What happens when two different species, live together, mate and reproduce? A new ‘hybrid’ species is sometimes born. Hybridisation between species plays an important role in evolution, though there is considerable debate about its significance. Around 25% of plants and 10% of animals are known to form hybrids with at least one other species. There is even evidence of hybridisation in modern humans, with up to 4% of DNA from all people living today attributable to Neanderthals.

In an astonishing discovery, on the remote St. Francois Atoll in Seychelles' Alphonse Group, conservationists have confirmed a rare hybridisation event between two different tern species: a Roseate Tern (Sterna dougallii or ‘Dyanman Roz’ in Creole) and a Black-naped Tern (Sterna sumatrana or ‘Dyanman Likou Nwanr’ in Creole). This is the second recorded time hybridisation between the two said species has been recorded anywhere in the world, the first being in Australia in 2004.

The colony of Black-naped Tern on St Francois - one of the three islands of the Alphonse group - has been monitored by conservationists from the Island Conservation Society (ICS) since 2008. According to a scientific publication published recently, using photographic evidence and field observations, ICS observed a single Roseate Tern that established itself within the colony of Black-naped Terns. It was then observed successfully mating with a Black-naped Tern to produce a ‘hybrid’ offspring with distinctive features of both parent species. For example, it had distinct red legs, a characteristic unique to Roseate Tern and not Black-naped Tern. The egg from this pairing also exhibited mixed features, for example, they were noticeably larger than that of both species, but also had pigmentation resembling the eggs of the Roseate Tern. Over time, Conservationists also observed a ‘hybrid’ mating with another ‘pure’ Black-naped Tern; a process called ‘backcrossing’.

Although hybridisation is known to occur in avian species, the study does not speculate why the two species have chosen to mate together. One suggestion is that Terns, as observed in Fairy Terns, are very loyal to their mate and have a tendency to return to a previously visited location or site fidelity. Hence, since this first pair of a Roseate Tern and a Black-naped Tern has been successful at mating and producing offspring at St Francois; they returned annually to repeat the same process.  

The findings have crucial implications for the overall population of both species. Seychelles hosts the only known breeding grounds for Black-naped Terns in the African region. In addition to St. Francois Atoll, both species of tern also breed on nearby St. Joseph Atoll, Étoile (an uninhabited low-lying coral cay of the Amirantes group) and Farquhar Atoll. Black-naped Terns also breed on African Banks, Bancs Providence, Aldabra and Cosmoledo Atoll in the Outer Islands. In contrast, Roseate Terns once bred at several of the inner islands, but survive today only at Aride Island.

Black-naped Tern is a relatively scarce species in the Seychelles with an estimated 245 to 345 pairs – the entire breeding population of the Afrotropical region.  As for Roseate Tern, although it has a wide distribution around the world, its population is declining. It is estimated that 1,600 pairs of Roseate Tern visit the Seychelles from May to August to breed only. The majority (over 1,200 pairs) breed on Aride Island and rely on the protection of this island as a Special Reserve for their continued survival.

With both species being relatively in low numbers throughout Seychelles, this study highlights the need for further study to assess the extent of genetic mixing and its consequences on the evolution of both species on St Francois. Since this occurrence is only in one location, there is a risk that the population of the ‘pure’ Black-naped Tern will continue to inbreed within itself and with the Roseate Tern. However, the conservation implications of hybridisation are disputed. While the process might threaten rare species, it could also save a rare lineage from extinction by introducing genetic diversity. A "pure" lineage could harm conservation by lowering genetic diversity and adaptive potential, particularly in species with low populations.

It is uncertain what will happen to the unique and distinct characteristic of both species if this continues. This is a fascinating and rare aspect of avian biology that is unique to Seychelles and the Alphonse Group and that merits more research. This hybridisation phenomenon also underscores the fragility and biodiversity of Seychelles' ecosystem, particularly in the protected St. Francois Atoll—a critical site for seabird conservation.   

Over the last 15 months the ICS Alphonse team has been working with the @Alphonse Fishing Company on 'accelerometer trials' on the Giant Trevally, a species which is resident within the lagoon. The tests assess reflex impairment and post-release condition to uncover the physiological and behavioural effects of catch-and-release. Last month the desired sample size of 40 was achieved. We would like to extend our gratitude to the many fishing guides who assisted ICS, in particular Trevor Sithole and Warren Graham for their enthusiasm. The data will be analysed by our partners at @UniMassachusetts and the results will help to further define the Fly-fishing Code-of-Conduct. Collaboration has been key to this @SeyCCAT funded project, and we are excited to continue working with our partners towards scientifically supported sustainable practices.

On the 22nd of September, much of the Alphonse Island community gathered to unload essential supplies from the first barge in six months. ICS were notified when a carcass was spotted floating in the lagoon. The 10m long-dead whale washed up on shore, allowing for the collection of measurements and DNA samples, with consideration not to burst the blubbery, bloated body, which was in a state of advanced decomposition. The tail fluke was missing and the underside eaten by sharks. Strong south-east winds kept the overwhelming stench at bay, while a JCB manoeuvred the whale into its final resting place buried in the sand. Unfortunately, the next day it was re-exposed, so staff set to work on plan B – towing it back out to sea. The shape of the skull and small anterior flipper suggest that this was a Sperm Whale and analysis of genetic samples could reveal its age and sex.

Each year, conservation staff working at the remote Alphonse Group, look forward to the shift in prevailing wind from south-east to north-west, which coincides with the return of migratory birds. Many species fly back from their breeding grounds in the northern hemisphere, to over-winter in the tropics. In particular, St. François Atoll is a magnet for many species of waders, including large flocks of Crab Plover, because the lagoons and mangroves are home to plentiful fish stocks and crustaceans. 

In this relatively short period, the Conservation Team on Alphonse Island recorded 110 various bird species, but more migrating birds such as the Madagascar Cuckoo and Grey-tailed Tattler.

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