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The secret life of Fairy Terns revealed

Updated: Apr 16, 2022

Over the past three years, I've had the immense privilege of studying the Australian Fairy Tern Sternula nereis nereis for my PhD. I learned about their lives in intimate detail, including their movements, natural history, behaviour and feeding ecology. On May 7, 2021, I presented my research findings at a public pre-completion seminar and with more than 100 people tuning in from around the world, you could say it was a great 'tern' out.

Unfortunately, I forgot to press record but here's a summary of my thesis findings entitled Natural history, ecology and population dynamics of the Australian Fairy Tern and implications for their conservation.

About Fairy Terns

The Australian Fairy Tern is one of seven closely related species in the genus Sternula. It is also one of three subspecies, which are all found within the Indo-Pacific Region, the other two being the New Zealand Fairy Tern Sternula nereis davisae and New Caledonian Fairy Tern Sternula nereis exsul.

Like most seabirds, Fairy Terns are relatively long-lived (the current longevity record is 24 years), they produce small clutches of eggs (usually one or two, three is rare) and most probably don't breed until three years of age. Thus, they have a slow life history.

Fairy Terns exploit a range of coastal habitats but most commonly select open, lightly coloured, coarse-grained sand spits and beaches. These sites are often near the mouths of estuaries, on nearshore islands or the mainland, adjacent to sheltered embayments, where small baitfish are abundant. Colony site fidelity is low and sites are often abandoned after one or more seasons, even where previously successful.

Nest failure is naturally high but a range of human-induced pressures, such as disturbance, exacerbate failures. Populations in the eastern states are thought to have been decreasing for at least three decades due, mainly, to low reproductive output. This led to the species being listed as Vulnerable to extinction under the Environment Protection and Biodiversity Conservation Act (1999) in 2011.

Project Rationale

The behaviour and life history strategies of terns vary widely among species. Thus, conservation strategies based on indicator species are not likely to be successful (Palestis 2014).

Despite being vulnerable to extinction and in urgent need of conservation action, there had been no major studies on Fairy Terns here in Australia. Much of what we knew about their biology, behaviour and feeding ecology was based on casual observations and nothing was known about their movements.

My project sought to address these knowledge gaps and inform future conservation efforts.

Movements and Meta-population Structure

This study aimed to determine the movement patterns of the Western Australian meta-population including the interchange of breeding adults and recruits at various spatial scales.

We mark-recaptured terns at 11 locations – colour bands denoted their place of capture. Dedicated surveys and coordinated community sightings were then used to locate banded birds.

From our recapture/resighting records, there appear to be two distinct populations of Fairy Terns in Western Australia. The first is a sedentary population that breeds during the winter months off the Pilbara coast. The second is a spring/summer breeding population that spends the winter months at the Houtman Abrolhos Islands. As the breeding season approaches, some birds move north, some remain at the Abrolhos Islands, while others move south. This two-way migration within a population is unusual.

Terns typically returned to the same region where they were banded but not necessarily the same colony sites. Based on the exchange of breeding adults and natal recruits among sites, there appear to be seven geographical regions (termed neighbourhoods) where the birds return each season.

We also found evidence of group adherence, with associations between birds at colonies persisting over multiple breeding seasons. Group adherence is thought to be an important behavioural trait of terns that nest in ephemeral habitats, allowing for the rapid reestablishment of colonies at new locations.

This work highlights the important role of citizen science and the opportunity to collect and coordinate data to help inform conservation. Overall, the banding project was an effective method to understand the seasonal movements and metapopulation structure of Fairy Terns in Western Australia. For more detail, see Dunlop & Greenwell (2021).

The Secret Life of Fairy Terns

This study aimed to investigate the behaviour of Fairy Terns between their arrival in south-western Australia to their migration to wintering grounds. Colony formation processes, habitat preferences, incubation, time to fledging, parental roles and post-hatching parental care were a major focus of this work.

There was a significant relationship between the date nests were established and the percentage cover of beach shells surrounding the nests. On average, birds that laid eggs earlier in the season selected territories with higher shell cover compared to birds that established nests later. We also found that the average distance from nearest neighbours increased over time.

Of the 92 nests I monitored, 82 (89%) hatched and the mean incubation period was 21 d. In two egg clutches, chicks hatched 0–2 d apart. Adults shared incubation duties almost equally and, on average, they brooded eggs/chicks for 1.27 h before changing over. Chicks fledged, on average, 22 d after hatching and left the colony with their parents within 8 days of fledging. For further detail see Greenwell et al. (2021a).

We also used a novel banding method (painted uniquely coloured nail varnishes onto ABBBS leg bands) to follow chicks in the field. This allowed us to successfully documented age-related behaviour and plumage changes up to 100 days of age Greenwell et al. (2021b).

Feeding Ecology

We investigated the diet of the Fairy Tern, a colonial, bill-loading seabird, at three colonies across an inner shelf seascape (Point Walter, Penguin Island and Rottnest Island) using non-invasive photo sampling. Fairy Terns feed by plunge-diving and during the breeding season, single prey items are carried, crosswise in their bill, back to colonies to feed mates and chicks.

I undertook ~240 hours of sampling over 57 days encompassing four different time periods. Over 9000 unique prey items were identified from 21 families. Fish accounted for 99.9% of all prey, while the squid Southern Calamari made up the remaining 0.1%.

Small pelagic schooling fishes were the most important prey caught by Fairy Terns at all sites and the timing of breeding in south-western Australia (October to February) overlaps the peak spawning period of several of the most important fishes. The abundance of these fishes in nearshore habitats is, undoubtedly, an important factor influencing the location of colonies.

Multivariate statistical analyses showed that the composition of prey differed significantly among locations, breeding seasons, life history stages, i.e. chick versus courtship feeding, and time of day. For more detail, see Greenwell et al. (2021c).

Cat Gets its Tern

If you needed any more evidence for why cats should be kept indoors, here it is. One desexed, free-roaming cat, observed by residents and wildlife cameras, visiting the colony at night was implicated in the death of six adult terns, 40+ chicks and, ultimately, the abandonment of an entire colony of ~220 birds.

We worked hard to protect the terns - deploying sonic deterrents and cat traps, sending letters to residents asking they keep cats indoors, and undertaking overnight stakeouts to keep the cat away. We thought the issue had been resolved after not seeing the animal for ~a week. Unfortunately, it came back and the terns abandoned the site.

This study highlights the significant negative impacts of free-roaming cats on wildlife. It also provides strong evidence against the practice of trap-neuter-release programs and demonstrates that desexed cats can continue to negatively impact wildlife post-release directly through predation, but also indirectly through fundamental changes in prey behaviour and a reduction in parental care (Greenwell et al. 2019).

This tragic event raised much awareness about the impact of cats in the environment but also the challenges Fairy Terns face during the breeding season. In addition, it led to the City of Mandurah adopting new local laws to help reduce the impact of cats within the community.

Please, if you own a cat - get a cat run or keep it indoors. This will improve welfare outcomes for your feline friend and reduce their impact on wildlife and your neighbours.

Research Implications

Overall, this research addressed important knowledge gaps about Fairy Terns and will be used to help inform future conservation strategies.

Our research into the effectiveness of audio-visual (decoys and call playback) cues, showed that while settling decisions of terns varied between sites, there is potential for call playbacks and, to a lesser extent, decoys, to be an effective tool for Fairy Tern conservation in the future. To learn more, see Greenwell et al. (2021d).


I acknowledge the traditional custodians of the land where I had the great privilege of conducting my research – those of the Whadjuk Noongar and Binjareb people. Sincere thanks to the long list of collaborators and the many volunteers who assisted with this research. Thank you to BirdLife Australia & the Stuart Leslie Bird Award, Ecological Society of Australia & Holsworth Wildlife Research Endowment, Fremantle Ports, Rottnest Island Authority and WA Parks and Wildlife Service for their generous support. Thanks also to the anonymous reviewers, who generously gave their time and provided feedback on manuscripts. Finally, to my supervisory team - Team Fairy - thank you for your guidance and support. It has been the most incredible journey.

I'm heading off to travel Oz for ~6 months. Come along for the journey by following my blog. Be sure to visit my website for more research and photos.

I'll post new images as I go.

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Jun 08, 2021

The banding colors are a brilliant idea. Are the terns banded as chicks?

The cat issue… sigh… this is a tough one. As a cat owner myself, keeping my cat indoors is a responsibility I take seriously and can eliminate so many issues including the life of your pet.

Jun 08, 2021
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We banded both adults and chicks. The colour bands were applied to adults so we could understand their movements and population structure. Since the colour banding project has met its objective, we won’t be colour banding in the foreseeable future. Regarding the free-roaming cats, they are a major problem but we can improve pet welfare and reduce their impact by confining them - either indoors or in a cat run.

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