waders and their estuarine food supplies - Vlaams Instituut voor de ...

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slowly-walking bird to feed on them without the need io make dashes or to wait for emerging prey. However, since Corophium only return to the surface 5 lo 10 minutes after a disturbance, birds feeding at high densities may severely depress the accessible fraction of the prey (Goss-Custard 1970b. 1976). When disturbed. Nereis also retreat down their burrows, where they are safe from waders with short bills. Curlews use two methods for preying on Nereis. Since their bill is long enough for them to extract worms from their burrows in summer. Curlews may ihen search for burrows and probe deeply. However, when many worms feed at the surface, the Curlews concentrate their feeding effort entirely on these eas) prey (Zwarts & Esselink 1989). The versatility in feeding techniques of both the worms and predatoi puts precise measurement of the accessible fraction of the prey beyond present capabilities. This is also true for plovers which peck at either an outflow of water from the hole when the worm is near the surface, or wait until it emerges from its burrow (Pienkowski 1983a. b. Metcalfe 1985). The feeding activity of the benthos, and therefore the proportion that is accessible on the surface, can change considerably within a very short time, even within minutes. Filter-feeding bivalves, whose valves are firmly closed at low tide, start feeding as soon as incoming water covets the surface (Vader 1964). This may allow Oystercatchers to stab the bill between the gaping valves of Mytilus and Cerastoderma and so take them in a fast rate (Zwarts & Drent 1981. Swennen et al. 1983). Corophium are very accessible to waders when they leave their burrows for a short period on the receding tide (Linke 1939. Vader 1964. Hicklin & Smith 1984, Boates & Smith 1989): this may explain the tendency for waders feeding on Corophium to follow the tide edge. In contrast, waders feeding on surface-feeding Nereis have no reason to follow the tide line. This worm remains in its burrow as long as food can be filtered from the overlying water but. at low tide, they emerge from their burrows to feed on the surface (Esselink & Zwarts 1989). This may explain w h\ () v sieic .ik hei s that Iced on Nereis at low tide take alternative prey as the tide ebbs (de Vlas et al. 1996) and Curlews vary their feeding method over the low water period (Zwarts & Esselink 1989). Il mav also explain why plovers are able to remain on the high- FOOD SUPPLY HARVESTABLE BY WADERS 66 level shores throughout the low water period, rather than move to lower levels with the receding tide edge. The main conclusion of this section is that the accessible fraction varies enormously, often by more than the variation in the total biomass. Il is also clear that the variation in accessibility differs between prey species, being relatively low in the more or less sessile bivalves 'Cerastoderma, Mya and Mytilus). larger in bivalves with a seasonal variation in burying depth (Macoma and Scrobicularia). and very large in invertebrates thai emerge from their burrows to defecate (Arenicola) or Io feed (Nereis. Corophium). The detectable prey fraction Waders may probe at random to locate buried prey which live within reach of the bill, as Oystercatchers are known to do for Cerastoderma (Hulscher 1976) and Macoma (Hulscher 1982). In randomly probing waders we can calculate the exact encounter rate with benthic prey, provided the surface areas of the shells and bill tip are known, along with the probing depth of the birds and the depths at which the bivalves live (Hulscher 1976. 1982. Wanink & Zwarts 1985. Mourilsen & Jensen 1992. Zwarts & Blomert 1992). Dunlin Calidris alpina and Sanderling can detect buried prey by taste (van Hee/.ik et al. 1983). thus enlarging the detection area of each probe. In contrast. Oystercatchers probably do not use taste perception since their encounter rate wilh experimental prey could be predicted precisely by touch alone (Hulscher 1982. Wanink & Zwarts 1985). Waders may also search for Hacks that betray the presence of pre) beneath the surface and concentrate their probing in such places. Thus an Oystercatcher look more time to locate Macoma when all Ihe surface clues on the mud surface had been erased experimentally (Hulscher 1982). The burrow entrance of Nereis is clearlv v tsible on the mud surface when they filter water through their burrow or after they have fed on the substrate and so left starlike feeding tracks around their burrow. Curlews searching for Nereis do not probe at random, but look systematically for these small tracks. Siphon holes of Mya can be very conspicuous, especially in muddy substrate (Linke 1939: photo 50 to 52). Curlews probably know the size of Mya before they probe, since there is good correlation between siphon diameter and shell size
(Zwarts & Wanink 1989). In quiet weather. Curlews walk from one siphon hole to the next but when waves have eroded the upper layer of the substrate, no siphon holes are visible and Curlews feed by touch by continuousl) making swift pecks at the surface. The detectabihty ol pre) affects the availability of prey to a bird in other ways. Prey must be detected within the visual range. Arenicola defecating within 2 m of a Curlew are accessible, but not detectable if the cast is produced behind the bird. Metcalfe (1985) found that Corophium was taken at a smaller distance by Lapwing Vanellus vanellus than Nereis and concluded that large prey may be delected at a greater distance than small and more cryptic prey. Though likely, there are two alternative explanations. First. Corophium may have been less accessible than Nereis, because it retreated more quickly into their burrows than Nereis. Second, the Corophium were smaller and less profitable than Nereis and so |x'rhaps less worthwhile walking any distance io attack. Waders may be able to increase their intake rate In increasing their rate of walking and thus, of encountering the prey. But. in doing so. more prey nia> be overlooked, and an opiiinal search rate would be a compromise depending on the crypticity of the prey (Gendron 1986). If so. waders would be expected to walk faster when they feed on easily detectable prey than when they search for cryptic prey or prey whose presence is only revealed by surface tracks. Redshank Tringa totanus do walk relatively fast when they Iced on prey, such as Corophium. which appear to be easily detectable at the surface and search more slowly for the less visible surface tracks of Nereis (Goss-Custard 1977a. 1977b). Search rate may also vary within one type of a prey, depending on its detectabihty as four studies have shown. First. Seniipalinated Sandpipers Calidris pusilla walk faster when they feed on Corophium crawling on the substrate on the ebbing tide than later al low water when mosl prey are found inside their burrows (Boates & Smith 1989). Second. Curlew s feeding on Mya walk iw ice as fast when they walk from one siphon hole to the next as when they search for the same prey by continuous probing (unpubl. observations). Third. Oystercatchers thai stab into Mytilus walk more slow |j than those that hammer a hole in the shell, perhaps because the cues used by slabbers (i.e. slightly gaping bivalves) are particularly FOOD SUPPLY HARVESTABLE BY WADERS 67 difficult to spot (Cayford & Goss-Custard 1990). Fourth. Curlews selecting Nereis adjust their search rate according to the number of conspicuous worms thev lake Irom the surface and cryptic ones they lift from the burrow (Zwarts & Esselink 1989). Although direct measurements of the conspicuousness and cryp licity of these prey are needed to avoid circular argument, these results do seen) to be consistent with the prediction that predators move faster when relatively more prey are conspicuous (Gendron 1986). Nonetheless, the main conclusion of this section must be thai the detectabihty of prey remains extremely difficult to measure, especially in waders hunting by sight. For such waders, the rate of encounter with prev needs to be measured as a function of prey size and the width of the search path needs to be determined as has recently been described for sparrows (Getty & Pulliam 1993). The ingestible and digestible prey fraction Curlews eating large Mya take some minutes to pull up the siphon and consume the flesh piecemeal from the shell (Zwarts & Wanink 1984). Curlews also dismember large Carcinus and eat the pincers and legs Separately from the carapace. In this way, some waders overcome the limit set by the width of their gape to the si/e of a food item that can be ingested. However, many prey cannot be broken into pieces, or it takes (oo long to do so. and must be swallowed whole. Thus. Knot eating whole bivalves and snails cannot swallow prey with a circumference exceeding 3 cm. This limit can be exactl) quantified from the relationship between shell circumference and length (Zwarts & Blomert 1992). The round shape of Cerastoderma means that Knot cannot eat shells greater than 12 mm long, but they can eat longer bivalves that are more slender. Thai is w hy Knot are able to ingest Macoma up to 16 mm long, Scrobicularia up to 17 mm long. Mya up to 19 mm long and Mytilus up to 21 mm long. Hence, because of this size limit set by the gape, a large part of the bivalve biomass preseni is not mgestibleby Knot. Other factors may limit prey size. Thus Purple Sandpipers Calidris maritima take hard-shelled prey but. as suggested by Summers et al. (1990). they may reject some large prey that are probably small enough to be swallowed, but too strong to be crushed in the gizzard. The average weight, and so strength, of the
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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
Page 19 and 20: 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
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Page 73 and 74: general law relating handling time
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
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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
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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
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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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