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their gut is full? Do they reduce their intake rate to the level of the processing rate of 0.66 mg s ' or do they stop feeding and start again when the alimentary tract is partly or completely empty? Apparently Oyster catchers do not lower their intake rale when the alimentary tract is full, since intake rates as low as 0.66 mg s ' are rarely recorded. Instead. Oystercatchers rest for a considerable part of the low water feeding period, just like Whimbrels Numenius phaeoptis eating fiddler Crabs Uca tangeri (Zwarts & Dirksen 1990). Active foraging is often restricted to the first and last hours of the low waler feeding period (Brown & O'Connor 1974. Swennen et al. 1989). The higher iniake rale on the incoming tide recorded in captive birds by Swennen et al. (1989) and in the field by Zwarts & Drent (1981) and Goss-Custard ct al. (1984) probably guarantees that birds arrive at the high water roost with a full gut We have shown that Oystercatchers continuously make feeding decisions lhat enhance their intake rate. At the start of the chapter, we assumed they do this in order lo minimize feeding time and thus maximize the lime they can spend on other activities such as preening and aggressive behaviour. However, when the processing rate is usually so much lower than the intake raie. ihe birds can preen or be aggressive during their inevitable digestive pauses. This implies that feeding and. for example, preening are not necessarily competing activities. The question, already raised by Kersten & Visser (1996a) is this: why should we continue to expect Oystercatchers to always try to maximize their intake rate and thus minimize the lime spent feeding.' Maximizing intake rate would only seem to be relevant when the birds have difficulties in achieving an iniake rale of about 1 mg S"'; only in these circumstances is the total consumption determined by the intake rate itself (Fig. 15). It seems unlikely to be important for Oystercatchers to attempt to increase their intake rate PREY SIZE SELECTION AND INTAKE RATE 172 and when ii has already reached levels of 3 or 4 mg s' 1 and vv hen the only apparent consequence ol doing so is that they nuisi pause earlier to allow for digestion. While these arguments seem to apply to many situations in winter, they do noi do so in the breeding season. The amount of food brought to the young often limits reproductive success {Ens etal 1992). Breeding birds have to feed in a hurry to return to their nest and/or defend their territory or young, and the rate of provisioning is not limited by the capacity of their own gut to process food. Indeed, breeding birds feed at higher average rates than non breeding Oystercatchers. which are not lime-stressed (Zwarts & Drent 1981. Hulscher 1982. Ens et al. 1996c). Moreover. Oystercatchers were able to handle prey faster and to increase their intake rales when they were experimentally forced to collect their food over short feeding periods (Swennen el al 1989). li seems that Oystercatchers do not normally Iced at the highest possible rate they can attain, even though they usually seem to make the most profitable choices in the sizes of prey they select. The task now is to investigate the trade-offs thai the buds presumably make under various conditions. One such trade-off might be to balance a higher intake rate against the need to minimize the risk of damage to the bill when breaking into their large and well-defended prey (Hulscher 1996). This would mean that the longterm advantage of an undamaged bill would be set against the short-term goal of a higher food intake rate and reduced feeding lime. Another trade-off might be between a lower intake rate but a reduced chance of being infected by parasites (Goss-Custard el al. 1996a). What is clear, however, is that a model in which maximizing the intake rate is the only consideration is not adequate. More sophisticated models, w Inch include trade-offs between a variety of changing influences, including the state of the bird's body reserves (Stephens & Krebs 1986). are now required.
Chapter 9 CAUSES OF VARIATION IN PREY PROFITABILITY AND ITS CONSEQUENCES FOR THE INTAKE RATE OF THE OYSTERCATCHER HAEMATOPUS OSTRALEGUS Leo Zwarts. Bruno J. Ens. John D. Goss-Custard. Jan B. Hulscher & Sarah E.A. le V. dit Durell published in: Anita 84 in press (1996) 173
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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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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 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 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
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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
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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
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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
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lion of Mussels Mytilus edulis hy O
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lulal Dal esiuanes: a comparison of
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