Repeated monitoring suggests shorebirds are disturbed consistently during winter at a globally important roost in tropical Australia

Roebuck Bay, in tropical Western Australia, is one of the most important non-breeding sites on the world’s most threatened shorebird migration route, the East Asian-Australasian Flyway. Important shorebird roosts along the bay’s northern shores are regularly visited by people and birds of prey, both of which may cause disturbance to shorebirds. We investigated this issue using a repeated, standardised citizen science monitoring program over two year-long study periods, fourteen years apart (2005–2006 and 2019–2020). This enabled us to quantify the number of alarm flights shorebirds made and estimate the amount of time they spent in flight, as proxies for measuring disturbance. We assessed factors that were associated with these metrics and identified the stimuli that led to alarm responses; the separation of our study periods provided valuable insight on changes over time. We recorded high numbers of alarm flights compared to studies elsewhere. These levels remained consistent, with no significant difference between the two study periods. However, they depended strongly on the location within the bay and the time of year: significantly more disturbance occurred during the austral winter, when there are fewer shorebirds in the bay (ca. 20,000 individuals comprising resident species and immature migrants; 100,000+ are present in summer, including breeding-age and juvenile migrants). Based on previous research, we conclude that disturbance in winter was likely to cause shorebirds to exceed critical energy thresholds on some beaches, and suggest this may drive previously described local movements away from the bay’s northern shores in winter. We found birds of prey the most common alarm stimuli (associated with 61% of alarm flights for which a stimulus was identified); a substantial number were also caused by people visiting the bay (20%), and a few were associated with aircraft (4%). The proportions of alarm flights attributed to different stimuli were highly consistent between the two time periods, but two raptor species significantly increased the rate at which they caused disturbance. We suggest measures to provide alternative roosting habitat and alter some visitor behaviours as possible courses of action to reduce the local impact of disturbance.