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ate of 1 prey min '. There are. however, two prey that weigh less. Both are taken at a high rate for Curlews: siphons of Scrobicularia i5 Io 10 mg) up to 9 min ' and juvenile Carcinus (40 mg) up to 7.4 prey min ' (Zwarts unpubl.). However. Oystercatchers and Curlews feeding solely on such small prey achieve a relatively low intake rate, so these prey are usually ignored or taken w In le searching for larger pre). The lower prey size acceptance rule may be tested by comparing its predictions with available measurements on both diet and si/e selection. By way of illustration, the following analysis considers waders feeding on one small prey species. Corophium. Fig. 13 shows the relation between size and weight in Corophium (from Table I), and the predicted lower acceptance threshold for several wader species. The lour lower panels give the observed size selection. expressed as 'index of selectivity' to correct for the varying frequency distribution of si/e classes on offer. As predicted, small waders, such as the Least Sandpiper Calidris minitiilla (19 g: bill length 18.5 mm) and Semipalmated Sandpiper (21 g: bill length 19.4 mm) took small Corophium (Gratto et al. 1984); (Corophium > 4 mm are due to their size probably uniugestible and/or unprofitable for these small waders). Redshank (110 g) was predicted to take prey larger than 5 mm, precisely as found by Goss-Custard (1969. 1977a: appendix b). Although Corophium is not a major prey of Bar-tailed Godwits (250 g). they do select the rare specimens larger than 7 mm (Zwarts unpubl.), which is also in line with expectation. A further prediction is that waders which are heavier than Bar-tailed Godwit should ignore Corophium altogether. In line wilh this, it has never been found in the diet of any of the larger species. Black-headed Gulls are as heavy as Bar-tailed Godvviis and eat Corophium (Curtis el al. 1985). They have a short bill in comparison to waders with a similar body weight and handle these prey quickly (Fig. 12); unfortunately no data are available on the size selection. Corophiitni is also one of the main prey of voting Avocets Recurvirostra avosetta. However, when they pass a body weight of 100 g they svi itch to ihe more profitable Nereis (Engelmoer & Blomert 1985) as would also be predicted. Adult Avocets (320 g) do not take Corophium either (Engelmoer & Blomert 1985). Were Shelduck Tadoma tadorna FOOD SUPPLY HARVESTABLE BY WADERS 72 (I l(K) g) only to pick up prey in ihe same way as waders, ihey would not be able to survive on a diet of such small prey as Comphium and Hydrobia. However. Shelduck are able to sieve mud through the bill lamellae and filter prey from the upper layer of the mud at the high rate of up to 3 prey s ' (Buxton & Young 19811, much faster than waders. Waders thai lake Corophium from the surface handle a prey in less than I s. Bui if waders have to take the same prey from beneath the mud surface, the handling time will be longer, due lo the time needed to probe the bill into the mud and to extract the prey. Moreover, it is likely that mud would stick to such prey, requiring the waders io spend additional time in shaking or washing the prey before it is ingested. The profitability rule may therefore explain why waders only peck Corophium from ihe surface and do not probe for them, though they are accessible to most wader species in burrows 3 to 5 cm deep (Meadows 1964. Jensen & Kristensen 1990). If it is assumed that the handling time is twice as long for a Redshank probing for Corophium rather than pecking them from ihe surface, only prey > 6.5 mm would still be profitable, representing a considerable reduction in the accessible prey biomass. In this way. prey thai are accessible may nonetheless be safe from predation if Ihey make themselves unprofitable to their predator. This is also probably one of the reasons why Hydrobia bury themselves just below the surface when not actively grazing at ihe surface (Vader 1964. Little & Nix 1976, Dugan 1981. Barnes 1986, Mouritsen & Jensen 1992). For the same reason Sanderlings. Curlews and Oystercatchers more often take shallow prey than just barely accessible prey. However, the risk of prey being taken is not solely a function of their depth, but also depends on their own density (Wanink & Zwarts 1985): an Oystercatcher offered a high density of Scrobicularia. became more selective and only consumed prey living in the upper 3 cm of the substrate. This is because the profitability of the prey decreased with depth due to the increase of the handling time with depth. The deep-living, less profitable prey were ignored at the higher prey density when the search time per prey decreased and the overall intake rate could be increased by concentrating on the shallow prey. This again shows that prey that under certain condition are known to be accessible can be ignored
4 5 6 7 8 9 size class of Corophium (mm) Kiij. 13. The relation between weight and size in Complinim voltilaior iironi Table I) and the observed size selection by four wader species The horizontal lines in the upper graph indicate the expected lower weight threshold according to equation £3)( see levi | and the vertical lines show the corresponding expected lower si/e threshold. The lower panels Live the measured size selection in Semipalmaled and Least Sandpiper idratto el al. \ l )H4). Redshank I iuss-Custard 1977K averaged for two sites) and Bar-tailed Godwit i/.wans unpubl.I. The index of selectivity, is obtained by dividing lass the numbers of prey selected by the numbers on oiler. set ihe maximum ratio to 100 and express all ratios relative to this maximum. FOOD SUPPLY HARVESTABLE BY WADERS 73 because their depth renders them unprofitable. The variable depth selection of Oystercatchers also makes clear the point that the lower acceptance threshold nuisi not be regarded as a fixed constant. By definition, the lower prey size acceptance threshold varies according to the intake rale during feeding Birds add less profitable prey lo ihe diet as their intake rale goes down. For example, as prey density declines. Oystercatchers accept all the Cerastoderma encountered and not only the open ones thai can be handled quickly (Hulscher 1976). Redshank and Oystercatchers include the smaller size classes in their diet as prey density declines (Goss-Custard 1977b. 1977c. Zwarts & Drent 1981) and. as mentioned, Oystercatchers eat the less profitable Scrobicularia lying at a greater depth. The profitability of prey may vary systematically between individual birds. Hulscher 11982) showed that Oystercatchers with blunt bills need more time to handle Macoma than birds with pointed bills. Similarly. Sutherland & Ens (1987) found that an Oystercatcher with a chisel-shaped bill was faster at slabbing Mytilus Uian a bird with a blunt bill, while ii was other wav around when (lie birds opened a Mussel by hammering. Again. Swennen et al. (1983) found individual differences in the time taken to handle Cerastoderma as did Wanink & Zwarts (1996) and Hulscher et al. (1996) for Oystercatchers eating Scrobicularia. Mya and Macoma. respectively. Individual differences in the handling efficiency would be expected to cause a variation in the lower acceptance level of size classes selected, but this possibility has still to be explored in the wild. In conclusion, smaller prey are ignored by waders since they are unprofitable: large waders usually take prey of at least 10 to 20 mg, but usually larger, and waders weighing less than l(K) g select prey of about 1 mg. Furthermore, prey that are not taken and so seem at first sight not to be available, may actually be detected but ignored because they are unprofitable. The harvestable prey fraction The information given in the former four sections, enables us to define the harvestable prey fraction. This will be illustrated in three examples. Fig. 14A shows the fraction of a benthic bivalve harvestable for Oystercatchers. Oystercatchers mav locate their prey by
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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
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 and 72: (Zwarts & Wanink 1989). In quiet we
Page 73 and 74: general law relating handling time
Page 75: nearly twice as much time (0.79 s).
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.
Page 122 and 123: siphon would not have to reach the
Page 124 and 125: 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
Page 367 and 368:
BreyT. 1989. Der Einfluss physikali
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latie tol chironoiiiiilen en de wai
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huch der Vogel Milteleuropas, Band
Page 373 and 374:
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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