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Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates

Published online by Cambridge University Press:  24 July 2006

John D. Goss-Custard
Affiliation:
Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorchester DT2 8ZD, UK
Andrew D. West
Affiliation:
Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorchester DT2 8ZD, UK
Michael G. Yates
Affiliation:
Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorchester DT2 8ZD, UK
Richard W. G. Caldow
Affiliation:
Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorchester DT2 8ZD, UK
Richard A. Stillman
Affiliation:
Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorchester DT2 8ZD, UK
Louise Bardsley
Affiliation:
Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorchester DT2 8ZD, UK
Juan Castilla
Affiliation:
Departamento de Ecologia, Facultad de Ciencias Biologicas, Pontificia Universidad Catolica de Chile, Codigo Postal: 6513677, Santiago, Chile
Macarena Castro
Affiliation:
Departamento de Biologia, Facultad de Ciencias del Mar y Ambientals, E-11510 Puerto Real, Spain
Volker Dierschke
Affiliation:
Research and Technology Centre, University of Kiel, Hafentörn, D-25671 Büsum, Germany
Sarah. E. A. Le. V. dit Durell
Affiliation:
Centre for Ecology and Hydrology, Winfrith Technology Centre, Dorchester DT2 8ZD, UK
Goetz Eichhorn
Affiliation:
Zoological Laboratory, University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands
Bruno J. Ens
Affiliation:
Alterra, P.O. Box 167, NL-1790 AD Den Burg (Texel), The Netherlands
Klaus-Michael Exo
Affiliation:
Institut für Vogelforschung “Vogelwarte Helgoland”, An der Vogelwarte 21, D-26386 Wilhelmshaven, Germany
P. U. Udayangani-Fernando
Affiliation:
29 Askam Road, Bramley, Rotherham, South Yorkshire S66 3YR, UK
Peter N. Ferns
Affiliation:
School of Biosciences, Cardiff University, Cardiff CF10 3TL, UK
Philip A. R. Hockey
Affiliation:
DST/NRF Centre of Excellence at the Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7701, South Africa
Jennifer A. Gill
Affiliation:
Schools of Biological and Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
Ian Johnstone
Affiliation:
RSPB North Wales Office, Maes y Ffynnon, Penrhosgarnedd, Bangor LL57 2DW, UK
Bozena Kalejta-Summers
Affiliation:
7 Mill Crescent, N. Kessock, Inverness IV1 3XY, UK
Jose A. Masero
Affiliation:
Departamento de Biologia, Facultad de Ciencias del Mar y Ambientals, E-11510 Puerto Real, Spain
Francisco Moreira
Affiliation:
Centro de Ecologia Aplicada ‘Prof. Baeta Neves’, Instituto Superior de Agraonomia, Tapada da Ajuda, 1349-017 Lisboa, Portugal
Rajarathina Velu Nagarajan
Affiliation:
Department of Psychology, University of Exeter, Exeter EX4 4QG, UK Present address: PG and Research Department of Zoology and Wildlife Biology, AVC College (Autonomous), Mannampandal-609305, India.
Ian P. F. Owens
Affiliation:
Department of Biological Sciences, and NERC Centre for Population Biology, Imperial College London, Silwood Park, Ascot, Berkshire SL5 7PY, UK
Cristian Pacheco
Affiliation:
Departamento de Ecologia, Facultad de Ciencias Biologicas, Pontificia Universidad Catolica de Chile, Codigo Postal: 6513677, Santiago, Chile
Alejandro Perez-Hurtado
Affiliation:
Departamento de Biologia, Facultad de Ciencias del Mar y Ambientals, E-11510 Puerto Real, Spain
Danny Rogers
Affiliation:
Johnstone Centre, Charles Stuart University, PO Box 789, Albury NSW 2640, Australia
Gregor Scheiffarth
Affiliation:
Institut für Vogelforschung “Vogelwarte Helgoland”, An der Vogelwarte 21, D-26386 Wilhelmshaven, Germany
Humphrey Sitters
Affiliation:
Limosa, Old Ebford Lane, Ebford, Exeter EX3 0QR, UK
William J. Sutherland
Affiliation:
Schools of Biological and Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
Patrick Triplet
Affiliation:
SMACOPI, 1 place de l'Amiral Courbet, 80 100 Abbeville, France
Dave H. Worrall1
Affiliation:
Countryside Council for Wales, Haverfordwest, Pembrokeshire SA67 8TB, UK
Yuri Zharikov
Affiliation:
SOLS, University of Queensland, Brisbane, QLD 4072, Australia
Leo Zwarts
Affiliation:
RIZA, P.O. Box 17, Lelystad, The Netherlands
Richard A. Pettifor
Affiliation:
Institute of Zoology, Zoological Society of London, Regents Park, London NW1 4RY, UK
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Abstract

As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro-invertebrates. Intake rate is measured as the ash-free dry mass (AFDM) per second of active foraging; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type II (‘disc equation’) formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching.

A review of 30 functional responses showed that intake rate in free-living shorebirds varied independently of prey density over a wide range, with the asymptote being reached at very low prey densities (<150/m−2). Accordingly, most of the many studies of shorebird intake rate have probably been conducted at or near the asymptote of the functional response, suggesting that equations that predict intake rate should also predict the asymptote.

A multivariate analysis of 468 ‘spot’ estimates of intake rates from 26 shorebirds identified ten variables, representing prey and shorebird characteristics, that accounted for 81% of the variance in logarithm-transformed intake rate. But four-variables accounted for almost as much (77.3%), these being bird size, prey size, whether the bird was an oystercatcher Haematopus ostralegus eating mussels Mytilus edulis, or breeding. The four variable equation under-predicted, on average, the observed 30 estimates of the asymptote by 11.6%, but this discrepancy was reduced to 0.2% when two suspect estimates from one early study in the 1960s were removed. The equation therefore predicted the observed asymptote very successfully in 93% of cases.

We conclude that the asymptote can be reliably predicted from just four easily measured variables. Indeed, if the birds are not breeding and are not oystercatchers eating mussels, reliable predictions can be obtained using just two variables, bird and prey sizes. A multivariate analysis of 23 estimates of the half-asymptote constant suggested they were smaller when prey were small but greater when the birds were large, especially in oystercatchers. The resulting equation could be used to predict the half-asymptote constant, but its predictive power has yet to be tested.

As well as predicting the asymptote of the functional response, the equations will enable research workers engaged in many areas of shorebird ecology and behaviour to estimate intake rate without the need for conventional time-consuming field studies, including species for which it has not yet proved possible to measure intake rate in the field.

Type
Review Article
Copyright
2006 Cambridge Philosophical Society

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