STUDIES OF THE FOODS AND FEEDING ECOLOGY OF WADING BIRDS Thesis submitted in part fulfilment of the requirements for the degree of Doctor of Philosophy of the Council for National Academic Awards .. by Malcolm E. Greenhalgh, B. A. Department of Biology Liverpool Polytechnic October 1975 s ABSTRACT In this thesis are described the populations of waders (Aves: Charadrii) occurring on the Ribble Estuary, Lancashire, special reference being made to the eleven species comprising the bulk of the shore wader population. The daily routine of these birds is described including the time spent in feeding. The feeding areas are described together with the foods taken from gut and pellet analysis and direct observation. The distributions of invertebrates, and especially those of major importance as wader food, are described as well as the factors affecting these distributions. Variations in density of prey in relation to O. D., general geography of the estuary, and time of year are included. Depth distribution and variations in prey size are outlined for the main species. Food intake was studied in the eight main waders. Daily intake through the year is described. in relation to energy requirements. Variations of feeding rates with several factors are included. All data are combined to enable calculation of the total biomasses of the main prey taken by waders in the course of a year. These are compared with total minimum annual production of the prey. Future work, including a computer study based on these and extra data, is outlined. Frontispiece a. The author counting a flock of 45,000 Knot 25 August, 1972 b. C rossens roost from the air 12 October, 1973 Bar-tailed Godwit top left Knot left middle Oystercatcher centre to right middle Dunlin centre to bottom right ACKNOWLEDGEMENTS Firstly, I must thank Dr. W. G. Hale who has supervised this work and given me, not only the use of the facilities in his Department, but also much encouragement and advice. Dr. P. H. Smith also gave much encouragement and also provided many data on wader numbers; for these I am grateful. I am also indebted to the following who accompanied me in the field in the course of this work: E. Coen, P. A. Greenhalgh, M. Greenwood and D. B. Collinge and many other members of King George V School, Southport. My thanks also go to the Headmaster and H. C. Davies of King George V School for encouragement and help, to Mrs. M. Contal for typing the Thesis and to Dr. P. J. Dare who, over ten years ago, sparked off my study on the Ribble and Morecambe Bay waders. Finally, this study would not have been possible without the encouragement of my family and especially my wife. She has tolerated my absence from home for days on end and, over the past ten months, has put up with the bulk of books and papers necessary to the writing of this thesis. CONTENTS Page INTRODUCTION 1 I THE STUDY AREA II WADER POPULATIONS Introduction 7 Methods 7 Results 15 Oystercatchers 15 Ringed Plover 21 Curlew 23 Black-tailed Godwit 25 Bar-tailed Godwit 29 Redshank 31 Turnstone 32 Knot 39 Sanderling 41 Dunlin 41 Discussion 41 III THE DIURNAL CYCLE OF WADERS 47 Methods 48 Results 1. High water roosts 49 2. Activities on the intertidal zone 59 3. Night feeding 61 Conclusions 64 Discussion 65 Page IV THE FEEDING AREAS Introduction 67 Results 68 Discussion 85 V THE FOOD OF WADERS Introduction 90 Methods 91 Results 94 VI FACTORS AFFECTING THE DISTRIBUTION OF PREY SPECIES Sediment size distribution 119 Water content of surface substrate 120 Loss on ignition 122 Salinity of water 122 Tide coverage of substrate 127 Temperature of substrate 129 Conclusion 129 VII THE DISTRIBUTION OF PREY SPECIES IN THE INTERTIDAL ZONE Introduction 130 Methods 133 Invertebrate size and biomass 134 Results 134 Polychaeta 146 Crustacea 150 Mollusca 154 Vertical distribution 159 Page Results 162 Seasonal variation in density 173 Biomass of four major prey and 175 calorific values VIII THE FOOD INTAKE OF WADERS Introduction 180 Methods 181 Results 183 Oystercatcher 183 Curlew 1914- Black-tailed Godwit 197 Bar-tailed Godwit 207 Redshank 205 Knot 212 Sanderling 219 Dunlin 225 IX IMPACT OF WADERS ON THE FOOD RESOURCES OF THE RIBBLE ESTUARY Preliminary intake 231 Effect of predation on invertebrate 236 populations Effects of season on wader predation 239 The development of a computer programme 241 X DISCUSSION 245 SUMMARY 251 REFERENCES 257 APPENDIX 263 INTIDDUCTION During the past decade several plans have been published or discussed which would result in the development of several major British estuaries. Some of these will possibly have been completed during the 1980 's if government and industry ignore the arguments put forward by conservationists and, as natural estuaries, these areas will be no more. There is a real possibility that many British estuaries, now in more or less a natural state, will be developed into major industrial complexes similar to the Rhine area of the Netherlands. Foremost amongst these, at present (May 1974) are the Maplin Sands, off Foulness Island (North Thames) where it is proposed to build London's third airport, a seaport, several road and rail links with existing transport systems and a city in South East Essex to house the half million people who would be employed by the two Maplin projects and also the industries which Maplin would attract. Elsewhere on the coasts of England and Wales the following projects have been proposed which would have similar effects on estuarine environments: 1. The Wash some form of barrage to form a reservoir for water storeage, 2. Medway and Swale a large industrial complex, 3. Humber a terminal for receiving North Sea gas and oil, and possibly an oil refinery, 4. Dee (Cheshire - a barrage for a reservoir including an Flint shi!'e ) extensive water-sports area, 5. Morecambe Bay plans, now suspended, for a reservoir with possibly a motorway across the barrage to improve communication between Furness and the main motorway network and hence attract industries to settle on the Furness coast (including N.W. Morecambe Bay), 6. Duddon a reservoir, 7. Solway Firth a reservoir, with separate schemes for a marina-water sports area. - 1 - Besides these the following estuaries are to a great extent affected by industry and are likely to be affected in a greater way in the future: Tees, Tyne, Wear, the estuaries entering Southampton Water, the estuaries of South Wales and the Mersey. In Scotland there is not the pressure that exists in England though many of the Firths, espeoially those on the east coast (e.g. Moray and Dornook) are likely to become increasingly affected as a result of the exploitation of offshore fuel deposits. Thus, few large estuaries are not scheduled for development, and one such is the Ribble Estuary, Lancashire. Estuaries have long been known as important areas for birds, especially non-breeding flooks of wildfowl (Anseriformes) and waders (Charad~iformes). The former order has been well studied in view of its interest to wildfowlers - in Britain there is the review of Atkinson-Wil14s (1963) and a series of booklets dealing with individual estuaries is being prepared by the Wildfowlers' Association of Great Britain and Ireland dealing with population~ whilst teams at the Wildfowl Trust and Royal SOCiety for the Proteotion of Birds have been investigating wildfowl feeding ecology. Until reoently however, the wader situation was poorly known. Rooth (1967) for instance stated that "in Britain it is particularly the Wash ••••••• and surrounding area with 150,000 waders that is important; also such areas as the Norfolk Broads ••••••• and the North Norfolk Marshes ••••••• " (l) The chief exception to this was the Fishery Section of the M.A.F.F. studies of the Oystercatcher Haematopus ostralegus dealing with population (Dare 1966), and feeding eoology (e.g. Drinnan 1957, Davidson 1967). It was beoause of this dearth of information and the inoreasing pressures on estuaries that it was felt that regular censuses ought to be oarried out of waders over the major estuaries and these started in the mid-1960's with the Ribble and Moreoambe B~ whilst in 1969 other estuaries were incorporated under the banner of the Birds of Estuaries Enquiry of the British Trust for Ornithology. At the same time studies were begun on the faotors whioh attract waders to estuaries. Out of the breeding season the main faotor is food. - 2 - It has been attempted, therefore, in this study to examine potential wader-food, species, populations, wader populations and the amount of food that these wader populations consume. From these it was hoped to gain an insight into whether the Ribble could hold more waders than it already does, and possibly whether it could absorb waders displaced from elsewhere. Furthermore, although no two estuaries are completely alike and thus wader feeding ecology is likely to vary to some extent from area to area, it is also hoped that the data presented here may supplement similar studies now in progress in the Tees Estuary and Lindisfarne areas (Durham University), ythan Estuary (Aberdeen University), Wash and Foulness (N.E.R.C.) and other projects. From such projects a clearer understanding is being obtained of the predator­ prey interactions in the wader-invertebrate food chains in the estuary ecosystem •. - 3 - The statistical tests of Bailey (1959) were mostly used in this study though reference was also made to Elliott (1971). Throughout the text means are given followed by standard error wherever possible. Graphs involving the plotting means also include one standard error either side of the mean by a vertical bar corresponding to the standard error.