BREEDING AND POPULATION ECOLOGY OF SOOTY TERNS ON ASCENSION ISLAND by B. JOHN HUGHES A thesis submitted to The University of Birmingham for the degree of DOCTOR OF PHILOSOPHY Centre for Ornithology School of Biosciences College of Life & Environmental Sciences The University of Birmingham December 2013 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. ABSTRACT Seabird breeding success and population size on many oceanic islands have declined in recent years as a result of predation by non-native mammals. On Ascension Island, South Atlantic the sooty tern Onychoprion fuscatus breeding population declined precipitously in size in the 1970s. Despite cat eradication in 2003 and a cull of predatory mynas, the tern population between 1990 and 2012 consistently contained 350,000 birds. Shortages of small fish within foraging range of Ascension driven to the sea surface by once more prevalent tuna rather than cat predation probably caused the 1970s decline. Abundance and size of rats increased in response to cat eradication with the impact of rats on the size of the seabird population similar to the combined impact of both cats and rats. Ascension sooty terns have adopted a life-history strategy of sub-annual breeding as a viable alternative to seasonal breeding. The population consistently bred every 289 days, as did most individual birds. Despite an increase in lifetime reproductive rate resulting from sub-annual breeding, I predict a slow but sure decline in the population size unless rats are controlled. Studies of sooty tern migratory ecology are required and the species’ current IUCN Red List category should be reviewed. ii This contribution to the study of tropical seabirds in the United Kingdom Overseas Territory of Ascension Island is dedicated to the citizen scientists in the Army Ornithological Society who gave freely of their time to collect the field data. iii ACKNOWLEDGEMENTS This thesis could not have been produced without the enthusiasm, energy and sheer hard work of more than 50 members of the Army Ornithological Society (AOS) and to them I owe a deep debt of gratitude. I am grateful for the tremendous support from my good friend Colin Wearn from the Royal Air Force Ornithological Society (RAFOS) for ringing some 25,000 sooty terns. I am deeply indebted to Andrew Bray, Roger Dickey, Tony Giles, Hilary Nash, David Vaughan and Mike Vincent who planned and organised field seasons on Ascension Island. My sincere thanks go to AOS members who helped to gather data in the field: Richard Agus, Richard Angove, Mike Battenbough, Billy Bowles, Lyn Camm, Tim Cowley, Tony Crowe, Chris Dickey, Tony Duroe, Tim Earl, David Earp, Beverley Fletcher, Dave Foley, Griff Griffin, Mike Hann, ‘H’ Harris, Andy Harrison, Bob Hayward, David James, Mike James, John Mc Inness, Peter Marsh, Chris Miles, Lynne Millard, Chris Mulligan, Mark Phillips, Rod Scott-Smith, John Self and Mark Winsloe. For valuable support during field seasons thanks goes to: Andrew Bray for rat indices; David Morrison and Gez Thompson for monitoring seabirds on the offshore stacks; Mark Varley for censuses of landbirds; Richard Seargent for monitoring frigatebird predation; Mark Easterbrook for censuses of white terns; Colin Holcombe for keeping the diary; Chris Etheridge for monitoring seabirds on the Letterbox peninsula; Rodney Walker for surveys of Mexican thorn; Barney Bainbridge, Andy Summers, John Tack and Will Williams for surveying the area of the tern colony; and John Walmsley for monitoring myna predation and recording levels of predation from feral cats. RAFOS support and photographs from Simon Croson, Andy Pickard and Martin Routledge are much appreciated. I am indebted to members of the Royal Navy Birdwatching Society (RNBWS) including Peter Carr and Martin Howells iv for assistance with ringing, and Mark Cutts and Tony Tindale for photographs. For commitment to ringing activities I am especially thankful to the ringing team led by Colin Wearn and ably supported by Roger Dickey. I am thankful to Dr Gale for ringing a cohort of 200 sooty tern chicks in 1975 and to Dave Boyle for ringing and re-trapping records in 2002. Special thanks go to Tony Giles for GIS analysis and survey work. I am very grateful to Dave and Lous Bell for teaching me the skills to monitor rat abundance and to the British Museum for identifying rat specimens. My research was unfunded but I am thankful to the Clare Miller and Sarah Sanders at the Royal Society for the Protection of Birds (RSPB) for airline tickets to Ascension. I am indebted to Patrícia Luciano Mancini and Ouattara Ramini for reporting re-traps. I am also extremely grateful to Helen Wilkinson for monitoring the survival rate of sooty tern chicks between AOS expeditions and to Robin Prytherch for maps of the tern colony. I thank Nathan Fowler, Derren Fox, Susanne Musick, Tara Pelembe née George, Jolene Sim, Drew Tetlow, Dane Wade, Richard White and Natasha Williams at the Ascension Island Conservation Office for their welcome to the island, transport on many occasions, e-mail facilities, photocopying, regular reports and help in the field. My research methods improved with professional assistance and advice from Professor Chris Feare and Drs Philip and Myrtle Ashmole, Phill Cassey, Alistair Dawson, Alison Johnson, Norman Ratcliffe, Ken Simmons and Jim Stephenson. I was incredibly fortunate to have the support of my supervisors Graham Martin and Jim Reynolds – thank you for giving me the opportunity to publish my research, for constructive criticism and encouragement. Lastly, thanks to my daughter Patricia for measuring eggs on Ascension, my loving wife and to my other children for their encouragement and IT skills. Without your support this thesis would not have been possible. v TABLE OF CONTENTS Chapter 1 GENERAL INTRODUCTION 1.1. Seabird breeding ecology on tropical Islands ............................................. 2 1.2. Seabirds in the United Kingdom Overseas Territories ................................ 3 1.3. Ascension Island ........................................................................................ 4 1.4. Services’ ornithological societies ..................................................................... 7 1.5. Historical research ........................................................................................... 8 1.6. Fauna of Ascension Island .......................................................................... 9 1.7. Sooty terns ................................................................................................ 13 1.8. Current knowledge ................................................................................... 14 1.9. Aims of the thesis ........................................................................................... 15 1.10. Thesis structure ....................................................................................... 17 Chapter 2 BREEDING PERIODICITY OF SOOTY TERNS ON ASCENSION ISLAND 2.1. ABSTRACT ............................................................................................... 19 2.2. INTRODUCTION ....................................................................................... 20 2.2.1. Patterns of seasonal breeding in animals ............................................... 20 2.2.2. Patterns in the breeding cycle of birds .................................................... 20 2.2.3. Patterns of periodicity in seabirds on tropical islands ............................. 22 2.2.4. Patterns of periodicity at population and individual level ........................ 24 2.2.5. Periodicity of study species .................................................................... 25 2.2.6. Sub-annual breeding hypotheses ........................................................... 27 2.3. MATERIALS AND METHODS .................................................................. 28 2.3.1. Breeding periodicity of the population .................................................... 28 2.3.2. Breeding periodicity in ENSO years ...................................................... 29 2.3.3. Breeding periodicity of individuals ......................................................... 30 2.3.4. Factors underlying sub-annual breeding ................................................ 31 2.3.5. Statistical analyses ................................................................................ 33 2.4. RESULTS ................................................................................................. 33 2.4.1. Breeding periodicity of the population .................................................... 33 2.4.2. Breeding periodicity in ENSO years ...................................................... 35 vi 2.4.3. Breeding periodicity of individuals ......................................................... 35 2.4.4. Factors underlying sub-annual breeding ................................................ 38 2.5. DISCUSSION ...........................................................................................