' U Lab Ina 1 Antitlara 73 1637 1.85 2 K(, 7.30 '••-1 19.16 SnmumetaL 1990 ,ii j 2 AII-III, ohi 309 4.12 VI 86/7 II Bunskoeke 1988 •eta 3 n i go s H Bun-k.vki* 1988 •dl a • nlemui 22 *J 1.13 2 73 8.50 58.8 l> Brawn & O'Connor 1974 .» 5 21 102 i 86 ll/l 61/5 8.50 55.0 31.30 w Dtvidtoo 1967 Inn 6 ,,'erma :: 154 2.31 1 54 • - . ' 42.0 26.20 349 V Drinnan 1957 mor 7 h ilemui HI 291 2.45 10 M 7.50 ll- \ Dnnnan1957 mor - Cermloderma 12 12 0.61 : 84 1000 al. 1996c lev 'J Ceraslodenrui is 316 2 12 2 SJ Pris,-.,./. 1995c tn IO derma |fi J13 1 SS : 84 Eat etal 1996c Ug II ,lenna 12 : 0.93 J 84 Ens el al. 1996c M 12 Ceraslodenrui 13 *3 2.87 J 84 ,,' 1996c le 13 Cerustoaeniui IS '.: J.7S s S.I l( BmetaL 1996c in 14 Ceraslodermu i- 92 2.38 '. sa B Ens«a". 1995c la 13 Ceraslodenrui 19 92 : 18 7 -j Emeial. 1996c l£\ 16 Ceraslodenrui H (II i"i '' S3 4 En\elal. I995d lev 17 Cerastoderma M 504 2.05 9 83 10 Ens etal. I995d lev 18 Ceraslodenrui 19 606 4.19 9 83 23 BmetaL I995d li-v 19 Cents Jl 338 3.09 9 83 49 Basel of, I995d lev 20 Cerastoderma 39 616 3.54 9 83 64 Eos etal. 199Sd in 21 Ceraslodenrui '•i 328 2S1 9 S3 n / I995d n'.. 203
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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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OPTIMAL FORAGING AND THE FUNCTIONAL
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prey density II III Fig. 12. Freque
Page 147 and 148: OPTIMAL FORAGING AND THE FUNCTIONAL
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
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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 and 182: 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: 5.0 - f-4.0 S 3.0 in I 20 a 2 a | 1
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
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PREDICTING SEASONAL AND ANNUAL FLUC
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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
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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