Foraging behaviour of the Eurasian oystercatcher in a challenging breeding site.

Grieveson, Isabelle (2018) Foraging behaviour of the Eurasian oystercatcher in a challenging breeding site. Open Access (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 31 pp.

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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.

Document Type: Thesis (Master thesis)
Thesis Advisor: Garthe, Stefan and Schwemmer, Philipp
Subjects: Course of study: MSc Biological Oceanography
Research affiliation: Kiel University
Projects: STopP
Date Deposited: 11 Sep 2018 10:52
Last Modified: 11 Sep 2018 10:52
URI: http://oceanrep.geomar.de/id/eprint/44210

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