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2 4 6 8 10 time on feeding area (h) Fig, I, Highest cumulative food consumption by Oystercatchers Ig AFDW) as a function of the time spent on ihe feeding area (hi. based upon the digestive constraint according to Kersien & vissej (1996a): 111 the storing capacity is 12 g AFDW. (2l defecation starts lis Ii aflei the start nl feeding, i.l) Ihe processing late i~ 0.66 m Thus, in a normal low water period of 5 I06I1. not nunc than 22.7 to 25.1 g can he eonsumed. From the predicted highest consuinpiion. the highesi crude intake rale img 5*' feeding, including iioii-tecilinc boms; CIRi|u, (left avis 11 can be calculated. In the first 0.5 h. CIRiio equals ihe storage capacity (12 g) divided by ihe time of spent feeding Subsequently it decreases according to an inverse relationship that approaches ihe processing rale of O.Mi mg s ' in ihe long term. eni light on the occurrence of inactive birds during the time spent on the feeding area. These are not necessarily birds that are idling away their time, but could be birds whose digestive tract has been filled to capacity si 1 that further feeding is impossible. Birds may not therefore always be able to fully exploit times of good feeding as. for example, when intake rate is high. and/or energy expenditure is low. and/or predation risk is low and/or risk of attracting parasites is low. If so, they may sometimes be forced to exploit less good feeding times as well. Evidently a bird that loses one hour of feeding time due to disturbance will suffer more if it has an empty gut than if its stomach is full because it loses irrecoverable processing time. The bottleneck hypothesis dales hack to Kenward & Sibly's (1977) work on Woodpigeons Colombo paliiiiibii.s eating vegetables and the work of Diamond et al. (1986) on hummingbirds feeding on nectar. The INTAKE RATE AND PROCESSING RATE IN OYSTERCATCHER 12 214 study of Zwarts & Dirksen 11990) on Whimbrel Nutnenitis phaeopus eating crabs seems to be the onlv other case of this idea being applied to a carnivorous shorebird. As Kersten & Visser (1996a) derived their conclusions from only a limited number of experiments on captive Oystercatchers, before it is accepted as a fact in future Oystercatcher studies, ii seems prudent to ihe hypothesis and assess the potential for variability iii the parameters. If there is a digestive constraint, it follows that for a given lengih of the feeding period the total food iniake cannot exceed the sum of the storage capacity and the amount of food that can be processed during that period ("broken siiek" in Fig. I). A necessary corollary is that maximal crude intake rates, or the intake rates calculated over a period which includes the digestive pauses, will decrease with an increasing length of feeding period ('curved line' in Fig. I). Since Oystercatchers stall to defecate 30 min after the beginning of feeding (Kersten & Visser 1996a). the only limit to the intake rale during the first 30 min of feeding is the 80 g storage capacity for wet food, equivalent to 12 g dry llesh. i.e. ash-free dry weight (AFDW). Therefore, if Oystercatchers feed for 10 or 20 min. the highest possible intake rate will be 20 and 10 mg AFDW s '. respectively, and will decrease linearly to 6.67 mg s ' if the birds feed for 30 min. If the feeding time is longer than 30 min. the intake rate further decreases with time but not any longer linearly because the birds start to defecate. There is thus an inverse relationship between the highest possible crude intake and the length of time spent on the feeding area (Fig. I). When the birds feed for an infinitely long period, the crude intake rate cannot exceed the processing rate of 0.66 mg s '. However, when the feeding time is limited, the influence of the storage capacity increases as the feeding time shortens, because the highest crude intake rate (OR ) can exactly be described by the equation: CIR =0.66+ 3 h', max The digestive constraint has one important consequence for the birds. If ihe daily requirement lor lood exceeds the maximum consumption that is predicted In mi the bottleneck hypothesis for a low-tide period, the birds will need to feed during both low-tide periods, irrespective of the intake rate lhat can potentially
e achieved. In this paper, we will address two questions: (I) Do the many studies on food intake of Oystercatchers conform to the predictions of the bottleneck hypothesis, i.e. do the crude intake rates not exceed the curved line in Fig. I? (2) Do birds rest more, or do they reduce their iniake rale, if their consumption is restricted by the digestive bottleneck, or do they both? To lesi the prediction of the bottleneck hypothesis. we use the data set on food intakes of Oystercatchers assembled from published and unpublished sources, of which a large part is summarized in Zwarts et al. (1996a). None of these studies was undertaken as an explicit test of the bottleneck hypothesis, but properly combining reported dala on intake rate, feeding activity and time spent on the feeding area should v ield figures that can be used for this purpose. The review shows that, except for a few cases, the prediction is met. In these anomalous cases, the intake rate exceeds the predicted maximum. This caused us to explore possible sources of error in the estimation of total food iniake and whether they are most likely to overestimate, rather than to underestimate, intake rate. Furthermore, to evaluate whether the digestive bottleneck prevents free-living Oystercatchers from fulfilling their daily energy needs during a single tide, we need to know how much food an Oystercatcher in the wild needs per day. Several papers measured precisely the daily food consumption in caged birds and we convert these data into an estimate of the requirements of freeliving Oystercatchers. Methods Data The data summarized in this paper have been taken from several sources, usually already published, but also unpublished theses, reports and data files. Zwarts et al. (1996a) give a full list of all sources. They also describe how these dala were combined and how all measurements on prey size, prey weight, intake rate, time spent on the feeding time, and feeding activity were assembled into one dala file, of which the essential measurements are given as an appendix; the intake rate were averaged per month of Oystercatchers feeding on a certain prey. In addition to these studies, their paper also includes long-term observations on individ INTAKE RATE AND PROCESSING RATE IN OYSTERCATCHER 215 ual birds studied in the Exe estuary (Ens & Goss-Custard 1984. Urfi etal. 19%), and the Dutch Wadden Sea (Blomert et al. 1983. Bis eta/. 1996a. 1996b. Kersten 1996). Nearly all field studies give food consumption of Oystercatchers as AFDW. Hence we also use this as the measure of food intake. Since, as discussed by Zwarts et al. (1996a I. the energy content of marine invertebrates usually varies between 22 and 22.5 kJ g' 1 and Oystercatchers digest 85% of the ingested energy, the factor 19 can be used as a common multiplier to convert gross intake (mg AFDW) into metabolized energy (kJ) if necessary. Definitions Food consumption is measured as dry llesh (AFDW). gross energy (kJ) or metabolized energy (kJ). Intake rate is defined as food consumption per unit time of feeding, excluding the time spent resting and preening over the period the bird is on the feeding area. The feeding period is defined as the time spent on the feeding area, including any time not spent feeding. Feeding activity is the percentage of the time actually spent in feeding ovei the whole duration ol Ihe leeding peinnl. Crude intake rate is the rate of food consumption over the entire feeding period, including all the non-feeding intervals. Finally, the highest possible crude intake rate, derived from the digestive bottleneck model and indicated by the curved line in Fig. 1. is called maximal crude intake rate (CIR Results Food consumption and feeding time Figure 2A shows that the food consumption by captive birds lies, on average, just below the maximum predicted by the food processing model of Kersten & Visser (1996a). This is also true for field studies in which the consumption by individual, usually colourbanded, birds was measured over long periods (Fig. 2B). Most birds were observed over an entire low water period, but all observations longer than 8 h. and some of the shorter observation periods, refer to breeding birds or to non-breeding birds feeding in grasslands by day. Grassland-feeding birds foraged for less than half of the observation time, sometimes for even less than 201. and therefore had very low crude intake
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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
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
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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
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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
Page 205 and 206: Notes lo appendix: PREY PROFITABILI
Page 207: Chapter 10 WHY OYSTERCATCHERS HAEMA
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
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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
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PREDICTING SEASONAL AND ANNUAL FLUC
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PREDICTING SEASONAL AND ANNUAL FLUC
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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
Page 379 and 380:
lion of Mussels Mytilus edulis hy O
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lulal Dal esiuanes: a comparison of
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