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stomach of different wader species may thus limit prey size. Three wader species that eat bivalves whole (Knot. Great Knot Calidris lenuimstris and Purple Sandpiper) have relatively heavier stomachs than species that eat soft prey (Piersma el al. 1993a). Moreover, Piersma el al. (1993a) also found indications that, within a species, individuals with a heavy stomach had taken more hard-shelled prey. To conclude, bivalves contribute most to the total biomass of the intertidal invertebrates (Figs. 4,5; Beukema 1976). but nonetheless, only a few wader species are Specialized to exploit them because of their protective shells. Most waders cannot digest these prey because they are either too large to be ingested or too strong to he cracked in the stomach. The profitable prey fraction Waders ignore small prey. These prey are unprofitable, or uneconomical, because they give too low a return in terms of energ) to be worth spending the time needed to handle them. Small prey are usually less profitable than large ones, because prey weight, and thus energy, increases exponentially with si/e while the increase in handling time is much less. In odier words, small prey must be handled extremely fast to make them as profitable as large prey. Optimal foraging theory (e.g. Krebs & Kacelnik 1991) shows that, for a given prey size class to be taken, its profitability (i.e. intake rate during handling prey) must always exceed the intake rate during feeding (searching + handling). This rule has been used to explain the value of the size acceptance threshold for small prey observed in Oystercatchers feeding on four prey species: My HI us (Zwarts & Drent 1981. Ens 1982. Sutherland & Ens 1982, Meire & Ervynck 1986. Cay ford & Goss-Custard 1990). Macoma (Hulscher 1982). Cerastoderma (Sutherland 1982c) and Mya (Zwarts & Wanink 1984), in Redshank preying on Nereis (Goss-Custard 1977b). in Curlews feeding on Mya (Zwarts & Wanink 1984). Nereis (Zwarts & Esselink 1989) and Uca (Zwarts 19851. in Whimbrels feeding on Uca (Zwarts 1985) and in Knot feeding on Macoma (Zwarts & Blomert 1992). These studies showed dial birds were more selective than predicted by theory, because barely profitable prey were usually taken much less frequently than expected. It is not easy to identify the fraction of the ma FOOD SUPPLY HARVESTABLE BY WADERS 68 crobenthos that is profitable for waders, the main problem being that the profitability of a given prey varies between wader species. Large birds are usually able to handle prey of a given size much faster than a small bird. For instance, the average time needed io handle a Fiddler Crab 20 mm wide decreased from 90 to 4 s in four bird species that varied in weight between 100 and 1500 g (Zwarts 1985). This makes prey of a given size much more profitable for larger waders. On the other hand, large waders also need more lood. so their lower prey size acceptance threshold will be elevated due to the need to maintain a higher intake rate. It is possible to determine for each wader species the minimum weight of prev that are sufficient!} profitable to eat. if we know the average intake rate and the handling time of small prey. The average intake rate of each wader species can be estimated because their total daily consumption is a function of metabolic requirements, and thus dependent on body weight. Zwarts el al. (1990b) estimated average daily consumption to vaiy with body weight of the wader (W. in gram) according to the equation: daily consumption (g dry flesh) = 0.322W" 7 -' 111 Waders in tidal habitats forage for 6 to 13 hours in each 24 hour period, with the time spent feeding being lower for the larger wader species (Goss-Custard el al. 1977a. Pienkowski 1977. Engelmoer el al. 1984. Zwarts ei al. 1990b). The average intake rate required to meet the energy demands equals the daily consumpiion divided by the feeding time. According to Zwarts et al. (1990b) this calculation gives the relationship between intake rate and body weight (W, in g) as: intake rate (mg dry llesh s-') = 0.004W OM (2) Although this is a crude approach, the formula predicted quite well the intake rate in four species (Zwarts et al. 1990b). Equation (2) can be used to derive a generalised lower prey size acceptance threshold. Accepting that the profitability (mg s' handlingi must at least be equal to the intake rate (mg feeding), the minimum prey weight can be defined as the product of intake rate and handling time. Thus, a
general law relating handling time to prey size in birds of different weight needs to be formulated. Waders can take up to 40 to 160 small prey per minute when ; ipidlv at one spot so thai they hardly spend any time searching for prey. The minimum time to take then a prey varies between 0.4 and 1.5 s. Film and video analyses showed that the time taken io transport a small prey up the bill to the gape amounts to 0.4 s for Knot taking a small piece of llesh (Gerritsen 1988). 0.40 s for a Common Sandpiper Actios hypoteucos taking a ehironomid larva (Blomert & Zwarts unpubl.l. 0.46 s for a Sanderling taking a small isopod (Myers et al. 1980), 0.70 s for a Blacktailed Godwit Limosa limosa taking a ehironomid larva and 0.79 s for a Black-tailed Godwit taking a rice grain (Blomert & Zwarts unpubl.). In contrast to ducks (e.g. de Leeuw & van Eerden 1992). waders have to pick up and swallow each prey individually. The total handling time must be longer than me time taken to mandibulate and swallow, because the prey must be (1) first recognized as edible. (2) grasped and. if necessary, lifted from the substrate. (3) shaken. or even washed, to clean it. (4) swallowed. (5) alter which the bill can be lowered again to recommence searching for. or handling, the next prey. Myers et al. (1980) distinguished three components in the handling tunc of Sanderlings and measured their duration: the orientation time of 0.12 s preceded the swallowing lime of 0.46 which was then followed by the time to return the bill to the surface, the down time of 0.12 s. The total handling lime was 0.69 s. thus 1.5 times the swallowing time. Since Sanderlings took 0.86 s to consume each prey when the prey density was high, the average search and non-feeding time between successive prey was only 0.86 minus 0.69. or 0.17 s. per prey. A similar calculation can be made for Blacktailed Godwits feeding on chironomids or rice grains. On average, ehironomid larvae were taken every 1.5 s (Blomert & Zwarts unpubl.>. exactly as found by Dirksen et al. (1992) and Szekely & Bamberger (1992). The handling time amounted to 1.0 s and was thus 0.5 s shorter than the time required to consume each prey. A quarter of this difference could be attributed to time lost in handling prey mat were subsequently rejected. Thus, less than 0.4 s per prey was spent in non-handling lime. The swallowing lime of a rice grain was 0.79 s and the total handling time FOOD SUPPLY HARVESTABLE BY WADERS 0.97 s, or 1.13 s when the lime wasted in rejected rice grains was included (Blomert & Zwarts unpubl.). A Black-tailed Godwit needed at least 1.71 s lo find and eai a prey, so the minimum search time was 0.6 s. The three analyses show that the feeding rate actually achieved was 20 to 35*31 below ihc maximum feeding rate the birds would have attained if they only spent lime in handling prey. Handling limes of small prey were rarely measured, but feeding rates of waders taking small prey have been determined in several studies. Therefore we will compare handling times estimated as ihe inverse ol ihe feeding rate and use the term 'composite handling time' to distinguish it from the 'true handling time'. 'Composite handling time" overestimates handling time', but il probably gives a more realistic description of the average minimum lime needed to consume a prey since it includes the wasted handling times and the time needed to move the bill from one prey lo the next. At first sight, it would seem obvious thai large waders handle prey of all size classes faster than small birds: for instance, their larger gape width would be expected to enable them to swallow prey much more easily. Surprisingly, the reverse trend has been found for small prey: it takes a large bird more time than a small one to eat tiny prey (Fig. 12A). The explanation may be found in Fig. I2B where long-billed birds are shown to take more time to handle a small prey than short-billed birds. Body weight and bill length are highly correlated in the sample of bird species used in Fig. 12 (r = 0.88). A mulliple regression analysis i sec- Fig. I2B) revealed that the effect of body weight on composite handling time disappeared completely when bill length was taken into account while the influence of bill length on composite handling time became even more pronounced than in the simple regression. As Fig. 12 includes the Black-headed Gull Lams ridibitndus and Glossy Ibis Plegadis falcinellus and their composite handling times do not deviate from the trends found in the wader species, the relationships in Fig. 12 may apply to all birds taking one prev at a time. How to explain that the composite handling lime depends on bill length ' Waders transport a prey from the bill lip to the gape by a series of "catch and throw movements' (Gerritsen 1988). Video analysis showed that the duration of the swallowing time, the major
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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
Page 21 and 22: the burrow depths and on the fracti
Page 23 and 24: INTRODUCTION Ms,i invest 409 of (he
Page 25 and 26: able lo sw itch 10 oiher prey which
Page 27 and 28: Long-term, broadly-based research c
Page 29 and 30: Chapter 1 SEASONAL VARIATION IN BOD
Page 31 and 32: SEASONAL VARIATION IN BODY WEIGHT O
Page 33 and 34: Tahle 1 Weight loss ('•- ± SE) m
Page 39 and 40: i 60 50 40 30 20 10 0 • INFESTED
Page 41 and 42: 240- SEASONAL VARIATION IN BODY WEI
Page 43 and 44: 20 10 0 -10 -20h 2 3 4 5 6 7 8 9 se
Page 45 and 46: a E 400 - S. plana 35 mm 320 240 16
Page 49 and 50: Chapter 2 HOW THE FOOD SUPPLY HARVE
Page 51 and 52: FOOD SUPPLY HARVESTABLE BY WADERS H
Page 53 and 54: Methods The study sites were situat
Page 55 and 56: less than that of bivalves, partly
Page 57 and 58: I Fig. 3). Peak condition was reach
Page 59 and 60: total biomass since most of those t
Page 61 and 62: on the basis of the preceding and t
Page 63 and 64: However, for obvious reasons, we ma
Page 65 and 66: prey. A decrease in the prey densit
Page 67 and 68: Tahle 2. The intake rate ol < lyste
Page 69 and 70: * ' % -. . ; a X - . - * • ^ •
Page 71: (Zwarts & Wanink 1989). In quiet we
Page 75 and 76: nearly twice as much time (0.79 s).
Page 77 and 78: 4 5 6 7 8 9 size class of Corophium
Page 79 and 80: and annually. Second, the lower siz
Page 81 and 82: Prey switching Waders feeding on ti
Page 83 and 84: autumn and spring, and the contrary
Page 85 and 86: where they switch to surface-living
Page 87 and 88: Chapter 3 BURYING DEPTH OF THE BENT
Page 89 and 90: DEPTH AND SIPHON CROPPING IN SCROBI
Page 91 and 92: I DEPTH AND SIPHON CROPPING IN SCRO
Page 93 and 94: DEPTH AND SIPHON CROPPING IN SCROBI
Page 95 and 96: Table 3. Three-Way analysis Of vari
Page 97: Chapter 4 SIPHON SIZE AND BURYING D
Page 100 and 101: 15 20 size (mm) Fig. 3. Cerasioderm
Page 102 and 103: 10 size (mm) SIPHON SIZE AND DEPTH
Page 104 and 105: o, 7 01 5 | * CL 5- SIPHON SIZE AND
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
Page 112 and 113: * ' 1 /-/ "».-^*«C
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.
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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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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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