The first multi-site assessment of white-tailed tropicbird population status and breeding trends in Seychelles

Seychelles supports the greatest abundance of seabirds in the tropical Indian Ocean, but regional seabird population breeding success and trend data are not readily available despite data collection spanning many years at several locations.

Combining Monitoring Data from Multiple Sites to Assess Population Status and Trends of White tailed Tropicbirds Phaethon lepturus in the Seychelles

A new paper from the organisations of the Seychelles Seabird Group (SSG) and published in the journal Avian Conservation and Ecology, brings together these monitoring datasets from five key nesting sites of white-tailed tropicbirds (Phaethon lepturus) to present the first nation-wide assessment of the status and trends of breeding success.

Overall, mean annual breeding success was lowest (15%) for Aldabra Atoll and much higher (33–55%) in the inner islands (Aride, Cousine, Denis, and Cousin). In the inner islands the species shows signs of potential population declines despite having stable and comparatively high breeding success. This contrasts with Aldabra where nesting activity has been relatively stable, but a decline in breeding success is causing concern.

The study, led by April Burt, former Aldabra Science Coordinator and SSG Coordinator, shows the value of pooling data from multiple islands to better understand species conservation status. Burt says “when we can only see the data from one site it may not tell us much about what is happening to this species at a wider level, but by bringing together data from multiple sites, we are able to view the bigger picture”.

The findings have highlighted key areas that require additional research; specifically, to investigate the causes of the declines in white-tailed tropicbird breeding population at Aride and breeding success at Aldabra. In fact, despite these findings only just being published, the results have already been used to leverage additional research. In 2018 and 2019, a seabird ecologist from the University of Oxford, Dr Annette Fayet worked with the Seychelles Islands Foundation and the Island Conservation Society (ICS) to deploy tracking devices on tropicbirds on Aride and Aldabra and set camera traps to capture the causes of nest failure.

Dr Fayet said: “It is great to see these important results published and to see proposals to continue and standardize tropicbird monitoring in Seychelles. The differences in productivity between populations highlighted by the study raise important questions about the factors driving these differences, which we are currently investigating. In particular, we are testing for potential differences in food availability and predation between populations in the inner and outer Seychelles”.

Commenting on the study ICS’ Chairman, Adrian Skerret, commented: ‘It is vital to understand trends and fluctuations in seabird breeding success as these reflect the health of our oceans and the impact of changes in climate and food supply. But we cannot interpret what is happening in the ocean environment as a whole without working together. There is a need for more studies such as this that bring together observations from multiple sites and multiple organisations to build up a global picture with global implications.’

The authors of the paper propose the adoption of standardised monitoring programmes and increased site coverage across Seychelles and the broader Western Indian Ocean. They believe this, coupled with increased pooling of data and regular analysis, will strengthen the collective effort towards the conservation management of this and other seabird species within this global seabird hotspot.

Burt added that “this study demonstrates the great benefit of collaborative multi-site research, monitoring and analysis; by joining forces we were able to do more with fewer resources than if we had all worked separately”.

There are additional recommendations to be found in the paper which is open access and can be accessed here: https://www.ace-eco.org/vol16/iss2/art28/

Full paper reference:

Burt, A. J., F. Cagua, C. Sanchez, L. Calabrese, J. van de Crommenacker, J. McClelland, N. Shah, H. Richards, and N. Bunbury. 2021. Combining monitoring data from multiple sites to assess population status and trends of White-tailed Tropicbirds (Phaethon lepturus) in the Seychelles. Avian Conservation and Ecology 16(2):28. https://doi.org/10.5751/ACE-01858-160228

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