Hybrid of Two Tern Species Discovered in Seychelles

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.