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lig. 2. Larger Mussels are less available than small ones lo Ov stercatchcrs hammering on Ihe dorsal or vemral side of Ihe shell, either because ihe shell is covered by barnacles or loo thick to break Cayford & Goss-OisiardllWO). enough to break (Fig. 2). Since Mussels often grow on top each other, an unknown proportion of the Mussels are not actually visible lo sight-hunting Oystercatchers (Goss-Custard et al. 1993. Meire 1996b, 1996c). This may affect the observed size selection, since large Mussels tend to cover small Mussels (Hulscher 1964. Norton-Griffiths 1967, Goss-Custard et al. 1993). Observed size selection Macoma balthica Oystercatchers feeding on Macoma detect them by PREY SIZE SELECTION AND INTAKE RATE DORSAL VENTRAL 5 10 15 20 length ol Macoma (mm) 50 fO 30 length of mussel (mm) V 50 ^T too ttiich na thin, but covered •*•• by Barnacles I | available touch. Size selection in birds feeding on this clam has been studied on many occasions. To construct Fig. 3, the frequency distributions of the mm classes laken and of those on offer in the substrate were combined from all the studies because the si/e class distributions taken and on offer were similar, apart from the very large variation in the density of prey smaller than 10 mm long. However, these small Macoma were never selected, regardless of their contribution to the total prey population. On average, 81% of the prey taken were 15 to 20 mm long, compared with onlv ^3' I on oiler al all depths in the substrate (Fig. 3A). In fact, Oystercatchers take Macoma only from the upper 4 cm tig. 3. Size classes of Macoma balihica selected by Oystercaicher i shaded i compared with the distribution ..I size classes in the prey population i solid line I: A. Total pre) population: B. Prey population in the upper 4 cm I sec Fig. 1):C Prey population in the upper 4 cm. corrected tor 'effective touch area" (see Table I) and so actually encountered. The graphs give the averages of 26 studies in which prey on offer and those laken were compared: Blomert el al. (1983), Cio-s l usiard et al. 11977b), Habekotte I 19X7). Hulscher (1982 and unpubl.). Triplet (1989a). 158
PREY SIZE SELECTION AND INTAKE RATE 10 16 18 20* io 12 14 16 18 20+ length of Macoma (mm) Fig. 4. Macoma balihica. The relative risk nl bivalves ol different lengths being laken by Oystercatchers. calculated as the ratio per size class of relative number of prev taken 10 the relative number in the prey population: data from Fig. 3. A. Relative risk (± SE) when Ihe density of large Macoma (18 to 22 mm long) was lower than 15 m : (n = 10) or higher than 30 m ; (n = 4); B. Relative risk (± SE) in April to June (n = 10) and in August to March (n = 6). tortile out the confounding effect of prey density shown in A., a selection is made for studies where ihe density of Manama 18 to 22 mm long is less than 30 m '. of ihe substrate (Hulscher 1982). When the frequency distribution of the size classes actually within teach ol the bill is calculated (Fig. 3B). (he selection for larger size classes is even more pronounced because all small prey, but only a small proportion of the large prey, are accessible. But when the increase in effective touch area with si/e (Table 1) is also taken into account, the probability that small prey will be encountered dropped to a third of the previous value. Even so. there still remains a remarkable deviation between the observed size selection and that expected ol" a randomly probing Oystercatcher (Fig. 3C). In all cases studied. Oystercatchers rejected Macoma less than 11 mm long, while the size classes 11 to 15 mm were taken much less frequentlv than would be expected on the basis of random searching. The relative risk of the medium-sized Macoma being taken appeared to depend on the density of the larger ones. 18 to 22 mm long (Fig. 4A). but not on their own densiiv. There was also a difference in size selection between summer and the rest of the year (Fig. 4B); any confounding influence of the variable density of large Macoma was removed by restricting the analysis to studies where the density of large Macoma was less than 30 specimens m : . In conclusion. Oystercatchers 159 concentrate their feeding on the largest Macoma. but when these are less common, llicv take relatively more of the medium-sized ones, especially in autumn and winter. Small prey are always rejected, however. Scrobicularia plana In contrast to Macoma. where different year classes regularly occur together, the frequency distribution of Scrobicularia si/e classes varies a lot depending on how many years have passed since the last recruitment took place. Three studies have been made of si/e selection in Oystercatchers taking Scrobicularia by touch. Blomert el al. (1983) studied Oystercatchers feeding on Scrobicularia between 24 and 48 mm long. The birds hardly took anv prey larger than 37 mm because they were inaccessible (see Fig. I). Prey smaller than 28 mm long were taken less than expected. whether or not a correction was made for the effective touch area and the fraction that was accessible. Hughes (1970a) found that Oystercatchers rejected prey smaller than 20 mm long when prev 25 Eo 40 mm long were available. Habekone (1987) showed that small Scrobicularia were taken when there were few large specimens. In his study the birds rejected prey smaller than about 13 mm long and selected the largest size
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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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Page 106 and 107: 4 Q) Si •a a. 16- 20- A predator:
Page 108 and 109: prey risk for an animal al a depth
Page 110 and 111: Table 6. Size al which there is a m
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Page 114 and 115: face and so expose themselves to a
Page 116 and 117: surface in die containers was varie
Page 118 and 119: 50 OOF 310.00 I 5.00 ^ 1.00 3=" 0.5
Page 120 and 121: Tahle 1. Macoma and Scrobicularia.
Page 122 and 123: siphon would not have to reach the
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Page 127 and 128: ACCESSIBLE PREY ARE OFTEN IN POOR C
Page 129 and 130: 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 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 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
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Page 199 and 200: situation arrived in the western pa
Page 203 and 204: • ' • 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
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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
Page 371 and 372:
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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