Stone Curlews Burhinus Oedicnemus and Human Disturbance: Effects on Behaviour, Distribution and Breeding Success
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Stone curlews Burhinus oedicnemus and human disturbance: effects on behaviour, distribution and breeding success Elisabeth C. Taylor Darwin College University of Cambridge September 2006 Thesis submitted for the degree of Doctor of Philosophy at the University of Cambridge ii Author’s declaration This dissertation is the result of my own work and includes nothing which is the outcome of work done in collaboration except where specifically indicated in the text. Elisabeth Taylor September 2006 iii Summary Previous studies have found that disturbance of birds by human activities can result in changes in behaviour, resource use and productivity. Changes in rights of public access through the Countryside and Rights of Way Act 2000 mean that much of the preferred open habitat of the stone curlew Burhinus oedicnemus in southern England is now designated as ‘open access’ for recreation. Consequently, there is concern that increased human activities near stone curlew breeding sites could impact the population. Systematic recording of behaviour showed that stone curlews respond to potential disturbance agents (PDAs) at large distances compared to many other birds. Responses varied with PDA type, time in the season, distance to PDA, and other factors. Logistic models were created that predict the probability of an active response (running or flying in response to a PDA event) being observed, in relation to attributes of the PDA. The models were then used to estimate the probability of response to any given specified PDA event. From this, the rate of responses per hour to an observed set of PDA events could be calculated. The main effect of disturbance on stone curlews in this study area was prevention of birds settling to breed in habitat that is suitable in other respects. Models predict that where there is no disturbance, the probability of site occupancy is 60%. However, with one expected active response per hour this is reduced to a 15% probability of site occupancy. Colour ringed pairs were observed moving from apparently suitable nesting sites where disturbance event rate exceeded 1.5 disturbance events per hour or 0.4 expected active responses per hour, and they selected a less disturbed site to breed. The average distance moved was 1.5km, although an extreme distance of 9km was observed where there were few other alternative sites. First egg date of pairs that moved was later by nine days and two pairs failed to breed that season. There was a suggestion that chicks grow slower on sites with disturbance but this had no impact on chick survival in the longer term. There were no other clear effects of disturbance on breeding success for the whole study area over two years of data collection. However, as stone curlews were already selecting the least disturbed sites to breed, variation in disturbance on breeding sites was low and any effects may not have been apparent. In order to guide conservation and habitat management whilst allowing recreational access, a management tool was developed from the behaviour and site occupancy models. The tool uses the models to calculate the reduction in probability of site occupancy under different disturbance intensities and offers a method to assess the effects of scenarios for future changes in disturbance type, routes and frequency. It is hoped that the tool will be valuable in making informed decisions about access and habitat creation to maximise stone curlew conservation. iv Contents Page Summary iii Acknowledgements v Chapter 1 General introduction and aims of thesis 1 Chapter 2 Potential causes of disturbance to stone curlews and factors 13 affecting their frequency of occurrence Chapter 3 The behavioural response of stone curlews to disturbance by 48 human activities in the breeding season Chapter 4 Predicting the probability of a behavioural response by stone 76 curlews to disturbance by humans Chapter 5 Using models of behavioural responses to disturbance to 133 improve the effectiveness of disturbance data Chapter 6 Determining factors that affect plot occupancy by breeding 148 stone curlews and predicting the effect of disturbance on occupancy Chapter 7 Effects of human disturbance on the breeding success of stone 199 curlews in Wessex Chapter 8 Movements of stone curlew pairs between sites before they 228 settled to breed Chapter 9 General discussion and management implications 258 References 273 v Acknowledgements My biggest thanks must go my supervisor Rhys Green who has given me enormous support and encouragement throughout. Rhys has a super-human ability to think about things and ways to analyse stuff and it is possible that I drove him to the edge of his sanity on numerous occasions due to my slower (more normal?!) working brain and less advanced statistical knowledge. But in spite of this Rhys always seemed to have faith in what I was doing. I really can’t thank him enough for everything he has done throughout out the PhD and giving me the opportunity to work on the greatest birds in the world. The project was fully funded by the RSPB and EN and I must thank the following from those organisations for help and encouragement along the way: Rowena Langston, Andy Brown, Allan Drewitt, Robin Wynde, and David Gibbons. Thank you also to Ken Smith at the RSPB for having faith I would finish this and offering me a job to do afterwards! The RSPB also funded me in my 4 th year to work on SCARE for which I am grateful. Nigel Butcher and Colin Gooch at the RSPB spent a considerable amount of time and patience making nest loggers and cameras for me and also provided me with a load of other equipment. They were also surprisingly tolerant when my equipment didn’t come back in a completely perfect state. My advisors Mike Brooke, Andrew Balmford and Andrea Manica provided valuable support and comments through the PhD. Andrea Manica was also an enormous and invaluable help by writing the R programme to run the bootstraps in Chapter 5. Thank you to those who read all or most of the thesis, in addition to Rhys, for improving my English and offering good advice: Jorn Scharlemann, Michael MacDonald, Pat Taylor and Katie Carr. Thank you to current and past members of the Conservation Science Group for advice, support and making the third floor a great place to be: Rhys Green, Andrew Balmford, Rob Ewers, Val Kapos, Ana Rodrigues (for the biscuits!), Tom Charman, Ali Johnston, Mila Plavsic, Jorn Scharlemann, Helen Hendry, Lauren Coad, Ben Phalan, Malvika Onial, Geoff Vielgus, Katie Carr, Frank Larsen. Special thanks must go to my final year office mates Ali Johnston, Lauren Coad and Ben Phalan for creating a great space to work in and who seemed to accept I needed to spread onto all the desks and also the floor to do my work! Thank you particularly to the fellow 4th years for enormous support: Lauren Coad, Mila Plavsic and Tom Charman. In Wessex I must thank the RSPB Wessex office and stone curlew team: Phil Sheldrake, Mike Austin, Trace Williams, Tracy Rich and Toby Branston, Julia Gallagher and Nick Adams. vi Phil Sheldrake in particular was fantastic to work with and a good friend, giving massive support to me during my field seasons. The MoD and Defence Estates allowed me to work on Salisbury Plain Training Area. Thanks to them and in particular, Dominic Ash of Defence Estates was a great support and very encouraging of the work. Thank you also to all landowners who allowed me access to stone curlew plots on their land. Thank you also to friends who visited me in sunny Wiltshire (who may or may not have been influenced by the proximity to the pub) – both those who dragged me ‘out of the field’ and those who trundled around my study sites expressing genuine or polite excitement for stone curlews: Matthew Lindow, Tom Charman, Pat and Meg Taylor, Catherine Joiner, Jorn Scharlemann, Arjun Amar, Emma McCabe, Becky Hodgkinson, Gill Gardiner, Joni Ward, Dave Fairhurst and John Mallord. Anne Heath was an outstanding volunteer, dedicating vast amounts of her time to my project. Sometimes I felt she knew better than me what we should do! Apart from the work she did for me, she also fed me several times a week. Other willing and not so willing volunteers were Pat Taylor and our wonderful border collie Meg, and Catherine Joiner. Thanks to them all. A great deal of friends have helped to keep me sane over recent months but I must thank two in particular – Tom Charman and Katie Carr. Katie always seems to know the right things to say at the right times and she has been amazing over the past few months - thanks for everything and I’ll definitely be repaying the favour your turn round! Tom has been there from day one of the PhD, and has become an incredible friend. Tom also knew my project inside out and could probably have written my thesis himself if he hadn’t been busy writing his own! I can definitely say that the PhD experience would not have been half as much fun without Tom there going through the same extreme emotions! Finally, I must thank my family, some of whom seem slightly bewildered by my choice of career. But special thanks to them – Cathrine, Caitlin, Jimmy and Richard. Extra special thanks are due to my mum for a million different reasons (don’t worry mum, I’ll never do another one!). She has been amazing all the way through my extended education and supported me every step of the way. I’m dedicating the thesis to her and in memory of my dad who after passing on those birdy genes I’d like to think would be proud of this.