Master thesis: Foraging behaviour of the Eurasian oystercatcher in a challenging breeding site Isabelle Grieveson 6th September 2018 Supervised by: Prof. Dr. Stefan Garthe and Dr. Philipp Schwemmer Declaration of Authorship I, Isabelle Grieveson, declare that this thesis and the work presented in it are my own and has been generated by me as the result of my own original research. Title of thesis: Foraging behaviour of the Eurasian oystercatcher in a challenging breeding site I confirm that: 1. This work was done wholly or mainly while in candidature for a research degree at this University; 2. Where any part of this thesis has previously been submitted for a degree or any other qualification at this University or any other institution, this has been clearly stated; 3. Where I have consulted the published work of others, this is always clearly attributed; 4. Where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work; 5. I have acknowledged all main sources of help; 6. Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself; 7. None of this work has been published before submission; 8. The printed and electronic versions of this document are identical. Signed: Date: I ABSTRACT The Eurasian oystercatcher (Haematopus ostralegus) population has declined dramatically in the Wadden Sea over the past 20 years. This top predator is an important indicator for the health of the ecosystem as a whole, but the causes behind this dramatic decline are not yet clear. It is thought that low breeding success and food availability may play a role, with the breeding season being a key period where birds must balance their time between foraging trips and defending their breeding territories from rivals or predators. Whilst previous studies have investigated this balance on island-breeding birds, here we aim to discover how this balance is shifted in individuals nesting in a more challenging breeding site. Oystercatchers nesting on the mouth of the river Elbe estuary on the mainland Kaiser-Wilhelm-Koog are subjected to increased levels of predation from nocturnal mammals that are not present on islands, together with an increasing frequency of storm floods that destroy nests. In this study global positioning system (GPS) data loggers were used to investigate oystercatcher foraging behaviour under these conditions, in particular the differences in foraging trip duration and distance during day and night time periods, and during the different tidal stages. Data on the abundance of benthic prey organisms available to the oystercatchers was collected at foraging sites identified using the GPS data, and at random sites on the tidal flats. Visual surveys were carried out in order to determine the hatching success of the oystercatchers in this area. Hatching success was very low, with only 3 chicks observed over the whole area. Although the birds were hypothesized to spend longer periods on the nest at night in order to defend their clutches from nocturnal predation, in fact similar behaviour to individuals breeding on islands was exhibited: foraging trips were of longer duration during the night than during the day. However, in contrast to previous studies on island breeding birds, oystercatchers at Kaiser-Wilhelm-Koog tended to have a relatively high foraging site fidelity and did not travel further when foraging at low tide periods. In addition, unlike several previous studies in different locations, the birds at Kaiser-Wilhelm-Koog did not target areas of high prey density. The most likely reason for this was the uniformly low level of benthic prey abundance across the study site, with an average biomass of only 3.34 gm-2 and with several key prey species of the oystercatcher absent entirely. This low-quality breeding site appears to result in significant changes in the foraging behaviour of the Eurasian oystercatcher, however more studies are suggested in order to untangle the multiple factors of low prey availability, high predation and flooding that may be causing these behavioural differences. II Table of Contents 1. Introduction .................................................................................................................................... 1 1.1 Population decline of the Eurasian oystercatcher ................................................................. 1 1.2 Balancing clutch defence with successful foraging ................................................................. 2 1.3 Kaiser-Wilhelm-Koog as a challenging breeding site .................................................................... 2 1.3.1 Mammalian predators ........................................................................................................... 2 1.3.2 Storm floods ........................................................................................................................... 3 1.4. Project Aims ............................................................................................................................ 4 2. Materials and Methods ................................................................................................................... 5 2.1 Study Site ................................................................................................................................ 5 2.2 Hatching success ..................................................................................................................... 5 2.3 Oystercatcher movement data using GPS loggers .................................................................. 6 2.4 Prey distribution...................................................................................................................... 7 2.5 Statistical analyses .................................................................................................................. 7 3. Results ................................................................................................................................................. 8 3.1 Foraging Behaviour ....................................................................................................................... 8 3.2 Benthic prey ................................................................................................................................ 12 3.2 Hatching success ......................................................................................................................... 16 4. Discussion ...................................................................................................................................... 17 4.1 Foraging behaviour ..................................................................................................................... 17 4.1.1 Tidal ...................................................................................................................................... 17 4.1.2 Site fidelity ........................................................................................................................... 17 4.1.3 Day-night .............................................................................................................................. 17 4.2 Benthos ....................................................................................................................................... 18 4.2.1 Prey density targeting .......................................................................................................... 18 4.2.2 Low prey density .................................................................................................................. 18 4.2.3 Prey composition ................................................................................................................. 20 4.3 Hatching success ......................................................................................................................... 20 4.4 Study limitations and future studies ........................................................................................... 21 5. Conclusion ......................................................................................................................................... 22 6. Acknowledgements ........................................................................................................................... 23 7. References ........................................................................................................................................ 23 8. Appendix ........................................................................................................................................... 30 III 1. Introduction 1.1 Population decline of the Eurasian oystercatcher The Eurasian oystercatcher (Haematopus ostralegus) is a wading bird with a large range across Europe and Asia. The European population is estimated to make up approximately half of the global population, with 568,000-708,000 individuals (Birdlife International 2015; Wetlands International 2012). In turn, the Wadden Sea supports 50% of the East-Atlantic Flyway oystercatcher population(Reineking and Südbeck 2007), and therefore the health of this population is not only of particular conservation concern, but also acts as an important ecological indicator for the quality of the Wadden Sea ecosystem (Reineking and Südbeck 2007). There has been a declining trend in the population of Oystercatchers