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

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spend time in opening them. In contrast, soft-bodied prey are ready to eat. Ihey are also buried and must be searched for. but they are an easy prey for Oystercatchers when ihey come to the surface to feed themselves (Ragworms Nereis) or to defecate (Lugworms Arenicola). The extent to which various prey species provide a staple food for the Oystercatcher varies dramatically with season. Inland fields are heavily exploited by breeding birds in spring and summer, but only used as a supplemental food resource by mosl Ovsiercatchers in winter (Heppleston 1971. Daan & Koene 1981. Goss-Custard & Durell 1984). On the tidal Hats, Macoma is laken in early spring, whereas Nereis predominates in late spring (Bunskoeke el al. 1996). Both prey are only locally important. In contrast as the papers reviewed below show. Mussels and Cockles are universallv important winter foods. The fact that Shore Crabs are only taken in summer is easily explained by their migration to deeper water in autumn (e.g. Beukema 1991). However, (he other prev occur year-round in ihe same habitat, with many individuals growing for scv - eral years. We lake as a working hypothesis that the seasonal changes in the utilization of these prey by the Oystercatcher population are primarily due to seasonal changes in harveslability of the prey. i.e. the prey fraction lhat is both accessible and profitable (Zwarts & Blomert 1991, Zwarts & Wanink 1993). By definition, harvestability is negatively related to the effectiveness of Ihe morphological and behavioural anti-predator defenses of the prey. Since each prey species has different adaptations to reduce predation risk, we may also expect differences between prey species in the time of year when the defenses are most effective. The intake rate is defined as mg s' feeding, thus ihe quotient of weight of the prey and feeding time. Feeding time consists ol two components: searching and prey handling. Hence the intake rate is the product of two ratios: prey weight/handling time x handling time/(search + handling time). The first ratio, the intake rate during prey handling, is called the profitability. The second ratio is the relative handling time, the proportion of the feeding lime during which the bird handles the prey. PREY PROFITABILITY AND INTAKE RATE 176 ll is obvious that the iniake rate will increase when the prey are large, arc handled in a short time, and/or when the search time between prey is limited. Several studies (Hulscher 1976. 1982, Wanink & Zwarts 1985. Habekotte 1987, Goss-Custard
catchers to take only certain prey from a mixture of different size classes and prev species. Methods Studies The data presented in this paper have been taken from 57 articles. 6 student reports. 4 unpublished theses, but also from unpublished data files of Anne-Marie Blomert. Klaus-Michael Exo, Kees Hulsnmn. Cor Smit and the live authors: all sources are listed in the appendix. The studies were performed in 20 areas (Fig. 1), of which ten are siiualed in Great Britain and Northern Ireland, six in the Netherlands and one in Denmark. France. Morocco and Mauritania. All studies were done in the field on free-living Oystercatchers. except those indicated as C in column 'Cap' of the appendix which refer to caged birds. Captive birds were either taken to the mudflats where they were allowed to Iced in temporary cages (Hulscher 1976. 1982. unpubl.) or BUR = Burry Inlet CON = Conway River EXE = Exe estuary FOR = Forth estuary FRI = grassland in Friesland MOR = Moreeambe Bay 0OS = Oosterschelde esluary PAE = Frisian coast neat Paesens SCH = Frisian island Schiermonnikoog SKA E Skallingen SKO = Skokholm SOM = Baie de Somme STR = Strangford Lough TEX = Frisian island Texel TRA = Traih Melynog VLI = Frisian island Vheland WAS • Wash Bay YTH = Ythan estuary PREY PROFITABILITY AND INTAKE RATE they were offered food on artificial mudflats (e.g. Swennen et al. 1989), Captive birds thus led in an almost natural situation, hut occasionally the food supply was manipulated either bv erasins* surface tracks that might reveal the presence of ihe prey (Hulscher 1982). or by implanting prey at different depths i Wanink & Zwarts 1985. 1996). Prey size Size classes taken were known because prey remnants could be collected, and/or the prey size was estimated when ihe birds held the prey in the bill. In the latter case, bill length or the size of ihe colour ring could be used as a ruler of known si/e. Calibration experiments showed lhat observers could estimate prey size this way rather consistently (Ens 1982, Goss-Custard et al. 1987, Boates & Goss-Custard 1989. Baser al. 1996b). Such estimates were usually accurate. In others, errors could be corrected. For instance, comparison of the size frequency distribution of remnants of fiddler crabs Uca tangeri taken by Oystercatchers (Ens unpubl.) and Fig. I. Map of the study areas in NW. Europe indicated hy three letter codes [too •Old* areas vi ere situated in Africa: ihe Bay olDakhla, Morocco (formerly Western Saharal and the Banc d'Arguin. M.HIM : 177
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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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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
Page 158 and 159: 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
Page 164 and 165: valves may vary by a factor of two
Page 166 and 167: optimal foraging model, the minimum
Page 168 and 169: their gut is full? Do they reduce t
Page 171: PREY PROFITABILITY AND INTAKE RATE
Page 175 and 176: licult error of estimate arose if p
Page 177 and 178: Counts of feeding and non-feeding b
Page 179 and 180: PREY PROFITABILITY AND INTAKE RATE
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
Page 195 and 196: winter. Similarly, handling time in
Page 197 and 198: 5.0 - f-4.0 S 3.0 in I 20 a 2 a | 1
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
Page 218 and 219: Wanink 1993). The energy content of
Page 220 and 221: (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
Page 371 and 372:
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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